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Choi, Seungbeom (Korea Institute of Geoscience and Mineral Resources) | Lee, Su-deuk (Seoul National University) | Jeong, Hoyoung (Seoul National University) | Jeon, Seokwon (Seoul National University)
Rock mass contains various discontinuities in terms of size and shape, such as fault, joint, and bedding plane. Among them, a joint is a planar discontinuity that has little strength and it shows smaller size but more frequency than a fault. In general, the joint exerts huge influence on mechanical behavior of rock mass since it acts as a weak plane. At the same time, the joint has several orders higher hydraulic conductivity that rock matrix so that the majority of fluid flow in rock mass occurs through the joint. Therefore, accurate understanding of characteristics of a rock joint is of great importance, as it affects both mechanical and hydraulic behavior of rock mass. Not only the mechanical conditions but also the geometric features affect the joint behavior. The geometric features can be explained by various properties, such as roughness, aperture, contact area, and so on. They exert complex influence on the hydromechanical characteristics of a joint, interacting with each other. Therefore, a series of laboratory experiments were conducted in order to investigate the hydraulic characteristics under various mechanical and geometric conditions. A fractal theory was used to generate coordinates of artificial rock joint surfaces so as to control the roughness of the joint. Then the coordinates were printed by a 3D printer and utilized to make cement mortar specimens. Four joint specimens, which had different levels of roughness and aperture, were prepared and tested. Before investigating the hydraulic characteristics, mechanical behavior of joint specimen was tested first. In order to consider various mechanical conditions, normal and shear stresses were applied and hydraulic tests were conducted with the same mechanical and geometric conditions. The results showed that flowrate per unit hydraulic head decreased with the increase of normal stress, increased with the shear displacement, and increased with the roughness of the joint. Also, comparison between hydraulic aperture, which was calculated based on the cubic law, and corresponding mechanical aperture was made and it showed more deviation from the cubic law when the roughness and stress were increased.
Moinfar, Ali (1 The University of Texas at Austin) | Sepehrnoori, Kamy (1 The University of Texas at Austin) | Johns, Russell T. (2 The Pennsylvania State University) | Varavei, Abdoljalil (1 The University of Texas at Austin)
Abstract The effect of geomechanics on fluid flow is more crucial in fractured reservoirs due to presence of fissures, which might be more stress-sensitive than the rock matrix. The flow characteristics of fractures are significantly affected by effective normal stress exerting on them. In spite of extensive experimental and field studies that have demonstrated the dynamic behavior of fractures, fracture properties have been often treated as static parameters in the simulations of naturally fractured reservoirs. Realistic modeling of production in fractured systems requires including the dynamic behavior of fractures into a discrete fracture model. We have incorporated the dynamic behavior of fractures into an embedded discrete fracture model, called EDFM. The coupled model allows inclusion of the impact of stress regime on fluid flow in a 3D discrete fracture network. We use empirical joint models to represent normal deformation of pre-existing natural fractures and couple them with the EDFM approach. Using these models, the aperture and permeability of an arbitrary-oriented fracture become functions of the effective normal stress acting on the fracture plane. In addition, we allow for fracture-conductivity tables to model dynamic behavior of propped hydraulic fractures in stimulated reservoirs. We present several examples in this study to show the applicability and performance of the coupled geomechanics-EDFM approach for the simulation of naturally fractured reservoirs. We examine the effect of pressure-dependent fracture properties on production leading to the conclusion that fracture deformation, caused by effective stress changes, substantially affects hydrocarbon recovery. Our simulations show that the significance of such effects on production strongly depends on parameters controlling the deformation behavior of fractures. Simulations also show that creating sufficiently high-conductivity fractures during stimulation treatment of unconventional reservoirs can mitigate the adverse effect of hydraulic fracture closure on production to a good extent. Furthermore, the coupled geomechanics-EDFM approach does not degrade the computational performance of EDFM, which is a promising new approach for modeling discrete fractures in a robust and efficient manner.