USE OF CLASS II INJECTION WELLS FOR DISPOSAL OF WASTE FLUIDS

Class II injection wells are designated by the US EPA for the injection of fluids associated with the production of oil and gas for either enhanced oil recovery (EOR) or fluid disposal. These wells currently can only accept liquid oil and gas exploration and production wastes, but repurposed wells offer an attractive potential for disposal of other waste fluids. Class II wells are most common in California, Oklahoma, Kansas and in Texas, where there are over 34,000 active Class II Injection wells (of these over 8,000 are disposal wells). The main concerns over injection of fluids are the risks of seismic activity and/or aquifer contamination. Disposal wells are generally shallower than the oil & gas reservoirs and well above the crystalline basement, so seismic risks are minimal. Residual desalination fluids require safe disposal because of high salinity and, in some, cases, concentrated organics. Three analytical and one numerical model predict the migration of desalination fluids in order to minimize the risk to productive aquifers. The Bear and Jacobs analytical model was found most appropriate. Comparison with numerical modeling results that incorporated fluid density effects and well partial penetration indicated that this model can predict safe buffer distances for such disposal. Abandoned EOR wells in depressurized petroleum reservoirs also offer a promising option for safe disposal of highly toxic liquid wastes because of their generally greater depth, stratigraphic barriers, and hydraulic constraints. Detailed mapping analysis is required to locate petroleum reservoirs that have minimal chance for additional future petroleum production and hydraulic isolation sufficient to retain fluid underpressures. Selected reclassification of Class II wells should be conducted.