Stress measurements by hydraulic fracturing for a rock mass with many planes of weakness

Abstract

When hydraulic fracturing is performed on a rock mass which contains many natural cracks and planes of weakness, most of the induced cracks are inclined transverse cracks. We propose a method which can be used under such a circumstance to determine the stress state. The method consists of two stages. The first stage determines the magnitude and orientation of principal stresses by using shut-in pressures of inclined transverse cracks and the circumferential position of longitudinal cracks on the wellbore surface. The second stage estimates the ranges of errors of the stress state determined in the first stage, by taking into account that the shut-in pressures and orientations of the cracks are the factors which give rise to errors of the determined stress state. We have applied this method to data obtained by the hydraulic fracturing performed at two wellbores drilled from a drift way at the Kamaishi mine. It was found that the ranges of errors of the principal stress magnitude were small. The orientation of the maximum principal axis of stress had a relatively high reliability in all directions, although the ranges of errors of the directions of the minimum and intermediate principal stresses showed a tendency to spread more widely in the N-S direction.