The widely distributed microfractures play an important role in bridging the gap between the matrix and hydraulic fractures during the gas production of shale. In this work, a newly dynamic apparent permeability (AP) model, coupling poromechanics, sorption-induced strain, and gas slippage, has been proposed to effectively reveal the real-gas flow mechanisms through microfractures of shale. More specifically, a dynamic aperture is innovatively incorporated into the Navier-Stokes (N-S) equation using the Maxwell second-order slip boundary condition to calculate the gas velocity and volume flux in single microfracture. Then the gas transport model for microfracture networks considering the distributions of aperture and tortuosity is obtained according to the fractal theory. The newly developed model is verified well with experimental data and network simulation. Results indicate that the gas conductance highly depends on the structure of microfracture networks (i.e., the maximum aperture and fractal dimensions). The AP presents a similar shape of "V" owing to the "negative contribution" of poromechanics at an early stage and the "positive contribution" for both gas slippage and desorption induced shrinkage at the late stage of gas production. Moreover, the "negative factor" of poromechanics is positively correlated with fracture compressibility coefficients at high pressures (>15[MPa]). Increasing gas desorption capacity, fracture spacing and internal swelling coefficient can enhance the "positive factor" of sorption-induced strain at low pressures (<15[MPa]). This work provides a comprehensive and theoretical guidance for the effective development of shale gas.
A New Dynamic Apparent Permeability Model for Gas Flow in Microfractures of Shale
Li, Yudan, Dong, Pingchuan, and Dawei Zhou. "A New Dynamic Apparent Permeability Model for Gas Flow in Microfractures of Shale." Paper presented at the SPE/AAPG/SEG Asia Pacific Unconventional Resources Technology Conference, Brisbane, Australia, November 2019. doi: https://doi.org/10.15530/AP-URTEC-2019-198302
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