A sharp initial decline in the production rate is being experienced in many shale gas plays. One reason is the closure of natural micron and micro fractures. The natural fractures, which widely exist in the over-pressurized source rock, react sensitively to the change of subsurface stress. The change in stress can be caused by the decrease of gas pressure during production. These fracture closure or re-open phenomena have significant effects on reservoir permeability and gas production.
In this paper, a fully coupled geomechanics and multiphase fluid flow model is presented to accurately simulate the fields of stress and fluid flow in shale gas reservoirs. Several relationships between fracture closure and applied stress are incorporated in this model, based on the experimental data from literatures. Therefore, the stress dependency of shale natural fractures is quantified and modeled in its full complexity. The natural fractures in this model are characterized as stiff, self-propped, and prone to closure. It represents an extension of our earlier “hybrid-fracture model” (DFN for hydraulic fractures, double-porosity for natural fractured domain inside the SRV, and single porosity or dual-continuum outside the SRV).