UNIVERSITY OF NAIROBI
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
GEOTHERMAL DEVELOPMENT, EXPLORATION AND DRILLING IN KENYA
PROJECT NO: 128
ROTICH BERNARD KIMTAI
F17/1844/2006
SUPERVISOR: DR. CYRUS WEKESA
EXAMINER: DR. N.O. ABUNGU
THIS PROJECT REPORT IS SUBMITED IN PARTIAL FULFILMENT OF THE
REQUIREMENT FOR THE AWARD OF A BACHELOR OF SCIENCE DEGREE IN
ELECTRICAL AND INFORMATION ENGINEERING
Submitted on: 18TH MAY 2011
ii
DEDICATION
To my family for the support and love they have given me……Thank you
iii
ACKNOWLEDGEMENTS
My heartfelt thanks, first to the almighty for giving me the gifts I have including people who
always support me in my life. Family is a gift. I also extend sincere gratitude to my
supervisor, Dr. Wekesa, for his unfailing guidance and assistance throughout this process. He
has shown so much guidance and patience to me in the past one year. His professional
knowledge and wisdom contributed much to this project. I could not ask for a better advisor
and mentor.
Huge gratitude also goes out to all my lecturers in the department of Electrical and
Information Engineering, for all the knowledge they have imparted on me throughout the
time I have been here. I will carry this knowledge everywhere I go. I would like also to thank
my auntie, Evelyn, who always stood by me throughout the whole process. Finally, special
thanks go to Janet, Judith and my colleagues for their academic support and constructive
criticism.
God bless you all.
Thank you.
iv
DECLARATION AND CERTIFICATION
This BSc. work is my original work and has not been presented for a degree award in this or
any other university.
……………………..
ROTICH BERNARD KIMTAI
F17/1844/2006
This report has been submitted to the Dept. of Elect and Info Engineering, University of
Nairobi with my approval as supervisor:
…………………
DR. CYRUS WEKESA
Date: 18th May 2011
v
TABLE OF CONTENTS
DEDICATION ................................................................................................................. ii
ACKNOWLEDGEMENTS ................................................................................................. iii
DECLARATION AND CERTIFICATION ............................................................................... iv
TABLE OF CONTENTS ..................................................................................................... v
LIST OF TABLES .......................................................................................................... viii
LIST OF FIGURES........................................................................................................... ix
ABSTRACT .................................................................................................................... x
CHAPTER 1: INTRODUCTION ..........................................................................................1
1.1 Background.................................................................................................................. 1
1.2 Statement of problem.................................................................................................. 2
1.3 Objectives of Study ...................................................................................................... 2
CHAPTER 2: LITERATURE REVIEW ...................................................................................3
2.1 Geothermal Energy ...................................................................................................... 3
2.2 Components of Geothermal Energy ............................................................................. 3
2.3 Forms of Geothermal Energy ................................................................................... 4
2.4 Advantages of Geothermal Energy over other forms of Energy.................................... 5
2.5 Uses of Geothermal Energy.......................................................................................... 6
2.5.1 Electricity Generation (power plants) .................................................................... 6
2.5.2 Heat Production..................................................................................................... 7
2.6 Stages of Geothermal Energy Development ................................................................. 7
CHAPTER 3: GEOTHERMAL ENERGY DEVELOPMENT IN KENYA..........................................9
3.1 Introduction................................................................................................................. 9
3.2 Geological Setting ...................................................................................................... 10
3.3 Geothermal Prospects in Kenya ................................................................................. 11
3.4 Exploration status of Geothermal Prospects .............................................................. 12
CHAPTER 4: GEOTHERMAL ENERGY EXPLORATION TECHNIQUES .................................... 13
4.1. Introduction........................................................................................................... 13
4.2 Objectives of Geothermal Exploration ............................................................... 14
4.3 Exploration Technologies ....................................................................................... 14
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CHAPTER 5: EXPLORATION TECHNIQUES COMMONLY USED IN KENYA............................ 19
5.1 Geophysical methods................................................................................................. 19
5.1.1 Thermal methods ................................................................................................. 20
5.1.2 Electrical Methods............................................................................................... 21
5.2 Geochemical Methods ............................................................................................... 24
CHAPTER 6: GEOTHERMAL WELL DRILLING ................................................................... 25
6.1 Introduction............................................................................................................... 25
6.1.1 Exploration.......................................................................................................... 25
6.1.2 Appraisal ............................................................................................................. 26
6.1.3 Production and re-injection.................................................................................. 26
6.1.4 Make-up .............................................................................................................. 27
6.1.5 Work-over ........................................................................................................... 27
6.2 Drilling Fluids ............................................................................................................. 27
6.2.1 Purpose of drilling fluids ..................................................................................... 27
6.3 Drilling Rigs ................................................................................................................ 28
6.3.1 Basic functions .................................................................................................... 28
6.3.2 Types of rigs ....................................................................................................... 29
6.3.3 Rig equipment systems ........................................................................................ 30
7.1 Objective of well plan ................................................................................................ 33
7.2 Classification of wells ................................................................................................. 33
7.3 Purpose of casing....................................................................................................... 33
7.4 Categories of casing strings........................................................................................ 34
7.5 Selecting casing depths .............................................................................................. 34
7.6 Hole geometry (well casing profile)............................................................................ 34
7.7 Casing design ............................................................................................................. 35
7.8 Cementing ................................................................................................................. 35
7.8.1 Purpose ............................................................................................................... 35
7.8.2 Primary cementing procedure .............................................................................. 35
7.8.3 Factors that influence slurry design ..................................................................... 36
7.8.4 Cementing additives ............................................................................................ 36
7.9 Well output optimization ........................................................................................... 37
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CHAPTER8: A CASE STUDY OF MENENGAI PROSPECT ..................................................... 38
8.1 objective .................................................................................................................... 38
8.2 Introduction............................................................................................................... 38
8.3 Methodology ............................................................................................................. 39
8.4 Results and discussions .............................................................................................. 40
8.4.1 Soil temperature measurements ........................................................................... 41
8.4.2 CO2 concentrations ............................................................................................. 41
8.4.3 Thoron-CO2 ratios .............................................................................................. 41
8.5 Conclusion ................................................................................................................. 41
CHAPTER 9: CONCLUSION AND RECOMMENDATION...................................................... 43
9.1 Conclusion............................................................................................................. 43
9.2 Recommendations for Future Work ........................................................................... 43
REFERENCES ............................................................................................................... 44
APPENDIX A................................................................................................................ 45
APPENDIX B: ............................................................................................................... 46
Geophysical Exploration Technologies.......................................................................... 46
APPENDIX C ................................................................................................................ 47
viii
LIST OF TABLES
Table 3.1: Exploration status of Geothermal Prospects 12Table 8.1: Descriptive statistics of CO2, Thoron ant Temperature from 275 sampling points
40Table 8.2: Rn220 radioactivity and CO2 concentration in fumaroles steam Error! Bookmarknot defined.
ix
LIST OF FIGURES
Figure 1.1: Geothermal Sites Worldwide............................................................................... 2Figure 2.1: some surface manifestations of Geothermal Resource ......................................... 3Figure 2.2 A schematic representation of an ideal geothermal system ................................... 4Figure 2.3: 3D cutaway showing electricity generation from a hot dry rock source ............... 5Figure 2.4: A systematic Diagram of a Geothermal Binary Power Plant ................................ 7Figure 3.1: Map of the Kenya Rift showing the geothermal prospects ................................. 11Figure 6.1: Typical conceptual model of a geothermal system in Kenya. ............................. 26Figure 6.2: Typical conventional rig ................................................................................... 30Figure 8.1: Location of Menengai Geothermal Prospect showing fumaroles, major faults andsampling points ................................................................................................................... 39
x
ABSTRACT
Geothermal is an indigenous and abundantly occurring resource in Kenya. Exploitation of
Kenya’s geothermal resources has environmental and social advantages over the other major
sources of electricity generation options available for the country. Geothermal is the least
cost source of electric power for base load in Kenya. Currently, 209 MWe of the installed
interconnected generation of 1350MWe is generated from geothermal sources at Olkaria
Geothermal is expected to contribute additional 5000MWe by 2030.
