HOW MANY CLASS II WELLS PRESENT A RISK FOR INDUCED SEISMICITY?

We examine the number and location of Class II wells in the central U. S. to constrain future work on the potential for induced seismicity. The EPA, state oil & gas commissions, scientific papers, and media stories frequently state that there are ~140 k -160 k Class II wells. Excluding California, we expected to find approximately 120 k wells; but instead found ~ 82 k active injectors in the available databases. State datasets vary in accessibility, availability, and content of well data. Lack of digitized well data also limited our online search, and several states require FOIA requests to be filed. State databases with poor searching and sorting functions further complicated data mining, requiring a well-by-well search, and for several states, well locations and injections were difficult to determine. Common discrepancies between EPA well totals and state database totals appear to be due to counting of plugged and abandoned wells, and wells that are permitted but not in use. No data has been retrieved for about 1,600 wells on tribal lands and Indian Country, and several states would not provide “confidential” well data. Of the active injectors, at least 55 k wells inject into producing, pressure-depleting oil and gas formations and are less likely to generate damaging earthquakes. Of the ~ 16 k non-EOR wells, we found 3,400 wells that inject at depths > 1.8 km, where most M > 3.0 midcontinent earthquakes occur. We will present examples of data from several states, that show the locations and depths of injectors, earthquakes, depth to basement, and we will provide an overview of the public file sharing system of the data. We will search for correlations between the depth of injection, the number of injection wells, recent seismic activity, the nature of the subsurface geology, and regional stresses. Of the 3,500 deep injectors are the most capable of generating induced seismicity; we will screen to determine their risks include depth, geology, hydrogeological structure of the site, the injection rates and volumes, regional stresses, and basement geology. Reducing the target from 140 k wells to ~3,500 will enable focused geomechanical and geophysically based seismic risk assessments analyses to be conducted. We aim to incorporate field and core data, where possible, to further constrain the risk of failure due to injection.