Sonatrach's _Well Testing

7/29/2019 Sonatrach's _Well Testing

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Development Phase

September October 2005abalt solutions limited - 2005

INTRODUCTION TO HYDROCARBON EXPLOITATION

Introduction to Hydrocarbon Exploitation

2005 Abalt Solutions Limited. All rights reserved

Well Testing

Section

By Pratap Thimaiah

WellTesting


Well Testing

Well testing procedures are a set of tools which

properly used can provide valuable clues as tothe condition of production or injection wells.


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WellTesting


Well Testing

Requirements of the following parties should benormally considered:

Senior Management

Explorationists

Petroleum & Reservoir Engineers

Production & Facilities Engineers

Drilling Engineers

Oil & Gas Trading Partners

Regulatory Authorities

WellTesting


Generally oil or gas well production tests may

be classified as:

1. Periodic Production Tests

2. Deliverability Tests

3. Transient Pressure Tests

Well Testing


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WellTesting


1. Periodic Production Tests

Determination of relative quantities of oil, gas and water producedunder normal producing conditions.

Aid in well and reservoir operations, meeting legal & regulatoryrequirements.

Productivity or Deliverability Tests

Performed on initial completions or recompletions to determinewell capability under various pressure drawdown.

Aid in selection of well completions & artificial lifts.

1. Transient Pressure Tests

High degree of sophistication and determines formation damage

or stimulation related to individual well or reservoir.

Reservoir parameters such as permeability, pressure, volume and

heterogeneities.

Well Testing

WellTesting


Periodic Production Tests

Run routinely to physically measure oil, gas &

water produced from a well under normalproducing conditions.

Provide physical evidence of well conditions.

Unexpected changes such as extraneous water

or gas production may signal well or reservoirproblems.

Abnormal production declines may meanartificial lift problems, sand fill up, scale etc.


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WellTesting


For Oil Wells, results reported as

Oil production rates in bbl/day

Gas-Oil ratio in ft3/bbl

Water-oil ratio in % per system

For Gas wells

Gas production in mscf(1000 of standard cubic feetper day)

Gas production in mmscf

Hydrocarbon liquids or water in bcpmm

Periodic Production Tests

WellTesting



Physical or empirical determination of produced fluid flow versusbottom-hole pressure drawdown.

They predict what the well should produce at other pressuredrawdowns.

Successfully applied to non-Darcy, below bubble point flow conditions.

Do not permit calculation of formation permeability or degree of

abnormal flow restriction near wellbore.

Used as an indicator of well flow conditions.

They involve measurement of BH Static and flowing pressure as well asfluid rates produced to the surface


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WellTesting


Types

For Oil Wells

Productivity Index

Inflow Performance

Flow after flow

Isochronal or modified isochronal

For Gas Wells

Flow after Flow

Isochronal or modified isochronal


WellTesting


Transient Pressure Tests

Essential to analyse the current behaviour and future performanceof the reservoir.

Designed to provide a quantitative analysis of the reservoirproperties.

Conducted by creating a pressure disturbance in the reservoir andrecording the pressure response at the wellbore (bottom-holeflowing pressure)

Most common tests are:

Pressure drawdown

Pressure build-up Multi-rate

Interference Pulse

Drill Stem

Fall off Injectivity

Step rate


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WellTesting


Information available from a well test includes:

Effective permeability

Formation damage or stimulation

Flow barriers and fluid contacts

Volumetric average reservoir pressure

Drainage pore volume

Detection, length, capacity of fractures

Communication between wells

Transient Pressure Tests

WellTesting


Drawdown Test

Series of bottom-hole pressure measurements during a

period of flow at constant producing rate.

Well is shut-in prior to the flow test for a period of timesufficient to allow the pressure to equalize throughout theformation.

Main objective

To obtain the average permeability k of the reservoir within

the drainage area of the well. Sub-objectives

Assess the degree of damage of stimulation induced in the

vicinity of the wellbore through drilling and completion practices.

To determine the pore volume and to detect reservoir

heterogeneities within the drainage area of the well.