Regional exploration for geothermal resources in Kenya indicates that the Quaternary
volcanic complexes of the Kenya Rift Valley provide the most promising prospects for
geothermal exploration. In this, the common geo-science disciplines used are geology,
geophysics and geochemistry. These methods are a primary tool for carrying out
investigation of the subsurface in prospecting for natural resources such as geothermal
energy.
Drilling at a geothermal prospect typically begins long before construction of the plant is
initiated. Thermal gradient holes (TGH) are usually drilled before drilling a deeper
exploration hole (“slim hole”), which are then followed by drilling a full-scale production
well in order to glean more information about the temperature of a geothermal reservoir at
depth. In certain regions, developers will conduct shallow and intermediate cored drilling to
obtain a temperature profile and measure thermal conductivity in order to estimate heat flow
in areas where insufficient prior data exists. The drilling of the first production well at a
geothermal resource is widely considered to still be an exploration phase activity.
The study found out that geochemical and geophysical exploration methods are the
commonly used techniques in Kenya due to their cost-effectiveness. Further, the study found
that five geothermal prospects, with an aggregate estimated potential size of 4350 MWe have
undergone completed surface exploration.
1
CHAPTER 1: INTRODUCTION
1.1 BackgroundGeothermal energy is the natural heat from the earth's interior stored in rocks and water
within the earth's crust. This energy can be extracted by drilling wells to tap concentrations
of steam at high pressures and at depths shallow enough to be economically justifiable. The
steam is led through pipes to drive electricity-generating turbines. Geothermal fields are
fairly widespread in the world (Figure 1.1) and are exploited in Italy, the USA, New Zealand,
Japan, Mexico, El Salvador, Iceland, the Philippines and Turkey. Italy pioneered the use of
geothermal energy for generating electricity in 1904 at Lardarello, near Pisa. However, the
world showed little interest in geothermal development until the middle of the century when
intensive exploration work was undertaken in New Zealand, Japan, and the United States.
These exploration activities led to the commissioning of geothermal power stations in these
countries in 1958, 1951 and 1960 respectively. Iceland joined the club in 1930.
A better appreciation of the benefits of geothermal energy occurred in 1970's after United
Nations Conference on New Sources of Energy in 1961. This meeting helped to publicize the
benefits and possibilities of using geothermal energy as a reliable source of electricity.
Following the meeting, interest in geothermal development grew steadily especially from
1964 when a number of countries started preliminary investigation projects. Kenya is the
first African country to tap power from the crust of the earth for national development. This
power is tapped at Olkaria East by the Kenya Electricity Generating Company (KenGen),
while that of Olkaria West by OrPower 4. KenGen is a public utility while OrPower 4 is an
independent power producer. Geothermal energy in Kenya lies beneath the vast, but
environmentally and culturally sensitive East African Rift Valley. The exploration and
exploitation of this resource should be done in a way that does not have negative impacts on
the environment and human life.
2
Figure 1.1: Geothermal Sites Worldwide
1.2 Statement of problemThe project seeks to establish an overview of geothermal development, exploration and
drilling in Kenya. The case study presented is for Menengai prospect located in the central
rift of Kenya, with an estimated potential of 1200MW of geothermal energy.
1.3 Objectives of Study
To establish the development of Geothermal energy
To establish the exploration techniques commonly used in Kenya
To study the well drilling of a geothermal system.
To study the use of geochemical method of exploration at Menengai site
3
CHAPTER 2: LITERATURE REVIEW
2.1 Geothermal Energy
Geothermal energy is formed deep within the earth’s crust, and is exploited for electricity
generation and other direct uses. The medium of this energy transfer is geothermal fluid. On
the surface, these are manifested as hot grounds, fumaroles, geysers, mud-pools and hot
springs. Some of these manifestations are shown in Figure 2.1 below:
Figure 2.1: Some surface manifestations of Geothermal Resource
2.2 Components of Geothermal Energy
Geothermal systems are made up of four main components: a heat source, a reservoir, a fluid
(the carrier that transfers the heat) and a recharge area. When defining a geothermal system,
the principal consideration is the practicality of how much power can actually be produced.
In most instances, electric power generation is the reason for developing geothermal energy
and the typical geothermal system must yield 10 kg of steam to produce one unit (kWh) of
electricity. Therefore, a geothermal system must contain great volumes of fluid at high
temperatures or a reservoir that can be recharged with fluids heated by the hot rocks.
Geothermal fields are found in rocks such as shale, limestone and granite, with the most
common rock type being volcanic. [2]
4
A schematic representation of a geothermal system is as shown in figure 2.2 below:
Figure 2.2: A schematic representation of an ideal geothermal system
2.3 Forms of Geothermal Energy
The basic forms of geothermal energy are:
Hydrothermal Fluids
Hot Dry Rock
Geo-pressured Brines
Magma
Ambient Ground Heat
5
A diagram showing electricity generation from a dry hot rock source is as shown in figure
2.3:
Figure 2.3: 3D cutaway showing electricity generation from a hot dry rock source
2.4 Advantages of Geothermal Energy over other forms of Energy
Geothermal energy is likely to become a major contributor to Africa’s electrical power,
especially in those countries that are endowed with this resource. Other countries may benefit
by means of high voltage direct current (HVDC) lines. Geothermal potential exists in east
Africa, the Horn of Africa, and parts of north and southern Africa. Key benefits of
geothermal energy include the following:
Geothermal energy is competitive in terms of cost. Estimates indicate that it
can even compete with large-scale hydro power.
Geothermal power plants have near zero emissions, (true for modern closed
cycle systems that re-inject water back to the earth’s crust) and very little
space requirement per unit of power generated. This makes geothermal energy
an attractive option compared to fossil fuel alternatives.
6
Geothermal energy is not susceptible to seasonal fluctuations, and is available
all year round. This is in contrast to hydroelectric power, which is affected by
low rainfall and oil fired power plants, which can be prohibitively expensive to
operate when oil prices are high.
Geothermal energy has other direct uses such as space heating and heating of
greenhouses for horticultural farming.
2.5 Uses of Geothermal Energy
Various uses of this type of energy are indicated below:
2.5.1 Electricity Generation (power plants)
Dry Steam Power Plant
Flash Steam Power Plant
Binary Cycle Power Plant
An example of a Geothermal Binary power plant is as shown in the figure2.4 below. The
flow of geothermal fluid is in red, the secondary fluid in green, and the cooling water in blue.
7
Figure 2.4: A systematic Diagram of a Geothermal Binary Power Plant
2.5.2 Heat Production
District Heating
Industrial Process Heat
Agriculture
Aquaculture
Electricity generation is the most important form of utilization of high-temperature
resources (greater than 150 °C), while medium to low temperature resources (less than 150
°C) are suited to many different types of applications.[2]
2.6 Stages of Geothermal Energy DevelopmentGeothermal development typically consists of several key steps. The prospective geothermal
fields undergo systematic investigation and evaluation processes from their initial
exploration and development until steam production mechanisms have been implemented.
These studies and evaluation processes are fairly similar worldwide with corresponding
modifications and innovations to suit the particular geothermal area of interest in each
country.[2]
8
From the perspective of the resource, a geothermal project can be divided into the following
phases:
Project definition and reconnaissance evaluation
Detailed exploration
Exploratory drilling and delineation
Resource analysis and assessment of development potential
Field development
Steam production and resource management
Upon confirmation of a resource for development, a complete feasibility study undertaken on
the project would also consider the corresponding geothermal power plant to be set up for
converting the energy from steam to power. The power plant will undergo the following
phases, in parallel with geothermal field development: bid tendering, design, manufacturing
and delivery, construction, commissioning and operation.
