Pressure Transient Analysis Using SAPHIR
Monitoring and maintenance is a continuous process in the various phases of a well’s life. One of the essential part of monitoring is well testing. Just as a regular medical checkup is advised for humans so is well testing advised for a target formation. Well testing provides a means of obtaining information about the well and the reservoir as well as diagnosing a problem if any is present. The objective for a well test determines the type and design to be carried out. In this study, build-up test was the main focus. Obtaining relevant data in order to characterize a reservoir and determine initial pressure is a major objective of a build-up test.
The results required from the tests cannot be directly obtained from the well test data. Hence, the importance of analysis. There are various methods used to analyze well test data such as manual plotting, excel analysis and software analysis. In this study, a build-up test data was obtain from an XYZ field for two dual string oil producing wells (4 wells) and was analyzed using a tool called Saphir. The result from tool showed the properties of the reservoir required for characterization i.e. pressure, permeability and boundary conditions.
It was seen that one well has undergone a successful stimulation and the other three damaged with one of them severely damaged. The highly damaged well was recommended for stimulation and the rest was recommended to have future plans for stimulation. Further studies were made to ascertain the effect of stimulation on the damaged wells and significant gain in barrels produced were observed. Well property such as bottom-hole flowing pressure was calculated using the results obtained from the analysis.
Table Of Contents
- Table Of Contents
- List Of Figures
- List Of Tables
- List Of Symbols
- 1.1 Background of study
- 1.1.1 Test and analysis principle
- 1.1.2 Types of Well Test
- 1.1.3 Uses of Bottom-hole pressure test Derived Information
- 1.2 Statement of the problem
- 1.3 Aim and objectives of the study
- 1.4 Relevance of study
- 1.5 Scope of study
- 1.6 Project outline
Materials And Methods
- 3.1 Introduction
- 3.2 Work flow
- 3.3 Operating data
- 3.3.1 Data requirement for the PTA to achieve the above objectives
- 3.4 Description of kappa saphir
- 3.4.1 Getting started
Results And Discussion
- 4.1 Results
- 4.2 Discussion
Conclusion And Recommendation
- 5.1 Conclusion
- 5.2 Recommendations
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
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):
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
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
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
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.
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.
Conclusion And Recommendation
Two dual string oil wells were tested and the resulting data run in a tool (Saphir) to obtain reservoir characteristics such as permeability, skin and formation capacity as well as reservoir extent and initial pressure. These results are shown in table 4.1. This fulfils the aim and objective of this work.
Saphir, the software used for the analysis is very user friendly and timely. It is also accurate and informative compared to manual calculations and plotting.
Bottom-hole flowing pressure of the wells was determined using equation 4.2 at the operating rate. Bottom-hole flowing pressure for 2L, 2S, 3L and 3S are 175.374, 5,991.85, 515.472, 709.356psia respectively.
The skin found showed a negative effect on the production rate for S>0 and positive effect for S<0. Stimulation was proposed as a remedial action for the formation damage.
Significant increase in production rate was observed for possible stimulation effect on the damaged wells. Three wells were proposed for stimulation with 2L being badly damaged and 3L and 3S slightly damaged. The slightly damaged wells (3L and 3S) need not be stimulated immediately but future plans and provision should be made to avoid drastic drop in rate. 2L needs adequate stimulation to increase the flow rate. 2S shows stimulation has been done and successful with S= 3.33.
An assumption of reduced skin value after successful stimulation was made to prove the effect of stimulation on production rate. For well 2L, skin was reduced from 45 to 15 showing an increase in production of 963.757bbls (130.835% increase) at its flowing bottom-hole pressure. For well 3L, skin was reduced from 6.65 to 2 showing an increase in production of 665.965bbls (46.82% increase) at its flowing bottom-hole pressure. For well 3S, skin was reduced from 1.94 to 0 showing an increase in production of 314.941bbls (24.465% increase) at its flowing bottom-hole pressure.
The tool used (Saphir) is not 100% accurate, a better software should be used to analyze well test data which does not require excessive modification of result during analysis.
Further studies should be conducted to determine the best stimulation method that is cost effective and delivers a high rate. The quantity of materials required for the stimulation should be considered as well.
A comparative analysis should be done using small data range to detect percentage of accuracy between manual plotting, excel plotting and/ or Saphir along any other software tool.
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