Types of Well Tests

Five test types are briefly discussed below: Pressure Build-Up, Injection/Fall-Off, Multi-rate, Multiple well, and Closed Chamber.

Pressure Build-Up Test (PBU) – A pressure transient is induced by producing a well for a period of time and subsequently shutting the well either downhole or at the surface for a buildup period. The rate schedule depends on the objectives of the well test.

Usually, data from the production periods of a test are noisy and, thus, not usable for pressure-transient analysis. Although a PBU test means a loss of revenue, it is preferred over a simple drawdown test when the objectives of the test involve obtaining reservoir properties and completion efficiency.

Injection/Fall-Off Test (IFOT) – Alternatively, a pressure transient can be created by injecting fluid into a well. When the well is shut-in after injection, the reservoir pressure falls off. Usually, the injection fluid is water. The different properties between water and reservoir fluid must be taken into account for the analysis. The shut-in time is generally shorter than for PBU tests because the main objective does not include a characterization of reservoir heterogeneity.

Similar to the advantages of buildup tests over drawdown tests, it is preferable to conduct a fall-off test whenever an accurate estimate of kh is needed. For injection wells that go on vacuum when shut-in, a rate-change test should be considered rather than a falloff because much of the test will likely be dominated by afterflow caused by the falling liquid level in the wellbore.

Multi-Rate Test – These tests can be conducted on both oil and gas wells. In these tests, several stabilized flow rates are achieved at corresponding stabilized flowing bottom-hole pressures. In gas wells flow rates are sufficiently high that turbulent or inertial pressure drops near the wellbore can be significant. In such cases, the additional pressure drop measured by the skin can be confused with the pressure drop caused by non-Darcy or inertial flow. In gas wells in which it takes a long time to achieve stabilized rates, wells are shut in and produced for a fixed time interval at several different rates. This test is usually considered for a gas well to characterize the degree of turbulence and to measure the parameters associated with deliverability. Flow after flow, isochronal, and modified isochronal are different versions of multi-rate tests. The isochronal test is conducted by alternately producing the well then shutting it in and allowing it to build to the average reservoir pressure before the beginning of the next production period. The modified isochronal test is conducted like an isochronal test, except the shut-in periods are of equal duration. The shut-in periods should equal or exceed the length of the flow periods.

Multiple-well Tests – These are used to establish communication between wells and determine the inter-well reservoir properties. The principle of multiple-well testing can also be applied between various sets of perforations in the same wellbore. Multiple-well tests between offsetting wells determine the reservoir properties areally. Tests that are run between various sets of perforations in the same wellbore usually determine the vertical reservoir permeability. Multiple-well tests are more sensitive to reservoir heterogeneity than single-well tests (SPE-10042-PA). There are two types of multiple-well tests: Interference and pulse.

Interference test – The flow rate of a production or injection well is changed abruptly and the pressure response of a static observation well is monitored. The time required for the pressure transient to reach the observation well and the associated pressure change can be measured to characterize the degree of communication between the two wells.
Pulse tests – An alternative to interference tests in situations where the physical separation between wells is small, such as in pattern floods. Instead of simply changing the rate at the active well, a series of pressure pulses are created by alternatively flowing and shutting in the active well. Pressure is monitored at one or several observation wells. This type of test is difficult to execute successfully in low-permeability, high-compressibility reservoirs with wide well spacing.

Interference tests are usually much more expensive than pulse tests because of the loss of revenue arising from having to shut-in a major portion or all of the tested reservoir to conduct the test. Also, ambiguity exists in interference test interpretation because it is uncertain that an observed response was actually caused by the active well.

In a pulse test, if a repeated signal is received in an observation well, there is little doubt that it was caused by the rate changes in the active well.

Closed-Chamber Test – The technique was originated to reduce operational and safety problems caused by hydrate formation during conventional drill stem tests in the Canadian Arctic. A closed-chamber test is conducted with the drill-string in the borehole. The surface valve is closed for the duration of the test. A downhole gauge is recommended. When the well begins to flow, air in the string is compressed and the volume of reservoir-fluid inflow is calculated as a function of time by monitoring the surface pressure. The downhole valve is closed to stop flow when the surface pressure reaches a value calculated a priori. This ensures that a known volume of reservoir fluid has been produced. No hydrocarbons are brought surface. The fluids are produced into the drill or completion string. Closed chamber tests are environmentally friendly and safe when H2S is expected.