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WellTesting


Drawdown Test

WellTesting


During flow at constant rate of Qo, the pressurebehaviour of a well in an infinite-acting reservoir

(unsteady state flow) is given by:

Can be re-written as:

Then:

Semi-log plot of pressure drawdown data

Drawdown Test


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WellTesting


Drawdown Test

Other parameters that can be obtained from plot

Semi-log plot of pressure drawdown data

If p wf=p1hr hich is found on the extension of the

straight line at log t (1 hr), then:

If pressure data measured at 1 hour do not fall on t hat line, the line must beextrapolated to 1 hour and extrapolated value of p1 hr must be used then.

WellTesting


Steps involved in drawdown test analysis:

1. Plot (pi - pwf) versus t on a log-log scale.

2. Determine the time at which the unit slope line ends.

3. Determine the corresponding time at 1 log cycle,ahead of the observed time in Step 2. This is thetime that marks the end of the wellbore storageeffect and the start of the semi-log straight line.

4. Estimate the wellbore storage coefficient from:

Drawdown Test


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WellTesting


Drawdown Test

Steps involved in drawdown test analysis continued

5. Plot pwf versus t on a semi-log scale

6. Determine the start of the straight-line portion assuggested in Step 3 and draw the best line throughthe points.

7. Calculate the slope of the straight line and determinethe permeability k and skin factor s by applying the

equations below:

WellTesting


8. Plot all the recorded pressure data after teia as afunction of time on a regular Cartesian scale. Thesedata should form a straight-line relationship.

9. Determine the slope of the pseudo steady-state line,i.e., dp/dt (commonly referred to as m) and use theequation below to solve for the drainage area A,

10.Estimate the time to the end of the infinite-acting(transient flow) period, i.e., teia, which marks thebeginning of the pseudo steady-state flow.

Drawdown Test

where m = slope of the semi-steady-state Cartesianstraight-lineQ = fluid flow rate, STB/day

B = formation volume factor, bbl/STB


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WellTesting


Steps involved in drawdown test analysis continued

11.Calculate the shape factor CA from a expression thathas been developed by Earlougher (1977).Earlougher has shown that the reservoir shape factorcan be estimated from the following relationship:

12.Use Table of shape factor to determine the drainage

configuration of the tested well that has a value ofthe shape factor CA closest to that of the calculatedone, i.e., Step 11.

Drawdown Test

WellTesting


Pressure Build-up Test

Describes the build up in wellbore pressure with timeafter a well has been shut in.

Serves to determine the static pressure

Also serves to determine:

Effective reservoir permeability

Extent of permeability damage around the wellbore

Presence of faults and to some degree the distance tothe faults

Any interference between producing wells

Limits of the reservoir where there is not a a strongwater drive or where the aquifer is no larger than thehydrocarbon reservoir.


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WellTesting



In pressure build-up and drawdown analyses,the following assumptions are usually made:

With regard to reservoir

Homogeneous

Isotropic

Horizontal of uniform thickness

With regard to fluid

Single phase

Slightly compressible

Constant mo and Bo

With regard to flow behaviour

Laminar flow

No gravity effects

WellTesting




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WellTesting


Horners Plot


WellTesting


The Back-Pressure Test-Flow after Flow

Conventional deliverability test: gauging the ability of the well to flowagainst various back pressures. Rawlins and Schellhardt (1936)

Shut in the gas well sufficiently long for the formation pressure toequalize at the volumetric average pressure

Place the well on production at a constant flow rate Qg1 for asufficient time to allow the bottom-hole flowing pressure tostabilize at pwf1 to reach the pseudo steady state.

Repeat step 2 for several rates and the stabilised bottom-hole flowpressure is recorded at each corresponding flow rate. If three orfour rates are used, the test may be referred to as three-point orfour-point flow test.


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WellTesting


The Back-Pressure Test

WellTesting


Conventional back-pressure test

Length of the flow periods is the most importantfactor to be considered in performing theconventional deliverability test.

Each rate must be maintained sufficiently long for thewell to stabilise to reach the pseudosteady state.

The stabilisation time for a well in the centre of acircular or square drainage area may be estimatedfrom:

The Back-Pressure Test


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WellTesting


Isochronal Tests

Developed to shorten test times in wells that take long time to stabilize

Series of single point tests developed to estimate stabilizeddeliverability characteristics without actually flowing the well for timerequired to achieve stabilized conditions.