9
CHAPTER 3: GEOTHERMAL ENERGY DEVELOPMENT IN KENYA
3.1 IntroductionExploration for geothermal energy in Kenya started in the 1960’s with surface exploration
that culminated in two geothermal wells being drilled at Olkaria. In early 1970’s more
geological and geophysical work was carried out between Lake Bogoria and Olkaria. This
survey identified several areas suitable for geothermal prospecting and by 1973, drilling of
deep exploratory wells commenced and was funded by UNDP.
Additional wells were thereafter, drilled to prove enough steam for the generation of
electricity and in June 1981, the first 15 MWe generating unit was commissioned. The
second 15 MWe unit was commissioned in November 1982 and the third unit in March 1985
which increased the total generation to 45 MWe. This was the first geothermal power station
in Africa and is owned and operated by KenGen. GDC (Geothermal Development Company)
is currently undertaking production drilling in the field to support an additional 140MWe
plant being developed by KenGen for commissioning in 2013. KenGen commissioned a
70MWe Olkaria II power plant in the Northeast field and was further increased by 35MW in
2010 giving the station an installed capacity of 105MWe. Greater Olkaria field is 55MWe
Orpower4 plant. The generator is currently undertaking production drilling to support a
further 36MWe in the field. GDC is also drilling production wells in the Olkaria IV area
earmarked for 140MWe plant being developed by KenGen. The plant is planned for
commissioning in 2013. All the developments at Olkaria will result in a total installed
capacity of 525MW by 2013 against a proven geothermal resource potential of well over
1,000MWe.
The Government of Kenya and other party have continued to explore for geothermal energy
in the Kenya rift. British Geological Survey and the government of Kenya carried out studies
in the Lake Magadi area, the area around Lake Naivasha including Longonot, Olkaria,
Eburru, and Badlands, and in the north rift. Geotermica Italiana also did some work in
Longonot and Suswa calderas and also described geo-volcanological features important in
geothermal exploration at Menengai.
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3.2 Geological SettingKenya rift is part of the eastern arm of the East African rift system. The segment referred to
as the Kenya rift extends from Lake Turkana in the north to northern Tanzania near Lake
Natron.
In this sector of the rift system, the activity began about 30 million years ago in the Lake
Turkana area and then migrated southward being more intense about 14 million years ago.
Formation of the graben structure started about 5million years ago and was followed by
fissure eruptions in the axis of the rift to form flood lavas by about 2 to 1 million years ago.
During the last 2 million years ago, volcanic activities became more intense within the axis
of the rift. During this time, large shield volcanoes, most of which are geothermal prospects,
developed in the axis of the rift. The volcanoes include Suswa, Longonot, Olkaria, Eburru,
Menengai, Korosi, Paka, Silali, Emuruangogolak, and Barrier Complex as shown in
figure3.1.
11
Figure 3.1: Map of the Kenya Rift showing the geothermal prospects
3.3 Geothermal Prospects in KenyaMore than 14 high temperature potential sites occur along the Kenyan Rift Valley with an
estimated potential of more than 15,000 MWe. Other locations include: Homa Hills in
Nyanza, Mwananyamala at the Coast and Nyambene Ridges. These prospects are at different
stages of development.
12
They include the following:
Menengai
Eburru-Badlands
Arus-Bogoria
Olkaria
Longonot
Suswa
Lake Magadi
Lake Baringo
Korosi
Paka
Silali
Emuruangogolak
Namarunu
Barrier
3.4 Exploration status of Geothermal Prospects
Table 3. 1: Exploration status of Geothermal Prospects
PROSPECT ESTIMATED SIZE SURFACE EXPLORATIONOlkaria 1000MW CompleteMenengai 1200MW CompleteLongonot 750MW CompleteBarrier 450MW Not doneSilali 800MW CompletePaka 500MW CompleteNamarunu 400MW Not done
13
CHAPTER 4: GEOTHERMAL ENERGY EXPLORATION TECHNIQUES
4.1. Introduction
A significant amount of the financial risk associated with geothermal power development
results from uncertainties encountered in the early stages of resource development; namely
geothermal exploration and the drilling of production wells. Costs associated with the
exploration and drilling stages of a geothermal project can account for at least 42% of overall
project costs. While current exploration and drilling technologies and practices are somewhat
effective, opportunities to significantly improve upon conventional technology as well as
develop more advanced “revolutionary” exploration and drilling tools exist. Increased
research and development (R&D) that is focused on improving current conventional
exploration and drilling technologies in the near term as well as bringing “breakthrough”
technologies will help to establish a strong geothermal industry base and improve project
economics. [1]
Success in initial geothermal exploration directly influences success in geothermal drilling
operations. Geothermal developers depend upon exploration of surface features to determine
first; whether or not a resource is worth the large amount of investment required to drill a
geothermal well, and second; where to place a drill rig so as to reach an optimal target depth,
permeability and resource temperature. Once surface features have been investigated, several
techniques are used to help identify drill targets without having to put a drill bit into the
ground. These exploration technologies not only need to better locate geothermal resources
but they must be able to provide more accurate imaging of the structure of the subsurface
reservoir and provide accurate reservoir temperatures at specified depths. In order to better
locate geothermal resources improved remote sensing technologies are necessary. The
development of more reliable geothermometers with the ability to detect soil/water gases or
dissolved minerals and isotopes would improve geochemistry information gleaned in the
exploration phase. [1]
14
The increased resolution and reliability of geophysical surveys would provide a much
needed improvements in imaging subsurface geothermal reservoirs.
Significant improvements in conventional geothermal drilling and well development
techniques are achievable in the near term. [1]
4.2Objectives of Geothermal Exploration
When pursuing geothermal exploration, it is important to consider the following objectives
as part of the program:
To identify geothermal phenomenon
To ascertain that a useful geothermal production field exists
To estimate the size of the resource
To determine the type of geothermal field
To locate productive zones
To determine the heat content of the fluids that will be discharged by the wells in the
geothermal field
To compile a body of basic data against which the results of future monitoring can be
viewed
To determine pre-exploitation values of environmentally sensitive parameters.
4.3 Exploration TechnologiesPrior to constructing a geothermal power plant and delivering power to the electrical grid, a
series of steps must be taken by a geothermal developer to ensure the successful completion
of a geothermal development project. The first of these steps is conducting a thorough
exploration program of any given geothermal resource site.
The only means by which a developer can know for certain whether or not their geothermal
site contains an economic resource is to drill at least one full size production well. However,
drilling a production well is costly. Most developers will drill their first production well only
after a complete exploration regimen has provided enough information to ensure some degree
15
of confidence of reaching a specific reservoir temperature, at a specific depth, with adequate
flow rates.
The exploration regimen consists of a combination of geological, geochemical, and
geophysical surveys, all of which are designed to provide increased rates of success in
drilling the initial production well. The exploration and drilling phases of developing a
geothermal resource are often compared to those of the oil and gas industry. [1]
However, while some of these techniques may be useful in geothermal exploration, the
subsurface characteristics of geothermal resources can significantly limit their effectiveness.
At the same time, as the oil and gas industry has had to locate resources in geological
regimes more similar to geothermal (i.e. oil or gas shale), and opportunities for technology
transfer between the industries may soon be possible. [1]
Future research and development in exploration stage technologies should focus on those
factors that affect project success rates regardless of extrinsic factors. It is certain that the
success of initial production well construction is directly related to the quality of previously
conducted exploration. While there is no substitute for the information gleaned and the
confidence gained from drilling an initial production well exploration technologies and
practices will certainly need to be improved in order to ameliorate the upfront risk associated
with the high costs of geothermal drilling.
