Effects of Anisotropic Permeability Evolution on Shale Gas Production: An Internal Swelling Factor Model

Abstract

Considerable attention has been paid to the anisotropic permeability of shale gas reservoirs, but the impacts of heterogeneous internal swelling have rarely been studied. In this research, we propose a new definition of the internal swelling factor (ISF) as the ratio of the increment in the pore adsorption strain to the increment in the matrix adsorption strain. In particular, we pioneered the study of the influence of the boundary conditions on the ISF. A new shale gas production model that considers the anisotropic ISF is proposed. The validity of the model was verified using field data sets obtained from the Marcellus shale and Barnett shale. We examined the influence of the anisotropic ISF on the permeability in all directions. In particular, the influence of the ISF on gas exploitation was characterized. In addition, the sensitivity of the ISF to Langmuir pressure was also analyzed. We obtained the following results. First, under in situ stress boundary conditions, the horizontal ISF was constant as a result of the constant-stress boundary conditions, whereas the vertical ISF increased as a result of the constant-volume boundary conditions during gas depletion. Second, the horizontal permeability was determined by the competition between the effective stress-induced strain and the adsorption strain because of the constant-stress boundary conditions. The horizontal ISF mainly affects the middle and late stages of depletion, whereas the vertical ISF has little effect on both. Third, the large permeability enhances the gas production rate in the early stages of depletion but has relatively little effect on the cumulative gas production volume. The horizontal ISF value is also proportional to the horizontal permeability value. Finally, as the Langmuir pressure of the matrix increases, the ISF value also increases, which leads to an increase in the gas production rate of the inorganic pores, whereas the gas production rate of the matrix decreases because of the decrease in the gas adsorption capacity. As compared with the existing models, this model provides a more accurate way to estimate the impact of the ISF on the shale gas exploration process and to predict the gas depletion characteristics.

Study on Multiscale Fluid–Solid Coupling Theoretical Model and Productivity Analysis of Horizontal Well in Shale Gas Reservoirs

“…When K n > 0.2, the flow regime is dominated by the diffusion effect; when K n < 0.01, the flow is dominated by seepage; and when 0.001 < K n < 0.2, the gas flow is in the form of coexistence of multiple flow patterns mainly including slip flow, surface diffusion, Fick diffusion, and Knudsen diffusion. − Shale gas is stored in various states in a shale reservoir, mainly including free gas, adsorbed gas, and dissolved gas; the gas mass transfer is the common result of the above multiple flow regimes. In addition, the fluid–solid coupling effect of shale reservoir is obvious during the exploitation process due to the large burial depth. − It is urgent to establish a productivity model of shale gas horizontal wells that can comprehensively considers the combined effects of multiple flow mechanisms and fluid–solid coupling effects of a shale reservoir, so as to achieve accurate productivity prediction and evaluation and optimize the location and spacing of fracturing wells.…”

A Review of Swelling Effect on Shale Permeability: Assessments and Perspectives

“…On the one hand, the gas adsorption layer on nanopore wall reduces the effective radius of nanopores − and thus influences shale permeability. On the other hand, gas adsorption on rock grains causes matrix swelling, compresses the nanopore radius, and thus changes the shale permeability. ,,− In addition to the above-mentioned three factors, shale permeability could also be influenced by other factors. For instance, Cai et al proposed an easy-to-implement model and disclosed that shale permeability could be influenced by heterogeneous pore size distribution.…”

Novel Shale Permeability Model Coupling Sorption-Induced Differential Deformation and Multiple Flow Regimes

“…In this study, we follow the basic framework of the DSI-based generic coal permeability model that can be used for different stress conditions , and proposed a new approach to solve the coupling between the flow regime and poromechanical deformation. In addition, a mathematical model of shale permeability was established.…”