RESERVOIR FRACTURE MAPPING USING MICROEARTHQUAKES - 2 OIL-FIELD CASE-STUDIES

Patterns of microearthquakes detected downhole defined fracture orient ation and extent in the Austin chalk, Giddings field, Texas, and in th e 76 field, Clinton County, Kentucky. At two sites in the Austin chalk we detected more than 480 and 770 microearthquakes, respectively, dur ing hydraulic stimulation, and more than 3,200 during primary producti on in Clinton County. The data were of high enough quality that 20%, 3 1%, and 53% of the events could be located, respectively. Reflected-wa ve data constrained microearthquake locations to the stimulated depths at the base of the Austin chalk. In plan view, microearthquakes defin ed elongate fracture zones extending from the stimulation wells parall el to the regional fracture trend. However, the widths of the stimulat ed zones differed by a factor of five between the two Austin chalk sit es, suggesting a large difference in the population of ancillary fract ures. Post-stimulation production was much higher from the wider zone. In Clinton County, microearthquakes defined low-angle, reverse-fault fracture zones above and below a producing zone. Associations with dep leted production intervals indicated that the mapped fractures had bee n drained previously. Drilling showed that the fractures currently con tain brine. The seismic behavior was consistent with stress changes as sociated with poroelastic effects above and below a drained volume or with the mass exchange of brine replacing produced oil.