Video: Computational Fluid Dynamics Modeling of Slip Flow Coupled with Gas Adsorption/Desorption Kinetics in Complex Pore Space
- Shanshan Yao (University of Regina) | Ronny Pini (Imperial College London) | Xiangzeng Wang (Yanchang Petroleum Group) | Fanhua Zeng (University of Regina) | Ning Ju (University of Regina)
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- Society of Petroleum Engineers
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- 2018. Copyright is retained by the author. This presentation is distributed by SPE with the permission of the author. Contact the author for permission to use material from this video.
- 5.8.2 Shale Gas, 5 Reservoir Desciption & Dynamics, 1.10 Drilling Equipment, 5.8 Unconventional and Complex Reservoirs, 1.10 Drilling Equipment
- Pore Space Reconstruction, Slip Flow, Adsorption/Desorption Kinetics, CFD
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At reservoir conditions, gas flow confined in submicron pores of shale falls within slip flow and transition flow regimes. Beyond the common instant equilibrium assumption, we believe that gas adsorption/desorption on rough pore surfaces could be in non-equilibrium status when gas pressure keeps decreasing during production. We investigate the interplay of gas slip flow inside complex submicron-scale pores and gas adsorption/desorption kinetics on pore surfaces with computational fluid dynamics (CFD) under unsteady-state flow conditions.
Different from previous studies, the gas adsorption/desorption is in non-equilibrium state, which is closer to real reservoir conditions. Given pore pressure Pp at time t, linear driving force model with gas desorption rate coefficient kd is applied to describe the difference between the equilibrium adsorption amount (calculated with adsorption isotherms) and the actual adsorption amount per unit pore surface area. Free gas flow inside 3D reconstructions of shale pore space is modeled by Navier-Stokes equations with Maxwell's first-order slip boundary conditions. To include gas contributions from desorption, extra source with strength equal to the gas desorption rate is added to the slip boundaries.
Any type of adsorption isotherms can be incorporated into our CFD modeling. We investigate the coupling of slip flow and Langmuir adsorption isotherms for methane in 3D reconstructed pore space. We observe that not all of adsorbed gas measured in adsorption isotherms contribute to gas production. In our study the pore pressure, Pp, decreases along with time t. One significant finding is that there exists a key time point, tk, after which adsorbed gas starts desorbing off pore surfaces and the decreasing rate of pore pressure becomes smaller. The higher the gas desorption rate coefficient, kd, is, the earlier tk occurs. But the decreasing rate of pore pressure is no longer sensitive to the coefficient, kd, when kd is larger than 0.0005. Another significant finding is that optimum gas production rate should be chosen according to kd values. It is always better to have high production rate when kd is larger than 0.0005. If the kd is smaller than 0.0005, simulations are necessary to determine optimum gas production rate by balancing the development efficiency and final amount of gas desorption.
Gas adsorption/desorption is always regarded as an instant equilibrium process in shale reservoir simulations. This study considers the non-equilibrium gas adsorption/desorption process, which is closer to real reservoir conditions. No studies in the literature have considered the influence of gas adsorption/desorption kinetics when choosing optimum production rates. CFD simulations in this study provide insight and guidelines on optimizing shale gas development with evaluating slip flow as well as gas adsorption/desorption characteristics.