Temporal Evolution of the Geometrical and Transport Properties of a Fracture/Proppant System Under Increasing Effective Stress

Abstract

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

Water-Gas Two-Phase Flow Behavior of Multi-Fractured Horizontal Wells in Shale Gas Reservoirs

“…where φ dc is dynamic porosity of shale gas reservoirs, dimensionless [21]; R int_max is the largest pore radius, m; R int_min is the smallest pore radius, m; R int is the pore radius, m; l b is the pore length, m; n p is the pore number, dimensionless; R dc is the dynamic pore radius considering stress sensitivity and organic shrinkage, m; R i is the ratio of rectangular pores, dimensionless; E i is the ratio of elliptical pores, dimensionless; ς i is the shape factor, dimensionless; ϑ ce is the specific area, dimensionless; d m is the diameter of the gas molecular, m and p L is the Langmuir pressure, Pa. The apparent permeability model considering the pore structure and pore size change can be expressed as [24,53]:…”

“…As studied, the pore systems in shale reservoirs are multiple-scale with nano-and micro-meter pores [18,19], and the micropores play important effects on geometric, topological and transport properties of the pore systems [20]. Scholars also have conducted a lot of research on the pore change during depressurization of shale reservoirs, including stress sensitivity [21][22][23], organic matter shrinkage [5,6,24] and adsorption layer changes [25]. Pore size shrinkage caused by pore stress sensitivity, pore size expansion caused by organic matter shrinkage and free gas expansion caused by a thickness change of the adsorption layer has attracted attention [24].…”

Investigation of Fracture Width Change under Closure Pressure in Unconventional Reservoir Based on the Hertz Contact Theory

“…Once the production starts, the pore pressure will decline obviously, and the fracture width will decrease. As the proppants partly remain in the fracture, the propping effect is motivated to keep the fracture open, which is expected [12][13][14][15]. The competition between the proppants and the closure pressure influences the fracture width and the flow conductivity there, which relates to the development efficiency [16][17][18].…”