RESERVOIR CHARACTERIZATION USING OIL-PRODUCTION-INDUCED MICROSEISMICITY, CLINTON COUNTY, KENTUCKY

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 geop hones were deployed in wells located within 120 to 250 m of new produc tion wells. Three tests were conducted sequentially for 9.5-, 20.5-, a nd 30-week periods during which 110, 180 and 3237 microearthquakes wer e detected, respectively. Moment-derived magnitudes ranged from -2.5 t o 0.9. Volumes extracted ranged from about 1300 to 1800 m(3); no injec tion operations were conducted. Gross changes in production rate corre late 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 dra ined depth intervals. Production history, well logs and drill tests in dicate that the seismically active faults or fractures are previously drained intervals that have subsequently recovered to hydrostatic pres sure via brine invasion. Storage capacity computed for one of these dr ained fractures implies that total oil production represents about 20% of total pore volume. Correlation of older production intervals and w ell-ion porosity anomalies with the seismically active faults indicate that the oil reservoir in the study area is primarily a set df compar tmentalized, low-angle thrust faults. Although low-angle fracture sets have not previously been considered in the exploration and developmen t of the area, the mapped thrust faults are consistent with other inve stigators' interpretations of oil associated with secondary fracture s ets occurring along deeper-seated, wrench-fault structures. Stress det ermined from composite focal mechanisms indicates a near-surface (<550 m) thrust regime. Maximum horizontal stress direction is N15 degrees W +/- 15 degrees, rotated approximately 90 degrees from regional orien tation. The seismic behavior is consistent with poroelastic models tha t predict slight increases in horizontal compressive stress above and below currently drained volumes. Pressure re-equilibration via brine i nvasion 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 app roximately 0.02 MPa, (C) 1998 Elsevier Science B.V. All rights reserve d.