A fracture/proppant system is used to mimic the interaction between the rock matrix and proppants during the process of fracture closing attributed to pore-pressure reduction during hydrocarbon production. Effects of rock type and bedding-plane direction are investigated. High-strength sintered bauxite proppants are placed in hydraulic fractures in sandstone and shale rock. There are two bedding-plane directions in shale rock: One is 90 , which is perpendicular to the fracture, whereas the other is 0 , which is parallel to the fracture. Increasing mechanical loading is imposed to close the fracture. Micrometer-scale X-ray tomography is used to visualize the internal structure. Cutting-edge image-processing methods are applied to extract patterns of both the fracture and matrix. A pore-scale lattice Boltzmann simulator, optimized with graphics-processing-unit parallel computing, is used to simulate the permeability tensor inside the fracture. Significant proppant embedment is observed in the sandstone rock when the effective stress is increased to 4,200 psi. Consequently, fracture porosity is reduced by nearly 70%, and permeability is reduced by two orders of magnitude. Embedded proppants are unable to create microscopic fractures on the matrix surface because of the low bonding strength between grains. In the shale rock with 90 bedding planes, when the effective stress is increased to 3,000 psi, significant microscopic fractures on the matrix surface are created because the lamination structure of the matrix is opened. In the shale rock with 0 bedding planes, noticeable microscopic fractures on the matrix surface are not observed until the effective stress is increased to 6,990 psi. Proppant embedment is insignificant because of the high bonding strength between fine grains. Significant anisotropy in the permeability tensor is observed during all experiments. This study is the first to use cutting-edge imaging and modeling methods to quantitatively study the interaction between proppants and the rock matrix during the stressincrease process. It has important applications, which help sustain production with adequate fracture conductivity in deep reservoirs (e.g., the Haynesville shale). Laboratory Materials and ApparatusAs illustrated in Fig. 1a, a cylindrical core plug (1-in. diameter and 2-in. length) was cut into two identical halves. The space between the two halves is the primary propped hydraulic fracture and hereafter is referred to as the primary fracture, in which 20-to 40-mesh high-strength sintered bauxite proppants were uniformly placed to form a monolayer. All the materials were retained in place by a cylindrical sleeve. Uniaxial mechanical stress was imposed to close the primary fracture (Fig. 1b) to mimic the
Natural fractures widely exist in shale samples, and natural opening-mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone. The existence of natural fractures is very important to the stimulation treatment and will eventually benefit the shale gas/oil reservoir recovery. The effect of natural fractures on a shale mechanical property study will serve as the basis for the formation evaluation and the sweetspot selection on the hydraulic fracturing treatment. By combining conventional destructive mechanical tests and novel digital rock nondestructive analysis, important conclusions can be determined regarding the mechanical properties changing trend as a function of the fracture structure. The more complex the natural fracture structures, the less resistant the rock. This paper presents, in detail, the changing trend from various shale samples. Studies on the mechanical properties of Eagle Ford shale samples (e.g., Young's modulus, Poisson's ratio, etc.) are performed in a laboratory using a hydraulic load frame, and the core internal fractures are computed tomography (CT) scanned before and after the mechanical tests. The effect of natural fractures on the mechanical properties is analyzed through the testing data. The induced fractures are characterized in several Eagle Ford shale core plugs in terms of orientation, size, and width. Substantial image processing techniques are employed to analyze the three-dimensional (3D) rock microstructures. The statistical properties, such as the width, length, and the tortuosity of the main fractures, are obtained through image analysis. The volume of the fractures and the permeability on all x, y, and z directions on the central part of the core plug are calculated based on the 3D CT images as well. The small fractures can serve as the planes of weakness and reactivate during a hydraulic fracture treatment. This paper elaborates on the data interpretation and measurement correlation methods used to study natural fracture behavior as well as how this behavior affects mechanical tests, which can be very important in the field of geomechanics as well as formation stimulation research.
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