INTRODUCTION

A Workshop on Hydraulic Fracturing Stress Measurements was convened by the authors in December 1981 in Monterey, California under the auspices of the U.S. Geological Survey and the U.S. National Committee on Rock Mechanics. There are now over a dozen groups of investigators around the world who are actively using the hydraulic fracturing method for making in-situ stress measurements. Because of the extensive recent experience of these different groups, the Workshop enabled forty investigators from eight countries to comprehensively assess the status of the method. Based on the presentations and discussions at the Workshop, in this paper we briefly discuss the current status of the hydraulic fracturing stress measurement method concentrating on problem areas where further research needs to be done. A complete account of the presentations at the Workshop will be available through the Proceedings of the Workshop which will be published by the U.S. National Committee on Rock Mechanics in early 1983. The most exciting aspect of the Workshop was the discovery by the various groups of investigators that others were having very similar experiences with the method. The consensus among the different groups of investigators at the Workshop was that they were recording very similar pressure-time data in the field, they were able to interpret them in similar ways, and they were achieving results that were quite encouraging because of several factors. First, the results at various locales were found to be internally quite consistent. This was reported by investigators from Germany (Rummel, Baumgartner, and Alheid, 1982), Japan (Tsukahara, 1982), Australia (Enever, 1982), and the U.S. (see, for example, Haimson, 1978, 1982a; Zoback et al. 1980). Second, the hydrofrac stress measurements generally agreed well with subsurface overcoring stress measurements wherever detailed comparisons could be made (Haimson, 1981, 1982; Li, et al. 1982; Doe, et al. 1982), and hydrofrac orientations seem to agree quite well with other stress measurement methods and stress field indicators (see Zoback and Zoback, 1980). Finally, the magnitudes of stresses determined by hydraulic fracturing seem to agree well with stress estimates based on the frictional strength of rock (Brace and Kohlstedt, 1980; Zoback and Hickman, 1982). Nevertheless, it was clear at the meeting that there are a number of areas where the method is more complex than is usually recognized, and other areas where further research is required to verify interpretation methods. There are discussed below.

DETERMINATION OF THE LEAST-PRINCIPAL STRESS

It has been well-demonstrated by laboratory and theoretical studies that hydraulic fractures propagate in a plane perpendicular to the least-principal stress (Hubbert and Willis, 1957; Haimson, 1970; Haimson and Avasthi, 1975; and others) and the magnitude of this stress can be determined simply from pressure in the fracture immediately after pumping has stopped (the shut-in pressure). Unless otherwise noted in this paper, we will usually refer to a vertical fracture propagating from a vertical well-bore and we assume that one principal stress is essentially vertical and results from the weight of the overlying rock.

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