Conducted by alternatively producing the well, shutting in the well andallowing it to build-up average reservoir pressure before beginning ofthe next production period.

Pressures are measure several time increments during each flowperiod.

Ex- BHFP could be measured at 0.5,1.0,1.5 etc hrs after beginning ofeach flow period.

More practical for low-permeability formations.

Although not required, a final stabilized flow point usually is obtained atthe end of the test.

WellTesting


Isochronal Tests


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WellTesting


Modified Isochronal Tests

Time to build up to average reservoir pressure before flowing for acertain period of time may be impractical.

Isochronal test is modified to shorten test times further and obtainsame data without using the sometimes lengthy shut-in periods

required to reach average reservoir pressure in the wells drainage

area.

The shut-in periods are of equal duration & should equal or exceed

length of flow periods.

Shut-in sand face pressures are recorded immediately before each flow

period and used instead of average reservoir pressures.

This test is less accurate than the isochronal test.

Note-As the duration of the shut-in periods increases, the accuracy ofthis test also increases.

Although not required, a final stabilized flow point usually is obtained at

the end of the test.

WellTesting


Modified Isochronal Tests


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WellTesting


Drill Stem Testing

Provides a method of temporarily completing a well and determining itsproductive characteristics of the specific zones

DST can provide

Good data to evaluate productivity zone

Completions practises

Extent of formation damage

Need for stimulation

Reservoir characteristics that can be estimated from analysis are:

Average effective permeability

Reservoir pressure

Wellbore damage Barriers, permeability changes & fluid contacts

Radius of investigation

depletion

WellTesting


DST Operations

Running in hole on drill pipe, bottom hole assemblyconsisting of a packer and a surface operated valve.

The DST valve is closed while running the drill string,thus pressure inside the drill pipe is very lowcompared to hydrostatic mud column pressure.

On bottom, the packer is set to isolate desiredformation zone from mud column and control valveis opened to allow formation fluids to enter the drillpipe.

After a suitable time valve is closed and pressurebuilds-up.

Control valve is opened again, and the flowing andshut-in periods are repeated to obtain additionaldata

Drill Stem Testing


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WellTesting


Sequence of Operation:

Tool is lowered on the drill pipe to a position opposite to theformation to be tested.

Packer expands against the hole to segregate the mud filledannular section from the interval of interest.

The test valve allows formation fluids to enter the drill pipeduring the test.

The equalizing valve allows pressure equalization after the testso the packer can be retrieved.

By opening the test valve a decline in pressure is obtained.

During the test, pressures and flow rates are measured as afunction of time.

Drill Stem Testing

WellTesting


Drill Stem Testing


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Pressure Vs. Time Plot Point A: As the tool is run in hole the Initial hydrostatic mud pressure is

recorded.

Point B: Initial shut in pressure, which is measured by closing the controlvalve and allowing pressure build-up towards static reservoir pressure.

Point C: Second flowing and shutting in periods permits the calculation ofreservoir parameters. As the control valve is opened, pressure falls to initialflowing pressure.

Point D: As fluids move up the drill pipe, pressure increases due to flowrestrictions in tools. After a suitable time, the tool is shut-in and termed finalflowing pressure.

Point E: Pressure then increases to final shut-in pressure

Drill Stem Testing

WellTesting


Recommendations We have to consider that the main objective is to record good data.

Therefore the DST must be planned to accommodate any specificsituation.

FIRST FLOW: (5-15 minutes), the objective is to remove any excesspressure which may have resulted from setting the packers.

FIRST BUILD UP: (30-60 minutes)

SECOND FLOW: (60 minutes)

SECOND SHUT IN: ( from 30 minutes to several hours) finding out thetransmissibility and other characteristics of the reservoir.

Drill Stem Testing

Pressure

Time

1 2

3

4

56

1. Going into the hole2. Initial flow period

3. Initial shut-in period4. Final flow period5. Final shut-in period6. Coming out of the hole


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WellTesting


Drill Stem Testing

Analysis methods:

Horners Plot method to determine permeability

Type curve matching method to account for wellbore storage.

Kohlaas type curve matching to determine

Correa & Ramey for multiphase environments

Computer matching DST

/kh

WellTesting


Injection Well Tests

Pressure transient testing during injection into

a well.

