Induced microearthquake patterns and oil-producing fracture systems in the Austin chalk

Abstract

Microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in Austin chalk oil reservoirs at two sites in the Giddings field, Texas. We deployed three-component, downhole geophone tools in production wells at depths of 2100 m and greater, one near Cook's Point, and two on the Matcek lease near Caldwell. At Cook's Point, we collected 482 microseismic events during a 4000 m 3 (25,000 bbl) hydraulic stimulation in an offset well. We collected 770 events during a similar operation on the Matcek lease. Many seismograms contained reflected phases that constrained location depths to the production zone at the base of the Austin chalk. By restricting all microearthquake 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 only the fracture wing closest to the observation stations. Locations formed elongated patterns extending up to 1 km from the stimulation well and trending N60°E, parallel to the known, regional fracture trend. The Cook's Point seismic zone measured over 100 m in width, while 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 following stimulation were much higher at Cook's Point, where we observed the wider of the two seismic zones.