In Situ Stress Magnitudes From Mini-Frac Records In Western Canada

Abstract

Abstract This paper lists instantaneous shut-in pressure and closure stresses derived from the records of 115 mini-frac tests run in western Canada in recent years. One hundred and eight of the mini-froes were run in Alberta wells and this data set is reviewed briefly. The measurements were made largely in reservoir sandstones, some of which exhibited formation pressures that had been lowered by recent production of hydrocarbons. There is considerable variability in the measured magnitudes of the smaller horizontal principal stress, SHmin both locally and regionally, although the average gradient is similar to that of the Gulf Coast. Many SHmin magnitudes are low enough to suggest that SV>SHmax>SHmin over much of the Alberta Plains. Higher SHmin values were obtained from wells within or near the foothills, painting to a regime where SHmax>SV>SHmin but these mini-fracs were largely from deeper tests than those run beneath the plains. Hence, it is not proven whether SHmin actually increases toward the mountains or whether there is only a change in gradient with depth. Introduction In recent years it has become clear that many practices in hydrocarbon exploration and production would benefit from an increased understanding of subsurface in situ stresses. It has long been recognized that principal stress orientation controls the propagation direction of hydraulic fractures(1). More recently, the effects of variations in horizontal principal stress magnitudes have been shown to affect fracture containment(2, 3) and borehole stability(4). The state of stress at a point within the earth can be completely described by three mutually perpendicular principal stresses. These stresses are assumed to be oriented vertically and horizontally(1, 5). Each can be considered as the sum of the effective stress component and pore pressure, so that:Vertical stress: S V = σ V + P oMaximum horizontal stress: S Hmax = σHmax + P o(1)Minimum horizontal stress: SHmin=σHmin+P0 where S = total stress σ= effective stress Po = formation fluid pore pressure Hydraulic fractures propagate perpendicular to the least principal stress. Although horizontal hydraulic fractures have been documented in the subsurface(6), the majority of hydraulic fractures are vertical, indicating that the least principal stress is usually horizontal(1). Therefore, earth stress measurements obtained directly from hydraulic fracture techniques are normally interpreted in terms of SHmin. In the Western Canada Basin, horizontal principal stress directions have been mapped fairly extensively by means of borehole breakouts, augmented by data from fracture orientations and overcoring(7). What has been lacking is equivalent published information on stress magnitudes. Over the past thirty years, thousands of hydraulic fracture jobs have been run in tight or sluggish western Canadian reservoirs and pressure/time records obtained. but these data have not been evaluated and interpreted in terms of in situ stress magnitudes. Mini-frac tests appear to be particularly suitable for estimating the smaller horizontal principal stress, SHmin For this reason, the Geomechanics Group of the Calgary Section of The Petroleum Society of CIM approached 33 operating companies in 1987 and requested the release of speci