PCB 3013 Chp 2 Introduction

8/12/2019 PCB 3013 Chp 2 Introduction

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Well Test Analysis (PAB 3013)

Introduction to Well Testing

AP. Dr. Muhannad Talib ShukerGPE Department

1PAB 3013 by Dr. Muhannad TalibShuker


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Introduction to Well Testing

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1. Description of A Well Test

2. Well Test Objectives

3. Information Obtained From Well Testing

PAB 3013 by Dr. Muhannad TalibShuker


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Description of a Well Test

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During a well test, a transient pressure response iscreated by a temporary change in production rate (1) .

R a

t e ,

q

P r e s s u r e ,

P

Time, t

drawdown build-up

P i



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4

The well response is usually monitored during a relatively shortperiod of time compared to the life of the reservoir, dependingupon the test objectives.

For well evaluation, tests are frequently achieved in less than twodays.

In the case of reservoir limit testing, several months of pressure

data may be needed.



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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 uniform inthe reservoir.

During the flowing period, the drawdown pressure response p isdefined as follows:



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Pressure Buildup and Pressure Drawdown Sequence



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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 elapsedtime t since the start of the period(time of opening or shut-in).



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Well Test Objectives

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Exploration well: To confirm the exploration hypothesis and toestablish a first production forecast: nature and the rate of produced

fluids, initial pressure and well and reservoir properties (DST DrillStem Testing)

Appraisal well: To confirm the well productivity, reservoirheterogeneities and boundaries, drive mechanisms. Bottom 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 descriptionand to evaluate the need for well treatment, such as work-over,perforation strategy or completion design, to maximize the well'sproduction life. Communication between wells (interference testing),monitoring of the average reservoir pressure are some usualobjectives of development well testing.



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Information Obtained from Well Testing

Well test responses characterize the ability of the fluid flow throughthe reservoir and to the well.

Test provide a description of the reservoir in dynamic conditions, asopposed to geological and log data.

As the investigated reservoir volume is relatively large, theestimated parameters are average values. From pressure curveanalysis, it is possible to determine the following properties (1) :



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

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



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Test Procedure

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Drawdown test :

The flowing bottom hole pressure is used for analysis. Ideally, the wellshould be producing at constant rate but in practice, this is difficult toachieve and drawdown pressure data is erratic.The analysis of flowing periods (drawdown) is frequently difficult andinaccurate.

Build-up test:The increase of bottom hole pressure after shut-in is used foranalysis. Before the build-up test, the well must have been flowinglong enough to reach stabilized rate.

During shut-in periods, the flow rate is accurately controlled (zero).

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



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Test Procedure

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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 oilwell is presented.

After some flow time at a constant rate, the well is shut-in for the finalbuild-up test.



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Typical Test Sequence

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Initially, the well is cleaned up by producing at different rates, until thefluid produced at surface corresponds to the reservoir fluid.

The well is then shut-in to run the down hole pressure gauges, andreopened for the main flow. The flow rate is controlled by producingthrough a calibrated orifice on the choke manifold. Several chokediameters are frequently used, until stabilized flowing conditions arereached.



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Input Well Test Data Required

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Test data:

Flow rate and bottom hole pressure as a function of time. The testsequence of events must be detailed, including any operationalproblems 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 someflow periods, it must be accurately estimated.

Well data: Wellbore radius r w

well geometry (such as inclined orhorizontal well), depths (formation, gauges).



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Reservoir and fluid parameters :formation thickness h (net), porosity , compressibility of oil c

o,

water c w , and formation c f ; water saturation S w , oil viscosity o andformation volume factor B. The total system compressibility c t isexpressed as:

The above reservoir and fluid parameters are used for calculationof the results; using interpretation model.

After a first interpretation, they may always be changed oradjusted if needed to refine the results, for the same theoreticalinterpretation model.



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Additional data can be useful in some cases: production log,

gradient surveys, reservoir temperature, bubble point pressureetc.

General information obtained from geologist and geophysicists

are required to validate the well test interpretation results.



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Flow Behavior

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Types of flow behaviorThe different flow behaviors are usually classified in terms of rateof change of pressure with respect to time.

1-Steady stateDuring 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 formation.

2-Pseudo steady stateThe pseudo steady state regime characterizes a closed systemresponse.



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Flow Behavior

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Usually, well test interpretation focuses on the transient pressureresponse.

Near wellbore conditions are seen first and later, when thedrainage area expands, the pressure response is characteristic ofthe reservoir properties until boundary effects are seen at late time(then the flow regime changes to pseudo steady or steady state).



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Flow Behavior

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Pseudosteady-state Flow

When the pressure at different locations of the reservoir isdeclining at a constant rate the flow regime is calledpseudosteady-state flow. Mathematically this condition isexpressed as:

constant t P i

This equation states that the rate of change of pressure P with

respect to time t at any location i is constant. The pseudosteady-state regime characterizes a closed system response.



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Flow Behavior

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Unsteady-state Flow (Transient Flow )Unsteady-state flow, also known as transient flow, is observedbefore constant pressure or closed boundary effects arereached.

The pressure variation with time is a function of the wellgeometry and the reservoir properties like permeability andheterogeneity. Mathematically this condition is expressed as:

t i f t P i,



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Flow Behavior

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Flow Regimes

Steady-state flow

Pseudosteady-state flow

Unsteady-state flow

0

it P

P r e s s u r e a

t l o c a

t i o n

I i n t h

e r e s e r v o

i r

Time

constant t

P i

t i f t

P

i

,


PCB 3013 Chp 2 Introduction

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