Shale gas mass transfer characteristics in hydration-induced fracture networks

Abstract

The hydration-induced fractures significantly enhance shale gas production after well shut-in, which reveals considerable gas mass transfer characteristics. However, few studies focus on multiple flow mechanisms coupling the fracture distribution and morphological properties. Therefore, a novel apparent permeability (AP) model, in which poromechanics and desorption-induced aperture evolution are captured, has been derived to precisely define gas mass transfer through fracture networks. In this study, the fracture distributions are derived by fractal law, and the morphologies are solved using the orthogonal decomposition method (ODM) and shape coefficient correction. Viscosity changes in confined channels are also considered, further upscaling volume flux, Knudsen and surface diffusion through fractal theory by discrete integrals and derivation of the AP model combined with Darcy's law. The proposed model is verified well by experiments and the literature. The results show that the viscous flow contribution ratio decreases with decreasing aperture, while the Knudsen flow ratio slightly increases, and gas desorption significantly increases permeability when pp < pL. Therefore, the viscous flow is the dominant flow regime at high pp, and Knudsen and desorption diffusion gradually dominate the transmission at low pp. The larger bmax/bmin obviously enhances AP, the more confined apertures, and the AP decreases obviously as pp decreases. The stronger desorption and diffusion capability represent that gas will be transported sufficiently, higher co and δ indicate that the aperture is close more effectively, causing the AP reduction to be fast, and hydration further lowers E and v denotes higher AP due to the aperture shrinkage being replaced by matrix parts. The real gas effect on AP reduction cannot be ignored. This study identifies the gas transport characteristics in hydration fracture networks, with the research method also being applicable to other structures.