2. Introduction to Well Test

AP Dr Muhannad Talib ShukerAP. Dr. Muhannad Talib ShukerGPE Department

Well Test Analysis, © UTP – MAY 2011

I t d ti t W ll T tiIntroduction to Well TestingDescription of a Well Testesc pt o o a e estWell Test objectivesInformation obtained from well testingg


D i ti f W ll T tDescription of a Well TestDuring a well test, a transient pressure response is g p pcreated by a temporary change in production rate(1).

e, q

Rate drawdown build‐up

Pi

Pressu

re, P

Time, t


Th ll i ll i d d i The well response is usually monitored during a relatively short period of time compared to the life of the reservoir, depending upon the test objectives. p g p jFor well evaluation, tests are frequently achieved in less than two days. I h f i li i i l h f In the case of reservoir limit testing, several months of pressure data may be needed.In most cases, the flow rate is measured at surface In most cases, the flow rate is measured at surface while the pressure is recorded down‐hole.Before opening, the initial pressure Pi is constant and

f huniform in the reservoir.During the flowing period, the drawdown pressure response ∆p is defined as follows:


response ∆p is defined as follows:

BU & DD Pressure SequenceBU & DD Pressure Sequence


When the well is shut‐in, the build‐up pressure change ∆p is estimated from the last flowing pressure p(∆t=0):

The pressure response is analyzed versus the elapsed time ∆t since the start of h d ( f h )the period (time of opening or shut‐in).


W ll T t bj tiWell Test objectivesWell test analysis provides information on the reservoir y pand on the well.Geological, geophysical and petro‐physical information is

d h ibl i j ti ith th ll t t used where possible in conjunction with the well test information to build a reservoir model for prediction of the field behavior and fluid recovery for different operating scenarios. The quality of the communication between the well and the reservoir indicates the possibility to improve the wellthe reservoir indicates the possibility to improve the wellproductivity. Usually, the test objectives can be summarized as follows:


Usually, the test objectives can be summarized as follows:

Exploration well: to confirm the exploration hypothesis and Exploration well: to confirm the exploration hypothesis and to establish a first production forecast: nature and the rate of produced fluids, initial pressure and well and reservoir properties (DST – Drill Stem Testing)properties (DST Drill Stem Testing)Appraisal well: to confirm the well productivity, reservoir heterogeneities and boundaries, drive mechanisms. Bottom hole fluid samples are taken for PVT analysis Longer hole fluid samples are taken for PVT analysis. Longer duration testing (production testing) is usually carried out.Development well: periodic tests to adjust the reservoir d i ti d t l t th d f ll t t t h description and to evaluate the need for well treatment, such as work‐over, perforation strategy or completion design, to maximize the well's production life. Communication b ll (i f i ) i i f h between wells (interference testing), monitoring of the average reservoir pressure are some usual objectives of development well testing.


Information Obtained from Well TestingWell test responses characterize the ability of the fluid flow through the reservoir and to the well. Test provide a description of the reservoir in dynamic conditions, as opposed to geological and log data. As the investigated reservoir volume is relatively large, the

i d l F

Reservoir descriptionPermeability (horizontal and vertical)

estimated parameters are average values. From pressure curve analysis, it is possible to determine the following properties(1):

Permeability (horizontal and vertical)Reservoir heterogeneities (natural fractures, layering, change of characteristics)Boundaries (distance, size and shape),Pressures (initial p; and average p ).

Well descriptionProduction potential (productivity index PI and skin factor S)


Well Geometry

Test procedurepDrawdown test: the flowing bottom hole pressure is used for analysis. Ideally, the well should be producing at

b i i hi i diffi l hi constant rate but in practice, this is difficult to achieve and drawdown pressure data is erratic.

The analysis of flowing periods (drawdown) is frequently The analysis of flowing periods (drawdown) is frequently difficult and inaccurate.

Build‐up test: the increase of bottom hole pressure after h i i d f l i B f h b ild h shut‐in is used for analysis. Before the build‐up test, the well must have been flowing long enough to reachstabilized rate. During shut‐in periods, the flow rate is stabilized rate. During shut in periods, the flow rate is accurately controlled (zero).

It is for this reason build up tests should be performed.


Injection test /fall‐off test: when fluid is injected into the Injection test /fall off test: when fluid is injected into the reservoir, the bottom hole pressure increases and, after shut‐in, it drops during the fall‐off period. The properties f th i j t d fl id i l diff t f th t f of the injected fluid are in general different from that of

the reservoir fluid, interpretation of injection and fall‐off tests requires more attention to detail than for producers.Interference test and pulse testing: the bottom hole pressure is monitored in a shut‐in observation well some distance away from the producer Interference tests are distance away from the producer. Interference tests are designed to evaluate communication between wells. With pulse tests, the active well is produced with a series of h fl h d d h lshort flow / shut‐in periods and the resulting pressure oscillations in the observation well are analyzed.