In order to better identify research and development needs in geothermal exploration,
commonly used technologies must be identified. The main technologies are discussed below.
4.3.1 Remote Sensing TechnologiesMany geothermal resources exhibiting clear surface manifestations (i.e. hot springs) have
already been located. The majority of future geothermal megawatts will come from resources
that exhibit more subtle surface features such as altered rock, salt crusts/evaporites, tufas,
travertine’s, sinter, and opal. These features are usually detectable over large areas using
remote sensing techniques.
Remote sensing is usually conducted by satellite or airborne observation which uses sensors
to detect different wavelengths of light to differentiate between different rock types.
16
The main advantage of conducting a remote sensing survey is that it can be done prior to
initiating the expensive and lengthy procedure of obtaining the land rights to a resource.
Thus remote sensing is an important “first step” in the exploration process. [1]
4.3.2 Geochemical Technologies
Geochemical, along with geophysical and other pre-drilling exploration methods, are
initiated upon the obtaining of land rights. If conducted previously, they are typically
continued after a developer gains access to the geothermal resource. An important issue in
developing geothermal resources is to determine how hot the resource might be at depth
without drilling. Fortunately, minerals and springs on the surface can indicate the
temperatures below necessary to create those mineral properties. [1]
The underlying assumption of geochemical analysis in geothermal exploration is that surface
manifestations of geothermal fluids can provide information on temperature and
physiological conditions in the subsurface geothermal reservoir. Obtaining this information
is accomplished by using geothermometers that are based on the relative amounts and ratios
of various elements or isotopes in the water. The levels of these elements within the
geothermal fluid and with respect to each other provide insights into the geothermal reservoir
temperature. Geochemists will gather and interpret data points from multiple
geothermometers in order to make the most reliable subsurface temperature estimates. Some
technologies and techniques which could be further developed to improve geochemical
exploration are soil and gas geochemistry and rare earth element (REE) geochemistry.
Soil and gas geochemistry entails the placement of detectors in or on the ground to detect
gases (such as mercury or carbon dioxide) associated with geothermal reservoirs at depth.
Background calibrations for local conditions must be taken into account and sometimes
detectors must remain in the ground for months to obtain useful data. The advantage of soil
and gas geochemistry is that it can be used to locate information on hidden geothermal
systems. More advanced technology is enabling the use of REE geochemistry in geothermal
exploration. Modern ultra-low-level measurement techniques enable geochemical explorers
to measure elements associated with geothermal activity that were previously below the
levels of analytical detection. [1]
17
4.3.3 Geophysical Technologies.Geophysical technologies can also provide clues as to what is happening in the subsurface.
Rather than identify potential temperatures, geophysical techniques provide indications of
the structure of subsurface geology and how those structures can be drilled to bring hot water
from the geothermal aquifer to the surface. Combined with geochemical studies, geophysical
analysis seeks to identify temperatures, permeability, and the orientation of fractures at
depth. While the oil industry has found 3D Seismic Tomography to be extremely effective in
locating subsurface oil and gas plays, no such geophysical “silver bullet” currently exists for
the geothermal industry.
Rather, geothermal developers will employ various studies from a suite of geophysical
exploration methods to better understand a geothermal reservoir prior to drilling.
The combination of geophysical survey methods employed in an exploration program are
dependent on intrinsic (i.e. local geology, hydro geochemistry) and extrinsic (local weather,
economic, and land issues) factors.
As such, some methods will be more effective in certain resource areas than others,
guaranteeing their widespread use. Still, certain geophysical technologies are more widely
used than others and incremental improvements in their technology or use could yield
improved exploration success rates. [1]
4.3.4 Seismic Imaging.
Seismic-imaging surveys. use explosive charges or man-made vibrations to direct waves into
the subsurface at the location of a suspected geothermal resource Waves that reflect off
subsurface structural features are used to render a 3D image of the geothermal reservoir.
The oil and gas industry began to experience great success in using 3D seismic imaging in
the 1990’s. Currently few oil or natural gas wells are drilled unless a thorough seismic-
imaging survey of the resource has been conducted. Seismic imaging had until the 1990s met
with less success in the geothermal industry. Some experts in the geothermal industry have
long asserted that rocks in geothermal reservoirs are too broken up for seismic waves to
render a proper subsurface picture.
18
Overcoming past difficulties with obtaining subsurface imaging from seismic surveys in
most geothermal prospects, the technology is being used successfully in areas such as
Imperial Valley and Coso, Calif., and throughout central and western Nevada. Additionally,
some experts believe that the industry is on the cusp of developing seismic imaging
technology that could significantly improve the economics of developing geothermal
resources by providing a more accurate 3D model of subsurface geothermal reservoirs. [1]
4.3.5 Other Geophysical Exploration Technologies and methods
While improved 3D seismic survey technology could significantly improve the success rate
drilling initial production wells, advances in other exploration technologies are also available
to geothermal developers.
Gravity surveys consist of gravitational field measurements being taken at several prospect
locations to identify the different density profiles of subsurface rock types and are especially
important in delineating buried granites, an important component of EGS. Airborne gravity
gradiometry technology has been developed that allows for the measurement of gravity
changes over large distances. This new development of gravity survey technology would aid
in imaging of the subsurface landscape and the location of faults in a geothermal reservoir.
Lastly, magnetotelluric (MT) surveys can be used to locate higher conductivity associated
with geothermal reservoirs at depths of 2 – 5 km. MT surveys have been used successfully in
geothermal exploration and are important in EGS as well as conventional hydrothermal
exploration.[1]
19
CHAPTER 5: EXPLORATION TECHNIQUES COMMONLY USED IN KENYA
5.1 Geophysical methods
Geophysical exploration of geothermal resources deals with measurements on the physical
properties of the earth. The emphasis is mainly on parameters that are sensitive to
temperature and fluid content of the rocks, or on parameters that may reveal structures that
influence the properties of the geothermal system. The aim can be to:
• delineate a geothermal resource;
• outline a production field;
• locate aquifers, or structures that may control aquifers in order to site wells;
• assess the general properties of the geothermal system.
The important physical parameters in a geothermal system are:
• Temperature;
• Porosity;
• Permeability;
• Chemical content of fluid (salinity); and
• Pressure
Most of these parameters cannot be measured directly through conventional geophysical
methods applied on the surface of earth. On the other hand, there are other interesting
parameters that can be measured which are linked with the parameters above and may thus
give important information on the geothermal system. [4]
Among these parameters are:
temperature
electrical resistivity
magnetization
density
seismic velocity
seismic activity
thermal conductivity
streaming potential
A distinction is usually made between direct methods, and indirect or structural methods.
20
The direct methods give information on parameters that are influenced by the geothermal
activity, while the structural methods give information on geological parameters which may
reveal structures or geological bodies that are important for the understanding of the
geothermal system. The direct methods include: thermal methods, electrical (resistivity)
methods and self potential (SP), while the structural methods include magnetic
measurements, gravity measurements, active seismic methods and passive monitoring of
seismicity. [4]
Different methods may be applied for the exploration of low-temperature fields compared to
high temperature resources. Furthermore, different methods are sometimes used from one
country to another despite similar geothermal surroundings, based on the routines that have
been developed at the different institutions. It is also important to combine different
methods, as relying on the results of measurements of a single parameter, usually, does not
give adequate information for good understanding of the geothermal system. [4]
5.1.1 Thermal methodsThermal methods include direct measurements of temperature and/or heat, and thus correlate
better with the properties of the geothermal system than other methods. However, as a (near-)
surface method, they are limited to shallow levels. To measure temperatures close to the
surface, in the uppermost metre or so, is fairly simple. Knowledge about status at deeper
levels is based on the existence of wells, usually shallow gradient wells (e.g. 30-100 m deep),
from which the thermal gradient can be calculated and possibly the depth to the exploitable
geothermal resource. Drilling is though usually fairly expensive, and puts practical limits to
the use of the method. Furthermore, shallow wells are not always adequate to get reliable
values on the thermal gradient.
