INTRODUCTION

ABSTRACT

The instantaneous shut in pressure often times is not a well defined feature of pressure-time records from in situ stress determinations by hydraulic fracturing. As the applications of in situ stress data become more sophisticated, the greater the need for an unambiguous method with which to identify the instantaneous shut in pressure becomes. Controlled experiments suggest that the instantaneous shut in pressure should be equated with the pressure at the inflection point of the pressure-time record following shut in. A method for determining this value is presented. Often times the minimum in stantaneous shut in pressure determined after several pressurization cycles is the best estimate of the stress acting perpendicular to the fracture plane.

Over the years that hydraulic fracturing has been used as a means of in situ stress determination it has evolved from an experimental technique to an engineering tool. It is being used with increasing frequency for practical applications within the mining (Enever, 1982), heavy civil construction (Haimson, 1978), and petroleum industries (Kry and Gronseth, 1982), as well as for scientific investigations (Zoback et al., 1980), which are increasing our understanding of tectonic processes in the Earth's crust. Regardless of the application, in situ stress determinations by hydraulic fracturing rely on the assumption that the instantaneous shut in pressure is equal in magnitude to the stress acting perpendicular to the plane of the induced fracture. While it can be shown experimentally that this assumption is justified, the instantaneous shut in pressure often times is not a well defined feature of pressure-time records from hydraulic fracturing tests. Identification of the instantaneous shut in pressure can be a highly subjective process. Several investigators interpreting the same pressure-time records can choose instantaneous shut in pressures differing by several MegaPascals. It has been observed that instantaneous shut in pressures obtained from multiple pressurizations of a zone do not always remain constant from cycle to cycle. The instantaneous shut in pressure obtained from the first pressurization cycle of a zone can significantly overestimate the magnitude of the stress acting perpendicular to the fracture plane. The purpose of this paper is to present the results of controlled laboratory and field tests which were performed to assess the reliability of the instantaneous shut in pressure as an estimate of the minimum stress acting perpendicular to the wellbore axis.

DETERMINATION OF THE INSTANTANEOUS SHUT IN PRESSURE

Experience reported in this paper suggests that for low flow rate hydraulic fracturing (<50 1/min), the instantaneous shut in pressure should be equated with the pressure at the inflection point in the pressure-time record following shut in. A series of laboratory tests was performed to determine whether reliable estimates of the stress acting perpendicular to the plane of an induced fracture could be determined from hydraulic fracturing data. Details of the testing program are given in Gronseth and Detournay (1979). This was done so that multiple runs could be made on a single block. The internal stress distribution was modelled using the Displacement- Discontinuity method.

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