G ll Gas well test: specific testing methods are used to evaluate the deliverability of gas wells (Absolute Open Flow Potential, AOFP) and the possibility of non‐Darcy flow condition (rate dependent skin factor S).The usual procedures are Back Pressure test(Flow after Flow), Isochronal and Modified Isochronal tests.In the following Figure, the typical test sequence of an exploration oil In the following Figure, the typical test sequence of an exploration oil well is presented.Initially, the well is cleaned up by producing at different rates, until the fluid produced at surface corresponds to the reservoir fluid.fluid produced at surface corresponds to the reservoir fluid.The well is then shut‐in to run the down hole pressure gauges, and reopened for the main flow. The flow rate is controlled by producing through a calibrated orifice on the choke manifold. Several choke through a calibrated orifice on the choke manifold. Several choke diameters are frequently used, until stabilized flowing conditions are reached.After some flow time at a constant rate, the well is shut‐in for the final


After some flow time at a constant rate, the well is shut in for the final build‐up test.

Typical test sequence


Input data required for well test analysisInput data required for well test analysisTest data: flow rate and bottom hole pressure as a function of time The test sequence of events must be detailed including time. The test sequence of events must be detailed, including any operational problems that may affect the well response.

Results of analysis are dependent upon the accuracy ofthe well test data.When the production rate has not been measured during some flow periods it must be accurately estimatedflow periods, it must be accurately estimated.

Well data: wellbore radius rw well geometry (such as inclined or horizontal well), depths (formation, gauges).or horizontal well), depths (formation, gauges).


Reservoir and fluid parameters f ti thi k h ( t) Reservoir and fluid parameters: formation thickness h (net), porosity ɸ, compressibility of oil co, water cw, and formation cf; water saturation Sw, oil viscosity µo and formation volume factor B. Th l ibili i d The total system compressibility ct is expressed as:

The above reservoir and fluid parameters are used for calculation of the results; using interpretation model After a first interpretation the results; using interpretation model. After a first interpretation, they may always be changed or adjusted if needed to refine the results, for the same theoretical interpretation model.dd l d b f l d l dAdditional data can be useful in some cases: production log, gradient surveys, reservoir temperature, bubble point pressure etc. General information obtained from geologist and geophysicists are


required to validate the well test interpretation results.

Types of flow behaviorTypes of flow behavior

The different flow behaviors are usually classified in terms of f h f h rate of change of pressure with respect to time.

1‐Steady stateDuring steady state flow the pressure does not change with During steady‐state flow, the pressure does not change with time. This is observed for example when a constant pressure effect, such as resulting from a gas cap or some types of water drive ensures a pressure maintenance in the producing drive, ensures a pressure maintenance in the producing formation.

2‐Pseudo steady stateThe pseudo steady state regime characterizes a closed system response


The pseudo steady state regime characterizes a closed system response.

With a constant rate production the drop of pressure With a constant rate production, the drop of pressure becomes constant for each unit of time.

3‐Transient stateTransient responses are observed before constant pressure or closed boundary effects are reached.Th i ti ith ti i f ti f th ll The pressure variation with time is a function of the well geometry and the reservoir properties, such as permeability and heterogeneity.g y


Usually, well test interpretation focuses on the transient pressure response.N llb diti fi t d l t h Near wellbore conditions are seen first and later, when the drainage area expands, the pressure response is characteristic of the reservoir properties until boundary effects are seen at late time (then the flow regime changes to pseudo steady or steady state).


Steady state FlowSteady‐state FlowThe flow regime is identified as a steady‐state flow if the pressure at every location in the reservoir remains constant. In other words, during steady‐state flow, pressure does not change with time. Mathematically this condition is expressed as:

0=⎟⎠⎞

⎜⎝⎛∂∂

tP⎠⎝ ∂ it

This equation states that the rate of change of pressure P with q g prespect to time t at any location i is zero. In reservoir this condition can only be achieved when the reservoir is supported by a strong aquifer, gas cap or pressure maintaining operations.


Pseudosteady state FlowPseudosteady‐state FlowWhen the pressure at different locations of the reservoir is declining at a constant rate the flow regime is called pseudosteady‐state flow. Mathematically this condition is expressed as:

constanttP

=⎟⎠⎞

⎜⎝⎛∂∂

t i⎠⎝ ∂

This equation states that the rate of change of pressure P with q g prespect to time t at any location i is constant. The pseudosteady‐state regime characterizes a closed system response.


Unsteady‐state Flow (T i t Fl )(Transient Flow)Unsteady‐state flow, also known as transient flow, is observed before

l d b d ff h d Th constant pressure or closed boundary effects are reached. The pressure variation with time is a function of the well geometry and the reservoir properties like permeability and heterogeneity. Mathematically this condition is expressed as:y p

( )tifP=⎟

⎞⎜⎛ ∂ ( )tif

t i

,=⎟⎠

⎜⎝ ∂

Well test interpretation focuses on the transient pressure response. p p pNear wellbore conditions are seen first and later, when the drainage area expands, the pressure response is the characteristics of the reservoir properties until boundary effects are seen at later


time (at which flow regime changes to pseudosteady‐state or steady state.

Flow RegimesFlow Regimes ⎞⎛ ∂P

Steady‐state flow

0=⎟⎠⎞

⎜⎝⎛∂∂

itP

rvoir

Pseudosteady‐state flow

in th

e reser

constanttP

i

=⎟⎠⎞

⎜⎝⎛∂∂

Unsteady‐state flowt loc

ation I

( )tiftP

i

,=⎟⎠⎞

⎜⎝⎛∂∂

Pressu

re at


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2. Introduction to Well Test

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