Pore pressure evolution and induced seismicity within the Permian Basin, Southeast New Mexico USA

Abstract:

We used three-dimensional hydrologic modeling to assess the potential linkages between crystalline basement seismicity (up to M3.2) beneath the Dagger Draw oil field in response to saline water reinjection. Production began in 2004 and preceded an increase in seismicity by about 5 years. Reinjection of produced brines occurred within the basal Ellenberger Group carbonate reservoir (yellow square). Published core permeability measurements for the Ellenberger vary between about 10^-15 to 10^-12 m². Evidence for seismicity being triggered by injection include observations that the largest injection rates (> 10⁶ barrels/month) occurred within wells closest to the induced seismicity (red circle about 15 km to the west of the injection well in A-C). Arguing against triggered seismicity is the apparent lack of temporal correlation between peak injection and felt seismicity as well as the extreme depth of the earthquakes (about 10-12 km below land surface). We conducted a numerical sensitivity study in which we varied the permeability of the basal reservoir as well as the crystalline basement rocks over several orders of magnitude. Assuming a crystalline basement permeability of 10^-16 m² and a basal reservoir permeability of 10^-13 m² produced about 50 m of excess heads in the seismogenic crust about 1900 days (D) after injection started. Prior studies suggest that excess heads of only a few meters could induce failure along critically stressed faults. The lag between injection and seismicity can be explained by the time required for the pressure envelope to propagate laterally 15 km and downward into the crystalline basement 11 km. Peak injection occurred 1900 days before recent increases in seismicity were observed. Future work will include assessing the potential role of relatively permeable Proterozoic faults in transmitting high fluid pressures into the crystalline basement.