A well test is a set of procedures, performed under specified conditions, to measure the factors affecting the capability of a well for production or injection of fluids and any possible damage. There are several different kinds of well tests that can be performed during the life history of a well, each with a specific purpose.
The testing timeline below illustrates the opportunities for various types of test during the productive life of a vertical well:
For a development well in an area where many other wells have already been completed, a simple assessment at the end of completion may sometimes be enough.
In contrast, for the first development wells, and even more so for appraisal and exploration wells, more thorough testing is needed. Based on these tests and further rock and fluid studies in the laboratory, the need for treatment and the suitability of one treatment over others can be determined.
Well testing is therefore quite often performed with temporary assemblies.
There are many different kinds of well tests. The most common are described below:
� PRODUCTION TESTS help determine how much and how fast a well will produce. The company representative may run production tests before or after setting casing, cementing, and perforating. Each test reveals certain information about a particular well and its reservoir. Accuracy is, of course, very important, and the data form these tests form the case history of a well. Production tests measure oil production rates, water production, gas/oil ratio, oil cut, API gravity, casing and tubing pressure, choke size, and separator pressure and temperature. They are used to determine wellbore damage and reservoir pressure in the well drainage. Therefore, the tests must be run long enough to compute these values.
Sometimes, when performing a production test, the fluids recovered are analyzed and this information is part of the well test. Production tests can be very elaborate � some information is collected in more than one stage, and at different states of pressure and temperature. The following illustrations show different types of production tests.
� PRESSURE TRANSIENT TESTS usually involve changing the rate that formation fluids enter the borehole and recording the resulting downhole pressure versus time. These tests are carried out in a well for the purpose of deriving information about the characteristics of the earth surrounding the interval being tested. During this procedure, measurements are made relating to the production or injection of fluids from or into a well for a given length of time for an established set of conditions. These measurements are made to assist in the estimation or prediction of production or injection capability. A well transient pressure test usually consists of causing rate stabilizations and fluctuations in a controlled manner, including terminating flow, and monitoring the variation in pressures that result. These activities may occur in a single well or in multiple wells simultaneously. Furthermore, they are used to determine the effect that a change in pressure in one well may have on other wells in the same field.
� POTENTIAL TESTS/PRODUCTIVITY TESTS are used to determine the maximum rate at which fluids may be injected into or produced from a well. It is the first dependable indication of the likely productivity of the well. It marks the change of the well from a drilling status to a production or other kind of status. These tests may be conducted over a series of rates with the intention of extrapolating to a maximum rate, or with all surface control valves wide open to measure the maximum rate. Measures produce oil, water, gas, tubing pressure, choke size, etc. If the well is artificially lifted, the specifications of the lift equipment are included.
� INITIAL POTENTIAL TESTS are used to determine the maximum rate at which fluids may be injected into or produced from a well when the well is first completed. It is also repeated periodically during the life of the well, especially after workovers.
� MULTI-POINT TESTS are conducted when the well is not shut-in, but it is flowed at many (unlimited) different rates to plot delta-p or delta-p-squared vs. rate. This is done in Texas for gas wells.
� DRAWDOWN TESTS evaluate the limits of a reservoir and the production and bottomhole pressure reduction of a well on the neighboring wells to determine the extent of the reservoir.
� BUILD-UP TESTS are used to determine the effective drainage radius of a well and obtain information about the permeability of the producing formation. The well is shut in for a period of time after it has flowed at a constant rate, and the rise of bottomhole pressure is recorded. This test can be determined whether skin or borehole damage exists.
� INJECTION TESTS are like drawdown tests, but they are conducted on injection wells.
� FALLOFF TESTS are like buildup tests, but they are conducted on injection wells. The injection rate is held constant, the well is shut in, and the decrease of bottomhole pressure is recorded.
� INTERFERENCE TESTS are like buildup/drawdown tests. You change the pressure on one well in order to see what happens to another well. They measure the pressure variation with time recorded in observation wells resulting from changes in rates in production or injection wells. In commercially viable reservoirs, it usually takes considerable time for production at one well to measurably affect the pressure at an adjacent well. Consequently, interference testing has been uncommon because of the cost and the difficulty in maintaining fixed flow rates over an extended time period. With the increasing number of permanent gauge installations, interference testing may become more common than in the past. [Oilfield Review]
� DRILLSTEM TESTS (DST TESTS) are tests that are conducted through the drillstem taken by means of special testing equipment attached to the drillstem to determine if fluid components in commercial quantities have been encountered in a wellbore. Although the drillstem test is a temporary well completion, it is not necessary to set casing or to remove the drilling fluid from the wellbore to conduct the test. Therefore, DST tests gather data on the potential productivity of a formation. DSTs are usually carried out as part of drilling operations to determine the economic and technical viability of a productive completion. A DST test is a test that temporarily completes a recently drilled well to the formation in order to determine the actual flow of a potential reservoir. It is used to calculate formation pressure, permeability, and the amount of formation damage. This is done because logs are indirect and can be misleading. The Society of Petroleum Engineers requires that the commercial productivity of reserves be supported by actual production or formation tests. They can be open holed or cased holed and must be done for a particular time period from 20 minutes to three days. Important measurements are initial/final hydrostatic pressure, flow pressure, and shut-in pressure. The test is recorded on a pressure-buildup curve. There are four stages: initial flow, initial shut-in, final flow, final shut-in. Flow rates are controlled by an adjustable choke. Samples are collected and sent to a separator where gas is metered and liquids gauged. If the fluid does not flow to the surface, the height of the liquid is measured in the drillstem. Backpressure can be applied by filling the formation with a cushion, such as water.
� FORMATION TESTS are used to evaluate possible producing zones encountered in a well. They are used to obtain fluid flow and reservoir pressure data. They are less expensive than DSTs. They are usually performed in a mud-filled open hole, after the total-depth is reached, but before running casing, on an electrical-conducting wireline test tool. They use a rubber packer seal, probe to go into formation, pressure builds up. These tests are repeated at successive locations providing a measure of reservoir pressure versus depth. Multiple zones can be tested on a single run into the hole. Types of tools used for formation test are formation tester, repeat formation tester, formation interval tester, modular formation dynamics tester, formation multi tester, and sequential formation tester tool.
� REPEAT FORMATION TESTS (RFT TESTS) measure the pressures at specific depths. The tester is a tool that the crew runs into the well on a conductor line. The tool perforates one or two holes in the casing. A valve opens a testing chamber to let in fluid and records its pressure. Finally, a sample chamber opens and draws in a few gallons or liters of formation fluid to bring to the surface. This test is good for quick readings of pressures, confirming porosity and permeability data from other logs, and predicting general productivity. (Can get up to 2 samples, but many different pressure readings).
� MODULAR FORMATION DYNAMICS TESTS (MDT TESTS) offer multiple sampling during a single wireline run and rapid pressure measurements. MDT tests comprise a number of modules, and they can be configured to collect many different kinds of data.
� The high-level information about a well testing activity will be stored in the Well_Test class.
� Information about each of the stages of a well test activity will be stored in the Well_Test_Stage class. At least one well test stage record must always be created for every well test.
� Interpretation of the data acquired during a well test will be stored in the Well_Test_Interpretation class.
� The boreholes and completions on which a particular well test or well test stage are performed are stored in the Well_Test_Site class.
� Raw curve data (bulk data) that is acquired during a well test is stored in the Well_Test_Trace class.
� The individual materials that are recovered during a well test or well test stage activity are stored in the Well_Test_Fluid_Recovery class.
� The equipment (Facilities) used during a well test or well test stage activity is stored in the Activity_Facility_Invl class.