Reservoir characterization using oil-production-induced microseismicity, Clinton County, Kentucky

Abstract

Microseismic monitoring tests were conducted from 1993 to 1995 in the Seventy-Six oil field, Clinton County, Kentucky. Oil is produced from low-porosity, fractured carbonate rocks at <600 m depth. Downhole geophones were deployed in wells located within 120 to 250 m of new production wells. Three tests were conducted sequentially for 9.5-, 20.5-, and 30-week periods during which 110, 180 and 3237 microearthquakes were detected, respectively. Moment-derived magnitudes ranged from −2.5 to 0.9. Volumes extracted ranged from about 1300 to 1800 m 3; no injection operations were conducted. Gross changes in production rate correlate with event rate: event rate lags changes in production rate by 2 to 3 weeks. Hypocenters and first-motion data have revealed previously undetected, low-angle thrust faults above and below the currently drained depth intervals. Production history, well logs and drill tests indicate that the seismically active faults or fractures are previously drained intervals that have subsequently recovered to hydrostatic pressure via brine invasion. Storage capacity computed for one of these drained fractures implies that total oil production represents about 20% of total pore volume. Correlation of older production intervals and well-log porosity anomalies with the seismically active faults indicate that the oil reservoir in the study area is primarily a set of compartmentalized, low-angle thrust faults. Although low-angle fracture sets have not previously been considered in the exploration and development of the area, the mapped thrust faults are consistent with other investigators' interpretations of oil associated with secondary fracture sets occurring along deeper-seated, wrench-fault structures. Stress determined from composite focal mechanisms indicates a near-surface (<550 m) thrust regime. Maximum horizontal stress direction is N15°W±15°, rotated approximately 90° from regional orientation. The seismic behavior is consistent with poroelastic models that predict slight increases in horizontal compressive stress above and below currently drained volumes. Pressure re-equilibration via brine invasion replacing previously produced oil along the seismically active faults should also be weakly promoting the observed seismic failure. Total estimated production-induced stress change promoting slip is approximately 0.02 MPa.