In-Situ Stress Determination From Wellbore Elongation Measurements

Abstract

SPE Members Abstract This paper explores the feasibility of making in situ stress measurements at depth from caliper measurements of deformed wellbores. The work has been carried out in three steps:theoretical development of viscoelastic constitutive equations necessary for calculation of stress directions and magnitudes,application of the equations to field data, andcomparison of results to stress measurements made by hydraulic fracturing and overcoring methods. The results of the theoretical analysis are equations that relate the minimum and maximum radii of a deformed wellbore to the maximum and minimum stresses perpendicular to the axis of the wellbore. Calculation of perpendicular to the axis of the wellbore. Calculation of absolute magnitudes of stresses requires the viscoelastic compliance and Poisson's ratio of the rock, but even when this information is not available relative magnitudes of stresses can be determined. The constitutive equations were applied to caliper measurements of a horizontal borehole in unwelded ash-fall tuff near an underground tunnel complex in Rainer Mesa on the Nevada Test Site. The horizontal borehole was aligned in the direction of the maximum horizontal stress. Over a 37 day period the wellbore showed time-dependent deformation, with maximum closure in the maximum stress direction (overburden). The ratio of principal stress calculated from caliper measurements of the deformed wellbore varied from 1.73 to 2.06, depending on the time interval used. The ratios obtained from overcoring and hydraulic fracturing were 1.79 +/- .22 and 2.12 +/- .12. Limitations of this approach to stress measurement are discussed, but results of the present study suggest that with further development of the viscoelastic consitutive model, stress variations along the length of the wellbore might be readily examined as part of the well logging process. process Introduction The overriding influence of stress an the orientation and geometry of hydraulic fractures has established the importance of knowing in situ stress to optimize stimulation treatments. 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. This survey reported that out of ten tight-gas R and 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. In addition, the production characteristics of many reservoirs are being recognized as stress sensitive, and thus even more emphasis is being placed on stress determination. The conventional method of determining in situ stresses at depth is with a small volume hydraulic fracture. The relation between the in situ stresses and the pressure record have been the subject of intense study since the benchmark work of Hubbert and Willis. Even so this approach has several limitations. For example, reliable results may be difficult or impossible to obtain when the wellbore is not closely aligned with one of the principal stresses, when the rock is strongly anisotropic, or when the packers induce fractures. packers induce fractures. One situation in particular which causes conventional hydraulic fracturing data to be of limited value occurs when the rock is not strong enough to support the stress concentrations near the wellbore. Under these conditions wellbore deformation occurs, making it difficult to accurately determine either the horizontal stress magnitudes or orientations at the wellbore. In brittle rock, wellbore deformation results in spalling of the wellbore (breakouts). Whereas in ductile rock, creep deformation results in wellbore closure. There is work to suggest that the geometry of deformed wellbores may provide information on the in situ stresses. The objective of this study has been to develop equations that relate creep deformation of a wellbore to the in situ stress acting perpendicular to the wellbore and to test the equations in the field. The equations are formulated so that stresses can be determined from measurements made by an oriented caliper or televiewer log. P. 279