REV-scale simulation of micro-fractured unconventional gas reservoir

Abstract

Multiscale fractures such as hydraulic fractures and natural micro-fractures coexist in unconventional gas reservoir among which micro-fractures have great effect on the effective matrix permeability at representative elementary volume (REV) scale. In addition, in the development of unconventional gas, matrix properties and fracture aperture are influenced by gas transport mechanisms including Klinkenberg effect, stress sensitivity and gas adsorption. To investigate the influence of micro-fractures and gas transport mechanisms, a new model for REV-scale unconventional gas flow simulation has been presented based on REV-scale Lattice Boltzmann Method model and gas transport mechanisms. And the effects of different fracture features were analyzed and the most important feature was filtered. In addition, the effects of gas transport mechanisms on matrix permeability and effective permeability were investigated. The results indicate that fracture density has the greatest effect on the effective permeability that the increase multiple can reach to 1 order of magnitude when matrix permeability is at 10−4 μm2 level while fracture aperture has little effect. Moreover, gas transport mechanisms have significant effects on matrix properties and effective permeability. With the decline of gas pressure, matrix permeability increases and fracture aperture decreases which lead to the decrease of the effective permeability. But the decrease of the effective permeability is not great which is approximately 50% under the gas pressure decline of 20 MPa.