Important for efficient planning & operation ofboth secondary and tertiary recovery projects.

For analysis, the mobility ratio of injected fluid

and in situ fluids must be near unity.

Mobility Ratio

M= (k/)w(k/)o

Ratio of mobility of the displacing (injected) fluid to the fluidbeing displaced.


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WellTesting


Procedure Well Shut-in to allow pressure to

stabilize at initial reservoir pressure

pi.

Test begins at t=0 with constant

injection rate q (taken ve).

For the constant rate, the injection

bottom hole pressure Pwf is

modelled.

Plot of BHP Pwf vs. logarithmic

injection time t is plotted and

forms a straight line with slope m.

Allows the estimate of formation

permeability k and skin factor s.

Injection Well Tests

WellTesting


Step Rate Injectivity Testing

Normally used to estimate formation or fracture

parting pressure, which is the injection pressurethat will initiate a fracture in an un-fractured well orextend an existing fracture.

Parting pressure determined from the test is

equivalent to fracture extension/propagationpressure in hydraulic fracturing.

Helps determine that any injection above parting

pressure will can result in pre-mature breakthrough, poor sweep, reduced oil recovery etc.

Procedure

Well shut-in before test begins .BHP nears

static reservoir pressure.

Injection rate stabilized at reduced andconstant rate,stabalization period should belong to achieve steady state or pseudo-steady

state flow.

Injection rates are increased step wise.

In low permeability formations (k10 mD)injection rates of 30 mins is adequate.


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WellTesting


Interference Tests

Involves creating pressure disturbances in one well and

measuring the pressure response in the other wells.

Done by either producing from or injecting into at least onewell, called active well, and observing the pressure response inanother well called observation well.

Characteristics of pressure behaviour as function of timereflects the reservoir properties.

Tests can be conducted with more than one active and more

than one observation well.

These tests are ideal for establishing continuity of pay zonewithin the reservoir ,estimating volume of hydrocarbons withindrainage area of wells and quantifying reservoir anisotropies.

WellTesting


Note that a time lag exists between the time when a rate

change is made at active well and time when pressure transientis seen in the observation well.

Interference Tests


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A coded signal is sent from anactive well to a shut-inobservation well.

Signal is generated by producingfrom or injecting into the activewell, shutting in the well,producing again, shutting inagain, and repeating in a regularpattern.

The well is pulsed so that we canidentify effective of active well onpressure response at observationwell amid noise of reservoirpressure trend.

Highly sensitive pressure gaugesare used.

These tests can quantifydirectional reservoir properties,such as permeability.

Advantages

Fewer problems withreservoir noise & pressuretrends that cause errors.

Pulse Tests

WellTesting


Surface Test Equipments

Surface Test Tree

Used in DST operations and are four-way cross with four valves.

Surface test tree should be able to support the full string weight

and all four valves should provide a seal with differential pressure.

Surface test trees with have either a master or production wingvalve or both, hydraulically operated.

Most trees include a swivel to allow pipe rotation to set packers

during tests.

Coflexip Hoses

High pressure hose used between surface trees & choke manifoldson tests conducted from floating rigs.

Allows good flexibility & better corrosion resistance.

Rig permanent Pipe works Information regarding line size, wall thickness, sour service, i nspection

reports must be checked.

High and low pressure pipe works need to be present.


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September October 2005abalt solutions limited 2005


WellTesting


Hydraulic surface safety valve Free standing valve incorporated for additional safety. Used where long high pressure flow lines are used.

Sand trap Functions as sand separator during testing and used when

excessive sand production is anticipated.

Consists of 2 vertical cylinders with screen meshes of varying s izes Normally installed between flow head and choke manifold.

Sand detection equipment Has an intrusive probe set into the flow line. Used to detect sand grains across multiphase flow conditions.

Data reader Used to monitor wellhead pressure and temperatures and chemical

injection upstream of choke.


WellTesting


Other equipments Gauge tanks

Surge tanks

Transfer pumps

Gas diverter manifold

Oil diverter manifold

Burners

Flare lines

Vent lines

Emergency shut down systems

Relief lines


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Sonatrach's _Well Testing

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