Predicting the Potential for Fluid-Induced Fault Reactivation: An Example From Wellington Field, Sumner County, KS

Abstract

Although historically stable, south-central Kansas 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 injection into a Cambro-Ordovician reservoir (Arbuckle Group) for long-term storage and a shallower Mississippian reservoir for enhanced oil recovery 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 pressure change magnitudes required to induce slip. Through the use of 3D seismic reflection data, 12-near vertical faults were identified with fault planes striking between 325° to 049° with the majority oriented NNE, consistent with nodal planes from moment tensor solutions in Kansas and Oklahoma. Fault lengths range from 140-410 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=42) identified from image logs and inversion of earthquake focal mechanism solutions (N=76) are consistent with the maximum horizontal stress (SHmax) oriented ~E-W. Estimates of stress magnitudes using step rate tests (Shmin = 18.4MPa), density logs (Sv = 36.5MPa), and calculations from wells with drilling induced tensile fractures (SHmax = 31.3-45.9MPa) are determined at the gauge depth of 1484m. Slip and dilation tendency analysis indicates that faults striking <020° are stable, 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 change of 1.1MPa to 7.6MPa at 1117m (Mississippian) and 1.31MPa to 9.8MPa at 1484m (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.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017