The heat exchange mechanism in the earth is important for interpretation of thermal methods.
A distinction is made between:
• Conduction, which is based on atomic vibrations, and is important for transfer of heat in the
earth's crust;
• Convection, which transfers heat by motion of mass, e.g. natural circulation of hot water;
and
21
• Radiation, which does not influence geothermal systems. [4]
5.1.2 Electrical Methods
5.1.2.1 IntroductionElectrical methods or resistivity methods are the most important geophysical methods in the
surface exploration of geothermal areas, and as such, the main methods used in delineating
geothermal resources and production fields. The parameter of interest is the electrical
resistivity of the rocks which correlates both with the temperature and alteration of the rocks
which are key parameters for the understanding of the geothermal systems.
The main principle is that electrical current is induced into the earth which generates an
electromagnetic signal that is monitored at the surface. The basic relationship behind
resistivity measurements is the Ohm’s law, which states that:
E = D j
Where E is the electrical field strength;
j is the current density; and
D is the electrical resistivity, which is a material constant.
For a unit cube/bar, the relationship for resistivity is defined as:
D = V / I
The reciprocal of resistivity is conductivity, thus it is also possible to talk about conductivity
measurements. However, in geothermal, the tradition is to refer to electrical or resistivity
measurements.
Electrical methods include many different types of measurements and varying setups or
configurations for the different types. The most important types are:
DC methods, where current is generated and injected into the earth through
electrodes at the surface. The measured signal is the electrical field generated
at the surface.
TEM, where current is induced by a time varying magnetic field from a
controlled source. The monitored signal is the decaying magnetic field at
surface from the secondary magnetic field.[4]
22
MT, where current is induced by the time variations in earth's magnetic field.
The measured signal is the electromagnetic field at the surface.
5.1.2.2 Resistivity of rocksElectrical resistivity of rocks in geothermal surroundings is a parameter which reflects the
properties of the geothermal system, or its history. Thus, a good knowledge on the resistivity
is very valuable for the understanding of the geothermal system. This relates to the fact that
the resistivity of rocks is chiefly controlled by parameters that correlate to the geothermal
activity, such as:
Porosity and pore structure, where distinction is made between:
Intergranular porosity such as in sedimentary rocks,
Fracture porosity, relating to tension, fracturing or cooling of igneous rocks,
Vugular porosity which relates to dissolving of material (limestone) or gas content (in
volcanic magma);
Alteration of the rocks, lining the walls of the pores, often related to as water-rock
interaction;
Salinity of the fluid in the pores;
Temperature;
Amount of water, i.e. saturation or steam content; and
Pressure.
The four listed first, namely fracture (secondary) porosity, alteration, salinity of the fluid,
and temperature, are the most important ones, and basic parameters for a geothermal system.
This explains why the parameter “resistivity of the rocks” is so important in geothermal
exploration, and especially in volcanic surroundings, and hence the important role of
resistivity soundings.
Generally, it can be said that electrical conduction is mainly through intercom-nected water-
filled pores. If the rocks are fresh, the conduction is mainly through the water, while the
alteration lining the walls of the pores is the decisive factor, when created at temperatures
between 50 and 200°C, due to its very conductive properties. [4]
23
5.1.2.3 TEM measurementsIn the late 1980s, a new method started to make its impact in geothermal exploration. This is
the TEM method (or Transient Electro-Magnetic method), which is an electromagnetic
method which uses a controlled-source to create the signal to be measured.
In the Central loop TEM sounding method, referred to as TEM, a constant magnetic field is
built up by transmitting current, I, through a big loop (grounded dipole). Then the current is
abruptly turned off. A secondary field is thus induced, decaying with time. This decay rate is
monitored by measuring the voltage induced in a receiver coil in the centre of the loop on the
surface. Current distribution and decay rate recorded as a function of time depend on the
resistivity structure below the measuring site, and can be interpreted as such. The signal can
also be based on a grounded dipole to create the primary magnetic field. TEM data are
presented on a bilogarithmic scale as DC data, but here the apparent resistivity is plotted as a
function of time after the current was turned off. [4]
TEM equipment is sophisticated and relatively expensive. Besides the receiver including a
data logger, a transmitter is required connected to a good electric generator able to generate
high currents (the order of 10 A) and thus a strong magnetic field through the transmitting
loop.
5.1.2.4 MT (Magnetotelluric)-measurements
MT or natural-source electromagnetic uses the earth’s natural electromagnetic field as its
power source. The variable natural magnetic field induces electrical currents in the
conductive earth. By measuring the signal of the fluctuating magnetic field and the electrical
currents (i.e. the electrical field) on the surface of the earth, it is possible to correlate this to
the resistivity of the earth below the measuring site. The frequency of the signal relates to its
probing depth, with low frequencies reaching deeper levels. Thus, frequencies of 0.00001 -
10 Hz are used for deep crustal investigations, while higher frequencies, like 10 - 1000 Hz,
for the upper crust. The MT equipment is fairly simple and portable with the electromagnetic
field monitored through magnetic coils and electric dipoles, through a data acquisition
system, usually connected to remote reference station for electro-magnetic noise. [4]
24
MT is a powerful method to probe deep resistivity structures, which gives it an advantage
compared to the other main electrical methods. The equipment is portable and the data
collection is simple, involving measuring magnetic field components B and the induced
electrical field E, both as a function of time for several hours, at each site. MT measurements
are, however, quite sensitive to cultural noise (power lines etc.). Similarly, the measurements
probe a large volume of rocks and are therefore sensitive to 3-D resistivity variations.
Detailed interpretation can therefore be difficult and may require 3-D interpretation. More
recently, the method has routinely been used in combination with TEM, with the TEM
measurements used for mapping of the uppermost kilometer in detail in order to enhance the
interpretation of the MT measurements and thus leading to better information at deeper
levels. This way, good information on the distribution of the resistivity into the deeper parts
of the geothermal system can be collected, reaching to 5-10 km depth. [4]
5.1.2.5 Gravity Measurements
Gravity measurements are used to detect geological formations with different densities. The
density contrast leads to a different gravitational force. The density of the rocks depends
mainly on the rock composition and its porosity, but partial saturation of the rocks may also
influence the values. Normally the density is between ~2 and 3 g/ cm3. Generally,
sedimentary rocks are lighter than crystalline rocks. The raw data needs to be corrected for
several factors. Methods of data interpretation are quite similar to those for magnetic
measurements. [4]
5.2 Geochemical MethodsThe major goals of geochemical exploration are to obtain the subsurface composition of the
fluids in a geothermal system and use this to obtain information on temperature, origin, and
flow direction, which help locating the subsurface reservoir. Equilibrium speciation is
obtained using speciation programs and simulation of processes such as boiling and cooling
to get more information to predict potential deposition and corrosion. Environmental effects
can be predicted and the general information is used as a contribution to the model of the
geothermal system. [4]
25
CHAPTER 6: GEOTHERMAL WELL DRILLING
6.1 Introduction
The drilling process, complex as it may be, basically involves breaking the ground and
lifting the rock cuttings from the resulting hole. The ultimate geothermal drilling objective is
to access the resource for exploitation. However, during resource development and
exploitation, drilling is used to confirm existence of the resource, obtain data for resource
assessment, provide adequate steam fuel for the power plant and resolve well production
complications.
Tri-cone tungsten carbide insert bits are very often used in geothermal drilling. Mobile and
conventional land rigs are predominantly used in the geothermal drilling industry. The rigs
are selected to technically fit the job at the lowest cost possible. The wells are made useful
by casing them. Several casing string are used for each well. They are cemented to bond
them to formation. Large production casing of 13 3/8” casing is increasing becoming
common where large well outputs are encountered and directional drilling is being employed
to target major faults that transmit fluids.[7]
Actual breaking of ground is achieved by use of a rock bit. The bit is rotated under weight.
