INDUCED MICROEARTHQUAKE PATTERNS AND OIL-PRODUCING FRACTURE SYSTEMS IN THE AUSTIN CHALK

Microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in Austin chalk oil reservoirs at two sites in th e Giddings field, Texas. We deployed three-component, downhole geophon e tools in production wells at depths of 2100 In and greater, one near Cook's Point, and two on the Matcek lease near Caldwell. At Cook's Po int, we collected 482 microseismic events during a 4000 m(3) (25,000 b bl) hydraulic stimulation in an offset well. We collected 770 events d uring a similar operation on the Matcek lease. Many seismogams contain ed reflected phases that constrained location depths to the production zone at the base of the Austin chalk. By restricting all microearthqu ake locations to production depths, we located 20% of the Cook's Point events and over 60% of the Matcek events. At both sites we observed o nly the fracture wing closest to the observation stations. Locations f ormed elongated patterns extending up to 1 km from the stimulation wel l and trending N60 degrees E, parallel to the known, regional fracture trend. The Cook's Point seismic zone measured over 100 m in width, wh ile long stretches of the Matcek seismic zone narrowed to 30 m or less . We believe that the width of the seismic zone reflected the density of conductive fractures and thus, the volume of the reservoir accessed by the stimulation. Indeed, production rates in the first year follow ing stimulation were much higher at Cook's Point, where we observed th e wider of the two seismic zones. (C) 1998 Elsevier Science B.V. All r ights reserved.