Introduction to Well Test Interpretation
Well test interpretation is the process of obtaining information about a reservoir through analyzing the pressure transient response caused by a change in production rate. This information is used to make decisions about how to produce the reservoir to maximize its net present value.
This Tip of the Month provides an overview of:
1. Types of well tests
2. Well test applications
3. Objectives of well test interpretation
Types of Well Tests
There is a wide range of well test types, as well as a number of different ways of categorizing well tests. Tests may be classified by the way in which they are carried out, whether they involve one or multiple wells, whether the test is run by opening up a well for production or shutting in a producing well, whether the well is used for production or injection, and whether the well is an exploration well or a well in a mature field.
Deliverability Tests Vs. Transient Tests
The primary objectives of deliverability tests are to obtain fluid samples, to determine well deliverability, to determine the well potential, or to develop inflow performance (IPR) curves for system analysis. Primary objectives of transient tests may include to estimate in-situ permeability to oil or gas, to estimate average drainage area pressure, to evaluate stimulation treatment effectiveness, and to estimate drainage area and/or fluids in place.
Multiwell Tests Vs. Single Well Tests
In a multiwell test, the rate is changed at one well (the active well), and the pressure response is measured at one or more offset, or observation wells. Multiwell tests are often more complex than single-well tests. Multiwell tests are usually run to quantify the degree of communication between wells or to estimate directional permeability.
In a single-well test, the rate is changed, and the pressure response is measured at the well being tested. Objectives of single-well tests may include evaluating the near-wellbore region to quantify the degree of damage or stimulation, to estimate the in-situ permeability, to estimate the drainage area or fluids in place, and to estimate the average drainage area pressure.
Buildup Tests Vs. Drawdown Tests
In a drawdown test, the reservoir is initially at uniform pressure and the well to be tested is shut in. The well is opened for production, ideally at constant flow rate, and the resulting pressure response is measured as the pressure “draws down”.
In a buildup test, the well has been producing (again, ideally at constant rate) for a period of time, thus creating a pressure gradient in the reservoir. The well is shut in, and the resulting pressure response is measured as the pressure “builds up”.
Exploration Well Tests Vs. Development Well Tests
For a well test on an exploration well, the focus is on the entire reservoir. Objectives of an exploration well test usually include fluid sampling, estimating initial reservoir pressure, evaluating well productivity, estimating distances to boundaries, and estimating fluids in place. Exploration well tests often must be conducted while the rig is on location, making such tests extremely expensive. The expense is justified by the major investment decisions that will be made based on the information obtained from the test.
In contrast, in development well testing, the focus is on the individual well and the near-wellbore area. Objectives of a well test on a development well might include estimating average drainage area pressure, evaluating stimulation treatment effectiveness, quantifying wellbore damage, and estimating reservoir permeability. Compared to exploration well tests, development well tests are relatively inexpensive. The results of development well tests are used in minor investment decisions such as whether or not to do a stimulation treatment.
Applications and Objectives of Well Test Interpretation
Well test interpretation plays a role in many stages in the life of a well including exploration, reservoir characterization, and production engineering.
Well testing plays an extremely important role during exploration. The information gained through well testing will be instrumental in making major investment decisions such as whether to set a platform, build a pipeline, develop a field, or sign a long-term production contract.
The two most important questions that well testing in exploration wells can address are “How much oil or gas does this reservoir contain?” and “At what rate can wells in this reservoir produce?”
Flow tests are often used during exploration testing; these tests are designed to investigate enough of the reservoir to assess the economic viability of the prospect. The deliverability is obtained from the flow rates achieved during the test. Distances to boundaries, reservoir pore volume, and estimates of oil or gas in place may be obtained from the pressure transient response.
In reservoir engineering, well testing may address questions such as “What is the in-situ permeability?”, “What are the nature of and distances to reservoir boundaries?”, and “What is the average reservoir pressure?”
A knowledge of the in-situ permeability is important in reservoir simulation and production forecasting. The nature of and distance to reservoir boundaries is important in reservoir simulation and in well spacing decisions. The average reservoir pressure is used in reservoir simulation, well optimization, and material balance calculations.
The focus in production engineering is on the individual well. The questions addressed by well testing in production engineering reflect this focus: “Is the well damaged?” and “How effective was the stimulation treatment?”
Poor well performance may be caused by reservoir characteristics, such as low permeability or low reservoir pressure, by near-wellbore conditions, such as damage from mud-filtrate invasion, non-Darcy flow, or plugged perforations, or by some combination of these factors. Well test interpretation can help identify the cause of poor well performance, so that the appropriate remedial action may be taken.
To learn more about well test application we recommend enrolling in a session of Well Test Design and Analysis - WTA. This course stresses practical application of well test theory to design and interpret pressure transient tests.
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