The bit both crashes and gouges the rock as it rotates. The broken rock pieces arising from
the drilling are lifted from the bore by floating them in a circulating drilling fluid. This
process continues until the well is completed. The ultimate goal for drilling is to access the
resource for exploitation. However, during the resource development and exploitation,
drilling serves various purposes as describe below:
6.1.1 Exploration
The very first evaluation of a prospect is achieved through detailed surface reconnaissance. It
is aimed at defining the resource by its key system characteristic namely: existence of a heat
source in the form of hot magmatic body near earth surface, existence of hydrological
system, characteristic of the geological setting and a real extent of the prospect (figure 6.1).
26
However, while the surface measurement and mapping and evaluation of the surface
manifestations provide great insight as regards the resource characteristics and potential,
results of the reconnaissance remain inferences and are inconclusive.
Figure 6.1: Typical conceptual model of a geothermal system in Kenya.
The initial employment of drilling in geothermal prospecting is aimed at providing proof of
exploitable steam and data required for further refining of the conceptual model.
6.1.2 Appraisal
Striking steam with the first well while is exciting opens up doors for more questions.
Having confirmed existence of the resource, the next question is its technical, economic and
financial viability. Further drilling (appraisal) is therefore carried out to delineate the
resource and establish production well and reservoir fluids characteristics.
6.1.3 Production and re-injection
At this stage of development, a decision to construct a plant is already made. The drilling is
therefore to provide sufficient steam to run the plant. Additional wells are drilled for
reinjection purpose. One reinjection well is required for every 4 to 5 production wells.[8]
27
6.1.4 Make-up
After commissioning of the power plant, with time, the reservoir suffers pressure decline
which affects well productivity. In addition, deposition may occur within the formation
around the wells, further reducing wells productivity. With time, therefore further drilling is
carried out to replenish the reduced steam delivery.
6.1.5 Work-over
Two types of problem may arise during exploitation. Steam depletion in the shallow
reservoir may necessitate deepening of the initial wells or deposition of scales within the
well bore may necessitate a mechanical removal of the scales. These two cases require some
form of drilling to accomplish.[8]
6.2 Drilling Fluids
6.2.1 Purpose of drilling fluids
Primarily, the drilling fluid function is to remove the cuttings from the bottom of the hole as
fast as they are created to facilitate further and efficient hole making process. In addition, the
fluid transports the cuttings to surface. The two functions constitute what is normally
referred as hole cleaning. The drilling fluid in real drilling situation is a complex subject with
consideration ranging from the basic hole cleaning, economics, availability, logistics,
chemistry, safety, fluid dynamics and reservoir management. As such the drilling fluids serve
many functions. The major functions include;
Cleaning of the hole bottom,
Carry cuttings to the surface
Cool and lubricate the bit and drill string
Remove cuttings from muds at the surface
Minimize formation damage
Control formation pressure
Maintain hole integrity
Assist in well logging operations
Minimize corrosion of drill string and casing
Minimize contamination problems
28
Minimize torque, drag and pipe sticking
Improve drilling rate
Cooling of the formation – unique for geothermal
6.2.2 Types of drilling fluids
The drilling fluids vary widely. The following are the various classifications of drilling
fluids:
6.2.2.1 Water based drilling fluid
a) Fresh water muds with little or no treatment. This include spud mud, inhibited muds
and natural clays
b) Chemical treated muds without calcium compounds added. This includes phosphate
muds, organic treated muds (lignite, chrome-lignosulfonate etc.)
c) Calcium treated muds which include lime, calcium chloride and gypsum
d) Salt-water muds which include sea water muds, saturated salt water muds
e) Oil emulsion muds i.e. oil in water
f) Special mud
6.2.2.2 Oil based drilling mud
a) Oil based mud
b) Inverted emulsion mud – water in oil
6.2.2.3 Gaseous drilling fluids
a) Air or natural gas
b) Aerated mud
c) Foam
6.3 Drilling Rigs
6.3.1 Basic functions
From a basic and simplistic view, the rig can be seen as that equipment that provides the
motive power to rotate the bit, allow weight on the bit to crash the rock beneath and circulate
the drilling fluid and hence achieve the drilling action.
29
Achievement of these basic rig functions requires systems and processes where various
individual pieces of equipment serve only as part of the function in the whole process and
system. Operational requirements and economics dictate the sophistication of drilling rigs.
6.3.2 Types of rigs
All rigs are categorized as either land or marine. Each of these categories, comprise various
types of drilling rigs.
6.3.2.1 Marine rigs
Drilling rigs used offshore (in water) are termed marine rigs. They fall under two categories;
those supported on water bottom and floating vessels. The marine rigs are not employed in
drilling of the geothermal wells.[8]
6.3.2.2 Land rigs
The land rigs fall under two main categories; the cable tools and the rotary rigs. The cable
tools accomplish the drilling action by raising a special drill bit and dropping it. The cable
tools are the predecessor of the modern rotary rigs and are hardly used anymore.
The rotary rigs fall under three categories:
The standard derrick where the mast/derrick was built on location and
dismantled after the drilling process.
Portable rig mostly truck-mounted for low rig up time
Conventional rig where key components are so large that they cannot be
transported on a single truck bed (figure 6.2).
30
Figure 6.2: Typical conventional rig
6.3.3 Rig equipment systems
The rig has six distinct systems:
1. Power system
2. Hoisting System
3. Circulating System
4. Rotary system
5. BOP System
6. Auxiliary Rig equipments
31
6.3.3.1 Power system
The power system consists of a prime mover, primarily diesel engines, and some means of
transmitting the power to the auxiliary equipment.
Transmission may be in the form of mechanical drives like chains, DC generators and motors
or AC generators, SCR (Silicon control rectifiers), Dc motors.[8]
6.3.3.2 Hoisting system
The hoisting system is one of the major components of the rig. Its primary function is to
support, lift and lower rotating drill string while drilling is in progress. It consists of:
Supporting structure: The support structure includes the mast or derrick,
the substructure and the rig floor.
The hoisting equipment: This includes the draw works, crown block,
travelling block, hook, links, elevators and the drilling wire-line.
6.3.3.3 Circulation system
The circulation system is another major component of the rig affecting its overall success. Its
main purposes are stated under the drilling fluid section. It consists of pumps, standpipe,
rotary hose, swivel, Kelly, drill string, shale shakers, tanks and mud pits.
6.3.3.4 Rotary system
The rotary system is responsible for imparting a rotating action to the drill string and bit. The
principle components are the Kelly, rotary table and drive bushing, swivel rotary hose and
drill string.
6.3.3.5 BOP system
The blowout preventer (BOP) are primarily used to seal the well to prevent uncontrolled
flow, or blowout, of formation fluids. Typically it consists of annular BOP, drill pipe or
casing ram BOP, blind ram BOP and accumulator system.
6.3.3.6 Auxiliary rig equipment
The auxiliary rig equipments are those items of the equipment that added to the draw works,
rotary, Kelly, swivel, blocks, drilling line, bits and prime movers, make it possible for the rig
to function more efficiently.[8]
32
They can be broadly be grouped as:
Drill string handling tools; spinning wrenches, power tongs, hydraulic torque
wrenches, power slips, automatic drilling, Kelly spinner, automatic cathead.
Instrumentation; weight indicators, mud pumps pressure gauge, rotary tachometer,
rotary torque gauge indicator, pump stroke indicator, tong torque indicator, rate of
penetration recorder
Air hoist
Rig floor tools.
33
CHAPTER 7: WELL PLANNING AND CASING DESIGN
7.1 Objective of well planThe main objective of planning a well is to drill safely, minimize costs and drill usable well.
