Hydraulic fracturing to determine the regional in situ stress field, Piceance Basin, Colorado

摘要

Several authors have discussed the theoretical aspects of using the techniques of hydraulic fracturing to determine the state of stress within the Earth. Few, however, have attempted systematic hydraulic fracturing for the specific purpose of determining the regional stress field. Using specially designed equipment, the state of stress was determined on a regional scale in the Piceance Oil Shale Basin of northwest Colorado. Structurally, the area is a simple basin that has been subjected to normal faulting. Seven oil-shale, test holes scattered throughout the Piceance Basin were logged with an acoustic borehole televiewer to select unfractured solution-free intervals. Fracture-free intervals, ranging in depth from 60 to 460 m, were isolated between packers and hydraulically fractured. Induced fractures were propped with ground lucite and relogged with the televiewer to determine their strike and dip. These data, combined with pressure measurements made during the fracturing experiments, were used to compute the orientation and magnitude of the stress tensor. Field observations showed that in a tectonically relaxed area, vertical fractures can be induced at hydraulic pressures as low as 0.6 of the overburden stress. Below a depth of 120 m, the induced fractures were approximately vertical, with a prominent strike of N70°W. This orientation parallels the major normal faulting in the basin as well as one of the prominent directions of jointing. These results also agree with those obtained at Rangely, about 50 km to the northwest. Key Words: engineering geology • principal stresses • structural analysis • fractures