The pore sizes of shale and tight-rock formations are on the order of nanometers. The thermodynamic phase behavior of in-situ hydrocarbon mixtures in such small pores is significantly different from that of bulk fluids in the PVT cells mainly due to effect of large capillary pressure, which has been a subject of great interest for shale reservoir study. For example, it has been recognized that the critical pressure and temperature of reservoir fluids may change due to the pore spatial confinement, thus the phase envelop shifts for unconventional shale reservoirs. Those shifts of phase envelop could favor the productions of shale reservoirs, especially for tight oil systems because of suppressed bubble-point pressure leading to longer single phase production.
On the other hand, the pore sizes, especially the pore-throats, are subjected to change due to rock deformation induced by the fluid depletion. As the fluids are being produced from the pore space, the pore pressure will decrease; thus the effective stress on reservoir rock increase, resulting in the reduction of the pore and pore-throat sizes. This reduction on pore spaces affects again the fluid flow through impacts on the thermodynamic phase behaviors, as well as stress induced changes on porosity and permeability. Thus a coupled flow-geomechanics model including pore confining effects is in general necessary to examine the unconventional shale reservoir behavior.
In this paper, we present a multiphase multi-component reservoir model, fully coupling fluid flow, geomechanics and pore confining effects for shale reservoirs. This model is based on general mass conservation law for each component. The geomechanical model is derived from the elasticity theory extended to porous and non-isothermal media. The Peng-Robinson EOS-based flash calculation is used to analyze the large capillary pressure effects on shale reservoir phase behavior. This theoretical model can be readily discretized and implemented with control volume based numerical methods, and generally applied to both dew-point and bubble-point systems. Eventually it could improve the forecast accuracy for long-term production rate and recovery factors of unconventional reservoirs.