INJECTION WELL TESTING
Injection wells are used to inject fluids into porous subsurface materials (as opposed to water wells and oil and gas wells from which fluids are discharged). The most common use is for disposal of wastewater, (especially water produced as a byproduct in oil and gas fields). Injection wells have many other uses, including injecting treated water in groundwater remediation systems, creating hydraulic barriers to saltwater intrusion, subsurface storage (sequestration) of carbon dioxide, increasing the recovery of oil and gas in producing fields, solution mining, disposal of geothermal fluids, land subsidence control, groundwater recharge, and Aquifer Storage Recovery (ASR).
Injection wells are highly regulated because there is some potential for contamination of potable groundwater, and testing is required for approval of subsurface injection projects. Other testing may be needed to plan the operation of the wells and to track the condition of the wells. One test that is often required for permitting is the step-rate injectivity test, which yields information on injection pressures that may be used without opening fractures in the injection formation or the overlying materials that separate it from potable water aquifers. Figure 1 illustrates how the test correlates injection rate with pressure and indicates the pressure at which fractures are opened by injection and closed when injection is decreased. Many other tests are used to get information needed to plan the operation of injection wells. Some are similar to pumping tests, but injection rate is substituted for pumping rate and pressure increase is used instead of water-level decrease (drawdown). Indeed the tests described in the web pages on step testing, unconfined aquifer testing, leaky aquifer testing, and fractured aquifer testing can be used for injection testing if appropriate modifications are made to the input and output of the respective computer programs used to analyze the test data.
Figure 1. Step-Rate Injectivity Test Graph.
Posted January 4, 2016