Multiscale and multiphase fractal model considering water reflux in shale gas extraction

Abstract

Hydraulic fractures and induced fractures occur in shale reservoirs during hydraulic fracturing. Hydraulic fractures and fracture-induced zones contain large amounts of water that can be returned to the surface. For the fracture induced zone with water backflow, the mechanism of water and gas flowing simultaneously is fully considered. Shale reservoirs are mainly composed of kerogen, inorganic matrix and fractures. A multidomain and multiphase fractal model is established by introducing fractal dimension which can characterize pore and fracture structure. The reservoir simulation results show that:(1) the correct degree of hydraulic fracturing can better promote the reservoir gas production. Too much or too little fracturing can result in reduced production due to water backflow or failure to access higher permeability fractures; (2) Increasing the maximum relative permeability of water or decreasing the inlet pressure of non-wetting phase can promote the development of fractures and pores, so as to obtain a good gas generation effect; (3) The variation of the hydraulic fracture aperture promotes the development of shale matrix pore structure, but the increase of the width of fracture induced domain has no significant effect on the pore structure.