ABSTRACT
A method to determine in situ stress directions and magnitudes from anelastic strain recovery measurements of oriented core has been used to determine the principal horizontal in situ stresses in the Rollins Sandstone in a deep well near Rifle, Colorado. The principal horizontal stress directions were determined directly from the principal horizontal strain recovery directions. The principal horizontal stress magnitudes were calculated from the principal strain recovery magnitudes, overburden stress, and Poisson's ratio of the rock using a viscoelastic model. The accuracy of the in situ stress directions and magnitudes determined from anelastic strain recovery measurements was substantiated by a direct comparison with open-hole hydraulic fracture stress measurements. The anelastic strain recovery method predicted the magnitudes of the maximum and minimum horizontal principal stresses to be 54.5 MPa and 51.9 MPa, respectively; with an azimuth of N63°W ± 8 ° for the maximum horizontal stress. The hydraulic fracture stress measurements yielded maximum and minimum horizontal principal stress magnitudes of 49.6 MPa and 46.8 MPa, respectively. The azimuth of the maximum horizontal stress ranged from N50°W to N70°W.
INTRODUCTION
Knowledge of the orientation and magnitude of in situ stresses is often critical to the design of hydraulic fractures for oil and gas stimulation. It was recognized early that the orientation of hydraulic fractures is predominantly controlled by the orientation of the principal stresses (Hubbert and Willis, 1958). More recently, the overriding influence of variations in the minimum, principal, horizontal stress magnitude on the containment of vertical hydraulic fracture growth has been recognized (Van Eekelen, 1982, Warpinski et al., 1982, and others). In addition, in naturally fractured reservoirs the ratio of the magnitudes of the horizontal stresses, relative to the hydraulic fracture treatment pressure, is important in determining the type of interaction between natural and induced fractures (Blanton, 1982). The economic importance of in situ stress measurements and their relation to fracture geometry is reflected in the results of an industry survey sponsored by the U.S. Department of Energy (Brashear et al., 1982). This survey reported that out of ten tight-gas R&D objectives, the ones given first and second rank in importance were (1) prediction and/or control of fracture geometry and (2) measurement of in situ stress to better predict fracture geometry. While knowledge of in situ stress is important, obtaining accurate measurements is often difficult. Of the several methods available for making stress measurements at depth, hydraulic fracturing is the only one that is not considered developmental. Even so, this approach has some limitations. First, the axis of the borehole must be closely aligned with one of the principal stresses. Even in this situation, information on both of the principal horizontal stresses is obtained only if the measurement is performed in an open hole. The oil and gas industry has been reluctant to conduct hydraulic fracture stress measurements routinely in uncased, open holes in deep wells because of the difficulty of attendant zone isolation and potential well control problems. In addition to these technical limitations, hydraulic fracturing is expensive and involves enough logistical complications to make any simpler, less expensive method look attractive, particularly if measurements of both principal horizontal stresses are required on routine basis.
