By Darrel Dunn, Ph.D., PG, Hydrogeologist - Geologist  

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(This is a technical page on injection well testing.  Click link to see a non-technical page on injection well testing.)

Introduction to Injection Well Testing Web Page

The purpose of this web page is to describe some techniques that may be applied to testing injection wells to determine the properties of the well and the formation receiving the injectate.  The petroleum industry uses a large proportion of all injection wells if one includes wells for disposal of produced water and injection wells used in enhanced oil and gas recovery operations.  Consequently, much injection well technology has been developed by petroleum engineers and applied to thousands of wells.  The petroleum engineering literature on injection well testing is extensive.  This web page is a limited treatment of the subject.  It does not include all tests applied to wastewater injection wells.  It does not include tests exclusively related to injection of fluids into oil and gas reservoirs for enhanced recovery, and it does not include injection tests exclusively used for pre-frac testing of oil and gas wells. 

Step-Rate Injectivity Test

The step-rate injectivity test (aka step-rate test, injectivity test, fracture step rate test, step rate injection test) is a common test used to estimate the threshold pressure at which fractures are opened in the injected formation and/or the overlying or underlying beds.  Some agencies that regulate injection wells require it.  This threshold pressure is sometimes called the "fracture pressure."  The test involves injecting fluid at a series of increasing rates with each rate preferably lasting the same length of time.  The fluid pressure at the formation-face (usually the bottom-hole pressure) is recorded and plotted on a graph of injection pressure at the end of each step versus injection rate.  Ideally, the points on the graph corresponding to low injection rates well plot as a straight line, and points corresponding to higher injection rates will plot as another straight line with a smaller slope.  Figure 2 is an example of a simple 4-step, sparse-data step-rate injectivity test plot corresponding to the injection test graphed in Figure 1.

Step-rate injection test graph.

Figure 1.  Example of a sparse-data step-rate injectivity test.  Data from Erlougher (1977).

Step-rate injectivity test graph.

Figure 2.  Step-rate injectivity test graph.

Some step-rate injectivity tests are not as simple as the one depicted in Figures 1 and 2.  In a simple case where skin effects and/or wellbore storage are not significant, the points below the fracture pressure conform to the inverse of the Jacob approximation of the Theis equation expressed in terms of gauge pressure rather than drawdown.  Consequently, an estimate of transmissiviy may be obtained from the slope of the line fitting these points if the steps are of equal duration.  Transmissivity may be converted to intrinsic permeability (darcys) by supplying values for dynamic viscosity and specific weight of the injectate.

Specifications for conducting step-rate injectivity tests vary.  A comprehensive specification would maximize the information that could be obtained from the test.  Items might include the following:

Local information may be used for the preliminary estimate of fracture pressure.  In the absence of local information, one could estimate the fracture pressure based on a fracture gradient of 1.0 psi/ft for depths less than 2000 feet, and ranging down to 0.4 psi/ft for greater depths.  Fractures approximately normal to the least principal stress open first.  At shallow depths the least principal stress tends to be vertical, but it tends to be horizontal at greater depths.  However, stress fields vary considerably, especially between cratonic areas, sedimentary basins, and tectonically active and inactive areas.

Interpretation of step-rate data my be more complex than suggested by Figure 2.  Damage to the casing cement bond can cause an inflection point that is not the fracture pressure.  Variable fracture geometry and orientation can conceivably produce a curve with no clear inflection point.  A pre-existing open fracture might result in a graph with no inflection point or a slight curve.

Injection and Falloff Tests

After an injection rate that will not induce fracturing is determined, a variety of test procedures are available to estimate the injection formation parameters.  Some of the procedures are the same as ones described in my aquifer testing web pages on step testing, unconfined aquifer testing, leaky aquifer testing, and fractured aquifer testing.  However, when these procedures are used for injection tests the discharge is negative and is called the injection rate; and the procedures only apply for injection pressures less than the fracture pressure.  In injection tests the drawdown is negative and is often expressed in terms of pressure increase.  Furthermore, hydraulic conductivity may be expressed in terms of intrinsic permeability (darcys) of the injection formation, and in terms of specific weight and dynamic viscosity of the injectate or various equivalent measures of the injectate properties.  Tests involving recovery after a well is shut-in are often called falloff tests.

Injection Well Testing Reference

Erlougher, Robert C. (1977): Advances in Well Test Analysis; Monograph, Institute of Mining, Metallurgical and Petroleum Engineers.

Posted January 4, 2016.  Revised May 16, 2023.