INJECTION WELL TESTING

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

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(This is a technical page on injection well testing.  Click link to see a nontechnical 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.