7.2 Classification of wells
Wells can be categorized as follows:
Exploration/discovery wells - No geological data or previous drilling records exist
Appraisal wells - Delineates the reservoir’s boundary; drilled after the exploration
wells
Production wells - Drill the known productive portions of the reservoir
Work-over wells - Re-entry of already drilled wells to deepen, clean etc.
Planning for the drilling of exploration wells takes more effort than appraisal wells and
production drilling. This is because the discovery wells are drilled in unknown area thus the
unexpected can happen.
7.3 Purpose of casingThe target resource is found for Kenya from around 500m to as deep as 3000m. The wells are
cased for the following reasons:
Isolate fresh underground water to prevent contamination
Maintain the hole integrity by preventing caving in to enable drilling further below
Minimize lost circulation into shallow permeable zone
Cover weak zones that are incompetent to control kick-imposed pressure (prevent
blowouts)
Provide a means for attaching and anchoring BOP and wellheads and thereby contain
resultant pressures
Provide safe conduit for the reservoir fluids to the surface
To prevent cooling of the reservoir fluids by shallow cooler fluids
Prevent well collapse.
34
7.4 Categories of casing stringsBefore the well is drilled to completion, several strings of casing are run and cemented in
place. The actual number used is depended on the drilling safety and operational problems
anticipated or encountered. The types of casing strings are:
Surface casing: Mainly used to isolate the shallow loose formation to enable further
trouble free drilling below.
Intermediate casing: This may be more than one string. They primarily isolate the
shallow potable water from contamination, provide anchorage for the wellhead and
seal off zones of loss of drilling fluid. They also protect the shallow formation from
high downhole pressure thus prevent blowouts.
Production casing: This primarily act as the safe conduit for the reservoir fluid to
surface, protect shallow formation from deep reservoir pressure thus prevent blowouts
and isolate cooler shall water from degrading the reservoir fluids
Slotted liner: This is primarily run to prevent the reservoir wellbore from collapsing
and blocking the well flow path.[7]
7.5 Selecting casing depthsThe first design task in preparing the well plan is selecting the depths to which the casing
will be run and cement.
The considerations made are the geological conditions such as formation pressures and
formation fracture gradient. Other considerations are policy and government regulations.
7.6 Hole geometry (well casing profile)Having decided on the casing depth, the next design aspect is to decide on the casing string
sizes to be run in the hole. The key consideration at this point is well productivity versus
costs. Small well bore may choke the well thus rendering it unproductive while on the other
side large wellbore cost much more. The drilling industry has developed several commonly
used geometries. These programs are based on bit and casings availability as well as the
expected drilling conditions. The most common casing geometry employed in geothermal is:
20” Diameter casing for 26” diameter surface hole
13 3/8”diameter casing for 17 ½” diameter intermediate hole.
9 5/8” casing for the 12 ¼” production hole
35
7” slotted liner for the 8 1/2” main hole
The considerations made are casing inner and outer diameter, coupling (collar) diameter and
bit sizes. Sufficient allowance is made to allow flow area between the casing and wellbore to
reduce washouts while provide sufficient velocity for drilling fluid to lift cuttings.[7]
7.7 Casing designThe casing is used for protection during the entire life of the well and therefore it is
designed to withstand many severe operating conditions.
Common problems often considered for casing design when drilling are kicks, lost
circulation, stuck pipe, wear, hydrogen sulphide environment and salt. Just like the drilling
string, the casing is designed to with stand burst, collapse, tension forces and biaxial effects
(combined effects). In General the thicker the casing, the more resistance it is to the above
factors. However, the more the well cost.[7]
7.8 Cementing
7.8.1 PurposeCementing of casings is one of the critical operations during the drilling of a well that affects
the producing life of a well. Casing strings are usually cemented in the hole to:
Bond the casing to the formation
Protect deeper hot producing zones from being cooled by cooler water
emanating shallow bearing zones
Minimize the danger of blowouts from deeper high pressure zones by isolating
weaker shallow zones
To isolate shallow troublesome formation to enable deeper drilling.
7.8.2 Primary cementing procedure
Primary cementing is the most important of all cement Jobs. It is performed immediately
after the casing is run into the hole. The objective is to deliver quality cement behind the
casing that is the annulus between the casing and the formation or previous casing strings.
Two methods are normally employed for the primary cement job, namely; the conventional
and stub-in (stinger) method. [7]
36
The conventional method could be single or multiple stage technique. In the single stage
cementing technique, cement slurry is mixed in the pumping truck and the cement slurry
pumped inside the casing string through the cementing head. After the entire slurry volume
has been pumped, the cement slurry within the casing is displaced to the float collar using
water. The two are separated by use of cementing plugs. The cement is prevented from
flowing back by the ball valve fitted on the casing float collar or casing float shoe or using a
valve fitted to the cementing head. After landing casing and before commencing pumping
cement slurry, a drilling fluid is circulated in the hole.[7]
7.8.3 Factors that influence slurry design
There are many factors considered in primary cement job slurry design. The key ones include
the well depth, well bore temperature, pumping time, slurry density, strength of cement
required to support the pipe, lost circulation, filtrate loss and quality of mixing water.
The mixing water should be clean for the resulting slurry to develop the desired properties in
particular strength. Deep wells require fairly long time to carry out and complete the
cementing jobs. This means that the cement slurry must remain pumpable for the entire
period during the cementing job. Temperature and pressure accelerates the setting of cement
slurry. Therefore it is very important to take into consideration the effects of these
parameters. Major losses of cement can result to very expensive jobs both on lost cement and
operation time. The density of the slurry is designed to effectively control blowouts and to
displace mud from the well bore.[7]
7.8.4 Cementing additives
The desired properties for a specific cement slurry design are achieved by adding various
chemicals and materials (additives) that alter the ordinary Portland cement normal behavior.
The additives are classified as follows:
Accelerators
Lightweight materials
Heavy weight materials
Retarders
Lost circulation control materials
37
Filtration- control agents
Friction reducers and
Specialty materials
Accelerators are used to shorten the cementing thickening time, light weight additives are
added to the slurry to reduce the slurry density while the heavy weight additives are added to
increase the density. The cement retarders are added to the cement to increase the slurry
thickening time for long jobs while friction reducers are added to the cement slurry to
improve flow properties of the slurry. The lost circulation control materials are added to the
slurry to bridge minor formation fractures that would take up cement while filtration control
additives are added to reduce the water loss from the slurry to the formation which would
result to early thickening of the cement slurry.
7.9 Well output optimization
The objective of drilling a well is to obtain the maximum output from the well. Where good
permeability have been encountered, it has been shown that the production casing size of 9
5/8” diameter has inhibited well output in some cases. In such fields, it is becoming
increasing more common to use the 13 3/8” casing as the production casing. It is now a
common practice to drill directional wells which target faults that control fluid movement
with the objective of increasing well output. Over 60% of the well cost is incurred drilling
the upper section of the well to the production casing (500 - 1500m). Drilling of forked or
multi-legged well completions may become increasing common as a way to optimize
investment economics.[8]
38
CHAPTER8: A CASE STUDY OF MENENGAI PROSPECT
8.1 objectiveThe main objective of this study was to establish the soil gas concentration of CO2 and Rn220
and relate with known geological structures at Menengai prospect using geochemical method
of exploration.
8.2 IntroductionThe Menengai Geothermal Prospect is located in the central section of Kenyan Rift Valley,
the area north of L. Nakuru and south of Lake Bogoria. The extend of the prospect covers an
area of approximately 600km2 characterized by a complex geological setting with Menengai
caldera being notably the major geological feature in the area and is also important for its
geothermal potential.
