Characterizing the potential for fault reactivation related to fluid injection through subsurface structural mapping and stress field analysis, Wellington Field, Sumner County, KS

Abstract

South-central Kansas, although historically stable, has experienced an increase in seismic activity since 2013. The correlation with brine disposal operations has renewed interest in the role of fluids in fault reactivation, specifically in the crystalline basement, where the majority of events have occurred. This study focuses on determining the suitability of CO2 and brine injection into a Cambro-Ordovician reservoir (Arbuckle Group) for long-term storage and a shallower Mississippian reservoir for enhanced oil recovery (EOR) in Wellington Field, Sumner County, Kansas. Our approach for determining the potential for injection-induced seismicity has been to (1) map subsurface faults and estimate in-situ stresses, (2) perform slip and dilation tendency analysis to identify optimally oriented faults relative to the estimated stress field, and (3) determine the pressure changes required to induce slip, both at reservoir depth and basement depth. Through the use of 3D seismic reflection data, 12-near vertical faults were identified with fault planes striking between 325° to 049° and the majority oriented NNE, consistent with nodal planes from moment tensor solutions from recent earthquakes in Kansas and Oklahoma. Fault lengths range from 210 to 4,450+ m and vertical separations range from 12-33 m. The majority of faults cut through both reservoirs, with a number that clearly cut the top basement reflector. Drilling-induced tensile fractures (N=40) identified from image logs and inversion of moment tensor solutions (N=70) are consistent with the maximum horizontal stress (SHmax) oriented ~EW. Stress magnitudes were estimated using step rate tests (Shmin = 18.4 MPa), density logs (Sv = 36.6 MPa), and calculations from wells with drilling induced tensile fractures (SHmax = 31.3-45.9 MPa) at the gauge depth of 1,484m. Slip and dilation tendency analysis indicates that faults striking <020° are stable under reservoir conditions, whereas faults striking 020°-049° may have a moderate to high risk for reactivation with increasing pore fluid pressure. These faults would require a pore fluid pressure increase of at least 1.1 MPa to 7.6 MPa at 1,117 m (Mississippian) and 1.31 MPa to 9.8 MPa at 1,484 m (Arbuckle) to reach failure. Given the proposed injection volume, it is unlikely that faults will be reactivated at reservoir depths. However, at basement depths, high rate injection operations could reach pressures beyond the critical threshold for slip, as demonstrated by the large number of injection induced earthquakes west of the study area.