Development of continuous estimates of the least principal stress with depth for application to multi-stage hydraulic fracturing

Abstract Obtaining accurate knowledge of variations in the magnitude of the least horizontal principal stress, S hmin , with depth is of great practical importance in the oil and gas industry. We demonstrate that accurate knowledge of the magnitude of S hmin with depth can be obtained utilizing two fundamental concepts. First, frictional faulting constraints place bounds on principal stress magnitudes in relatively stiff (high Young’s modulus) rocks. These bounds depend on depth, pore pressure and whether a site is characterized by a normal, strike-slip or reverse faulting stress state. Second, in more ductile clay-rich formations such as those typical of most US unconventional plays, varying degrees of viscoplastic stress relaxation (VSR) reduces differences between principal stress magnitudes. We focus in this paper on the importance of how layer-to-layer variations of lithology control the magnitude of S hmin and how the two concepts above can be used to accurately predict variations of the magnitude of S hmin with depth. It is well established that the effectiveness of multistage hydraulic fracturing is critical to stimulate production in unconventional oil and gas reservoirs. Using several case studies, we illustrate how predictable variations of stress magnitude with depth affect hydraulic fracture propagation. We also demonstrate that widely used elastic loading models do not accurately predict variations with depth. In the context of VSR, essentially complete stress relaxation also helps explain occasional observations of the least principal stress being nearly equal to the overburden stress in some clay-rich formations. In such cases, sub-horizontal hydraulic fracture propagation is expected due to the low tensile strength of bedding planes.