The general exploration for geothermal resources in Kenya indicates that the Quaternary
volcanic complexes of the Kenya rift valley provide the most promising prospects for
geothermal exploration, and as a result, intensive exploration work has focused on areas with
volcanic centres located within the rift valley. Studies show that these volcanic centres have
positive indications of geothermal resource that can be commercially exploited. Menengai
geothermal area is one of the priority prospects in the current prospects ranking. Eleven
locations with fumaroles have been identified in the region (Figure 8.1).
A number of thermal anomalies and diffuse degassing assessments have been carried out in
most of geothermal prospect areas in Kenya, including Menengai. Results for CO2 and Rn-
220 soil diffuse concentrations have been found to discharge more in areas that coincide with
faults. The purpose of this study is to establish the soil gas concentration of CO2 and Rn-220
and relate with known geological structures.
39
Figure 8.1: Location of Menengai Geothermal Prospect showing fumaroles, major faults and
sampling points
8.3 MethodologyThe procedures employed in the study are divided into: fumarole steam condensate and gas
sampling and soil gas sampling to determine mainly the concentrations of carbon dioxide
and radon radioactivity. For fumaroles, steam and condensate samples were collected for
various analyses. Soil CO2 concentrations measurements were performed using an Orsat
apparatus whereas Rn-220 soil gas concentrations were measured with a portable radon
detector (emanometer). A total of 275 sampling points were measured.
The fumarole gases were sampled by directing the steam into two evacuated gas flasks for
each fumarole.The CO2 as a percentage of the total gas was measured using the Orsat
apparatus.
40
Soil gas samples were obtained using a spike, equipped with a steel outer jacket to penetrate
the ground to a depth of 0.7m.The same procedure was repeated for the acquisition of Rn220
radioactivity.A random separation of about 300m in average was used for the soil gas survey
sampling.The soil gas sample containing Radon was pumped into the decay chamber of the
Emanometer,consisting of a cylindrical copper can and the readings recorded in counts per
minute(CPM).Three background counts were recorded at three-minute interval prior to
introduction of sample into the emanometer.
An infra-red thermometer was used to take the temperature by directly pointing it into the
hole made by the spike and the resultant temperature recorded.
8.4 Results and discussionsThe results of descriptive statistics of CO2, Rn-220 and Temperature collected are presented
in Table 8.1.
Table 8. 1: Descriptive statistics of CO2, Thoron ant Temperature from 275 sampling points
Range Minimum Maximum Mean Std.
Deviation
Variance
Statistics Statistics statistics Statistics Statistics Statistics
Rn-220 6427 0 6427 386 603 363391
CO2 12.30 0.00 12.30 1.12 1.54 2.37
Temp. 55.90 21.20 77.10 28.70 6.99 211.52
Table8. 2: Rn220 radioactivity and CO2 concentration in fumaroles steam
Fumaroles
NO.
Easting’s Northing’s Elevation CO2(%)
vol.
Temp.
(oC)
Rn-222
(cpm)
Rn/CO2
ratio
MF-1 172400 9975900 2160 15.1 60.0 1650 109.3
MF-2 175324 9977356 1992 25.6 94.0 12194 476.4
MF-3 171868 9977563 2081 16.7 77.1 2200 131.7
MF-4 169080 9984679 2016 48.2 81.2 3700 76.76
MF-5 167712 9985324 1896 43.8 86.1 2890 66.0
MF-6 173208 9979555 1908 32.6 84.6 16848 516.8
MF-7 173682 9977020 2106 18.3 73.5 1600 87.4
41
8.4.1 Soil temperature measurementsThe mapping of temperature variations at or below the earth’s surface is an essential
geothermal exploration instrument. Quite a number of anomalies can be identified in relation
to the major structural setting. And as indicted by results, a higher temperature anomaly in
the caldera is evident around the fumaroles with the highest fumaroles temperature.
Similar to other soil surveys, soil temperature results in the caldera is mainly affected by the
lava that covers the caldera floor and apart from the areas where the lava did not cover.
8.4.2 CO2 concentrationsThe concentration of CO2 in the soil gas in the surveyed area is given as a percentage of the
total gas in the soil. The degassing of CO2 through the soil in the prospect area is considered
to be mainly supplied by two sources, the one of volcanic origin emanating from deep
environment and a shallow one that result from organic activity.
8.4.3 Thoron-CO2 ratiosThe ratio of the Rn-220/CO2 gases would be a good indicator of the magmatic source of the
gases since the ratio is not expected to change if they are from the same source. A mantle
source of CO2 is a possibility of the source of this gas due to intense faulting in the area, and
these faults around Ol’Rongai are of great importance in localizing degassing of CO2 and
Rn-220 as well thermal manifestation anomalies. If a mantle source is inferred due to intense
faulting, then the faults should be deep seated. These areas apparently coincide with high
CO2 and high Rn-220 absolute values
8.5 ConclusionDespite the erratic distribution of sampling points, the results from acquired data can be used
to make some conclusions relative to the objective. The fumaroles show close relations with
geologic structures, which provides means of access for hydrothermal fluids to reach the
surface. In the same way, most of the gas anomalies are located in fault zones. CO2 and Rn-
220 emanations from the presently quiescent faults in Menengai were measured to be higher
than background values.
The two fault-lines at Menengai are considered in this study to be of great importance in
geothermal exploration and any future development as it reflects intensity of subsurface
permeability in the area, and enhances the confidence to invest.
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The caldera itself as a geological structure displays a positive evidence of the existence of
geothermal resource, with its thermal anomalies that coincides with CO2 and thoron
anomalies. Generally, CO2 concentrations are structurally controlled but may have been
modified by changes in land use.
As seen from the findings, the study of soil CO2 and Rn-220 composition as proved to be
important geochemical method to identify vertical zones of high permeability. The
employment of these methods in other geothermal fields could give important and relatively
cheap information for field utilization and development.
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CHAPTER 9: CONCLUSION AND RECOMMENDATION
9.1 ConclusionThe project was successful since;
1. The study of geothermal development was achieved
2. Geochemical and Geophysical are the commonly used exploration techniques in
Kenya
3. Geothermal well drilling study was accomplished
4. Kenya has 14 geothermal sites with 5 of them with complete exploration done
5. Menengai site has an estimated potential size of 1200MW with geochemical method
used as the main method of exploration.
9.2 Recommendations for Future WorkA number of improvements could be carried out on this project. These include;
1. Econometrics of Geothermal process - This includes the cost of exploration and
drilling of geothermal energy in Kenya.
2. Electrical equipments needed for use in Geothermal exploration - this includes the
instruments use to carry out geophysics, magnetics, resistivity and geochemists.
Electrical methods or resistivity methods are the most important geophysical
methods in the surface exploration of geothermal areas. The parameter of interest is
the electrical resistivity of the rocks which correlates both with the temperature and
alteration of the rocks which are key parameters for the understanding of the
geothermal systems.
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REFERENCES
[1] Dan Jennejohn, 2009,”Research and Development in Geothermal Exploration and
Drilling”
[2] Prospecting for Geothermal Resources, Lumb, J.T., 1981.
[3] Geothermal Energy Association, 1999, “Geothermal Energy, the potential for clean
, power from the earth”.
[4] Geophysical Methods in Geothermal Exploration by Ludvik S. Georgsson.
[5] Arnason K. and Flovenz O.G., 1992: Evaluation of Physical methods in Geothermal
Exploration of rifted Volcanic Crust.
[6] Mbuthi P., 2004, HBF Geothermal Study: Kenya.AFREPREN, NAIROBI.
[7] Adams N.J., 1985: Drilling Engineering, A complete Well Planning Approach.
[8] Moore P.L., 1986: Drilling Practices Manual, 2nd Edition.
[9] Smith K.D., 1996: Cementing, 2nd Edition.
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APPENDIX A
Description of key rig parts
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APPENDIX B:
Geophysical Exploration Technologies
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APPENDIX C
Location map of geothermal fields in Kenya
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