Pressure Transient Analysis Using SAPHIR

Introduction

1.1 Background Of The Study

Well tests are carried out as a form of formation evaluation to provide confirmation and detailed fluid properties, ability of the formation to produce and accurate pressure measurements. Well testing can be done during the exploration or appraisal stage for reservoir characterization. It is the last step of evaluation before a well is produced.

Information obtained in this stage helps in the design of production and completion facilities. Well testing can also be carried out during the production stage of a well in order to monitor reservoir and well performance as well as understand causes of deviation in performance. Well test can be carried out using two technologies namely;

Wireline formation testing

Well testing using a packer lowered on a drillpipe or tubing.

 

During a well test, a selected flow rate is applied to the reservoir using a particular method depending on the technology used. Pressure changes resulting and the selected flow rates are recorded versus time. From these measurement one can determine properties such as drainage area, average pressure, skin factor, permeability and other related parameters .The data recorded during the well tests (pressure, flow rate and time) are of high frequency and resolution. The process of analyzing these data is called PRESSURE TRANSIENT ANALYSIS. From the analysis, information about the reservoir is obtained.

In this study, interpretation of a test carried out on a well is done using an industry standard pressure transient analysis software called SAPHIR. Therefore, information on well productivity and reservoir performance over a large radius of investigation (permeability, reservoir pressure, well dame, connected volume, etc…) are obtained after pressure transient analysis.

1.1.1 Test and analysis principle

Figure 1.1: Test and Analysis Principles

The test principle involves allowing some known rate changes to occur in the reservoir and measuring the resulting pressure changes. Note that some characteristics of the reservoir such as the permeability and skin are not known.Analysis principle involves applying the same rate changes to a mathematical model whose characteristics are known and observing the resultant pressure changes. The model characteristics can be changed until the pressure changes from the model become equivalent to pressure changes obtained when the same input (rate changes) were applied to the reservoir (Figure 1.1). We now conclude that the model characteristics are equivalent to the reservoir characteristics.

1.1.2 Types of Well Test

There are various reasons for designing and conducting oil well tests and the type of test required depends on the objective of the test. Common well tests include:

Potential test

Gas-oil ratio test

Productivity test

Bottom-hole pressure test

 

Potential test involves measurement of the amount of oil and gas a well produces during a given period (normally 24 hours or less) under certain conditions fixed by regulatory bodies. The information obtained from these tests is used in assigning producing allowable of the well.

The gas-oil ratio test is carried out to determine the volume of gas produced per barrel of oil in order to ascertain whether or not a well is producing gas in excess of permissible limit.

Productivity tests are made on oil wells and include both the potential test and bottom-hole pressure test. Its purpose is to determine the effects of different flow rates on the pressure within the producing zone of the well. This helps in determining producing characteristics of the formation.

Bottom-hole pressure test involves measurement of sandface pressure and flowrate variation with time. Such tests are quite economical to run and they yield valuable information about the reservoir characteristics and well characteristics. Hence, bottom-hole pressure tests are usually referred (Earlougher, 1977; 1982) to as well tests. Types ofBottom-hole pressure test are:

Pressure Drawdown

Pressure Build up

Multi-rate

Injection/fall-off

Interference

Pulse

 

During conventional well tests, fluid is extracted to the surface or injected into the well at controlled rates. A program of flow and shut in periods is used to establish deliverability and completion efficiency of the well. Tests can involve a single well or many wells. Depending on test objectives and operational considerations, a range of well tests can be carried out. The tests usually fall into the following categories (Horne, 1995; Bourdet, 2002):

Build-up test:

This test is conducted in a well that has been producing for some time at a constant rate and is then shut in. The build-up down-hole pressure is then recorded for a given time.

Figure 1.2: Build up test

Drawdown test:

This test is conducted when a well is flowed at a constant rate. The flowing down-hole pressure and the production rate are measured as functions of time and analyzed to estimate the reservoir properties. The major difficulty of the drawdown is the inability to maintain a constant flow rate.

Figure 1.3: Drawdown test

Injection test:

This test is identical to a drawdown test, except that the flow is into the well rather than out of it. Fluid is injected into the well at a constant rate and the injection rate and the down-hole pressure are measured as functions of time.

Figure 1.4: Injection test

Falloff test:

This test is analogous to a build-up test and it measures the pressure decline as a function of time subsequent to the shut in of an injection.

Figure 1.5: Falloff test

The combined response can be interpreted in a number of ways to estimate the permeability-thickness and the skin factor.

Interference Test:

Unlike the first four tests (drawdown, injection, buildup, falloff) which are tests involving only one well (single well tests), the interference test involves the use of more than one well (multiple well test). During interference tests, pressure changes due to production or injection or shut-in at an active well is monitored at an observation well. Interference tests investigate the reservoir properties and establish pressure connectivity between wells.

Interference tests are primarily used to establish sand continuity between the active and observation wells. In situation where more than one observation well is used, interference test can be used to find maximum and minimum permeability and their directions.

Figure 1.6: Interference test

1.1.3 Uses of Bottom-hole pressure test Derived Information

Results obtained from BHP tests are used for the following purposes:

Reservoir Surveillance.

Determination of Stimulation Candidates.

Gaslift Optimization.

Input for Reservoir Simulation.

Material Balance Calculation.

 

1.2 Statement Of The Problem

During the process of drilling a well, foreign particle may invade into the porous space and plug it, there may be formation of clay dispersion and migration etc. which have great effect on the flow of the reservoir fluid. When this occurred in the wellbore and reservoir, it cannot be known without the help of well testing. Furthermore, for every test conducted, there is an objective to be arrived at but there are challenges of improper design of the test and accurate analysis and interpretation to meet objective.Hence, the need for an effective and accurate tool “Saphir” to determine if there is a damage to the wellbore and impairment to flow.

1.3 Aim And Objectives Of The Study

The importance of well testing in the oil and gas industry cannot be overemphasized just as a doctor trying to treat a patient without carrying out a test to actually diagnose the problem to know the right prescription for the situation. This is also the case of an oil and gas well. Thus, the main aim of this study is to determine well and reservoir properties from a well test data. In order to achieve this aim, the following objectives need to be carried out:

Test a producing well and obtain relevant data from the test.

Characterize reservoir such as permeability, skin and formation capacity.

Estimate average reservoir pressure and define reservoir limit.

 

1.4 Relevance Of Study

Prevent wrong decision making as a result of accurate estimation of well and reservoir properties.

Help monitor reservoir and well performance.

Detect possible causes of performance deviation.

Data obtained will help in reservoir surveillance, input into reservoir simulation study and for production optimization.

 

1.5 Scope of study

This study is basically carried out to characterized reservoirs in terms of permeability, skin estimate to know if the wells are damaged or ascertain the success of stimulation jobs. Also determine the average pressure and drainage volume with the use of well test analysis software “Saphir”. Because of the difficulty of getting data from the industry, only build up will be analyzed.

1.6 Project outline

The outline of this thesis is in five chapters. Chapter 1 gives an introduction to the whole work which includes the aim and objectives, the scope and the relevance of the project. Presented in Chapter 2 is an extensive literature survey on the process route involved in pressure transient analysis. It looks at well testing, operation of Saphir software and interpretation of results. Chapter 3 presents the collected data and the sequential procedure by which these data were implemented in SAPHIR as well as figures for clarity of such processes. The results obtained from pressure transient analysis using SAPHIR and the discussion on how it monitors reservoir performance and well deliverability makes up Chapter 4. The conclusions made after analyzing the final results and future work recommended for further investigations are stated in chapter 5.