3D Finite Element Modeling of Laboratory Hydraulic Fracture Experiments

Abstract

Abstract Laboratory experiments are an excellent way to visualize and improve our understanding of the hydraulic fracturing process. However, in order to truly apply the results of laboratory work to the field, an understanding of what artificial conditions are being created during the laboratory process must also be developed. This paper describes 3D finite element modeling and associated results using actual laboratory block tests as a basis. These models consisted of seven basic block systems, including a single layer system, with and without a wellbore, as a control case; a three-layer model with and without a wellbore; and a seven layer system, with a wellbore, without a wellbore, and with a cased wellbore. These seven systems were based on actual triaxial hydraulic fracturing experiments that were performed on mid-sized blocks (11 × 11 × 15 in) consisting of the same single, three, and seven-layers. The fracture growth patterns in the actual laboratory tests were extremely complex, not at all like the planar fractures dictated by hydraulic fracturing theory. The intent of the modeling was to 1) determine the spatial stress contrasts being created by the material property contrasts of the layered systems in the triaxial system and 2) determine if the complex fracture growth could be accounted for by these modeled contrasts. Results were also used to determine the potential for shearing across the layered systems. A commercial, modeling software, capable of numerically determining the 3D stress distributions in the various porous media, was used. Numerical results were validated with analytical calculations. The results of the modeling provide insight into the time-dependent application of stresses in a laboratory setting. It helped to explain the complex fracture growth in the actual experiments and gave some insight into the conditions that would cause vertical or horizontal fracture growth in a layered system of different material properties. The modeling also documented the creation of shear stresses which accounted for some fracture path deviation. The results of this paper aid in understanding the mechanisms of complex hydraulic fracture growth in reservoirs settings. Additionally, insight into the shear and tensile fracture mechanisms that generate acoustic events was gained.