Hydraulic Fracture Height Growth Under the Combined Influence of Stress Barriers and Natural Fractures

Abstract

Abstract Fracture height control is an important concern in stimulation design. Existing knowledge of interaction mechanisms between hydraulic fractures, natural fractures, and source rock fabric has been largely obtained from 2D or pseudo-3D models. The recent advances in full-3D hydraulic fracture modeling capabilities reveal previously-unknown 3D interaction mechanisms on fracture height growth. A fully coupled finite element/finite volume code is used for modeling 3D hydraulically driven fractures with arbitrary geometries under the influence of natural fractures and stress barriers. Both hydraulic fractures and natural fractures are represented in a unified framework by split interfaces between solid elements that represent the rock continuum. Fracture flow elements and frictional contacts are deployed along the split interface, generating fluid pressure and contact stress applied to the rock faces on both sides as traction boundary conditions. Fracture dilation/propagation/sliding and the resultant stress alteration are natural outcomes of the model. The current study focuses on the influence of strong variations in closure stress interacting with natural fractures upon hydraulic fracture propagation. We observe that the slipping of a natural fracture, triggered by elevated fluid pressure from an intersecting hydraulic fracture, can induce both increases and decreases of normal stress in the minimum horizontal stress direction, toward the center and tip of the natural fracture respectively. Consequently, we expect that natural fractures can both encourage and inhibit the progress of hydraulic fractures propagating through stress barriers, depending upon the relative locations between the intersecting fractures. Once the hydraulic fracture propagates above the stress barrier through the weaken segment near a favorably located natural fracture, a configuration consisting of two opposing fractures cutting the stress barrier from above and below forms. The fluid pressure required to break the stress barrier under such opposing-fracture configuration is substantially lower than that required by a fracture penetrating the same barrier from one side. Sensitivity studies of geologic conditions and operational parameters have also been performed to explore the feasibility of controlled fracture height. We demonstrate that the interactions between hydraulic fractures, natural fractures, and geologic factors such as stress barriers in 3D are much more complex than in 2D. Although it is impossible for a specific study to exhaust all the possible configurations, we demonstrate the ability of a 3D, fully coupled numerical model to naturally capture these processes.