Reservoir Fracture Mapping Using Microearthquakes: Two Oilfield Case Studies

Abstract

SummaryPatterns of microearthquakes detected downhole defined fracture orientation and extent in the Austin chalk, Giddings field, Texas, and in the 76 field, Clinton County, Kentucky. At two sites in the Austin chalk we detected more than 480 and 770 microearthquakes, respectively, during hydraulic stimulation, and more than 3,200 during primary production in Clinton County. The data were of high enough quality that 20%, 31%, and 53% of the events could be located, respectively.Reflected-wave data constrained microearthquake locations to the stimulated depths at the base of the Austin chalk. In plan view, microearthquakes defined elongate fracture zones extending from the stimulation wells parallel to the regional fracture trend. However, the widths of the stimulated zones differed by a factor of five between the two Austin chalk sites, suggesting a large difference in the population of ancillary fractures. 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 depleted production intervals indicated that the mapped fractures had been drained previously. Drilling showed that the fractures currently contain brine. The seismic behavior was consistent with stress changes associated with poroelastic effects above and below a drained volume or with the mass exchange of brine replacing produced oil.