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 tha