A pore-scale mechanistic investigation of shale gas condensate at near saturation pressure on fluid flow in shale

Shale gas condensate is known to form more readily in smaller than larger pores at the same reservoir conditions and can reduce the mobility of the gaseous phase significantly not only in individual pores but also in a pore system, to limit gas production. To investigate the interplay of fluid confinement factors on effective gas flow behaviors in shale, in this work we developed a new phase equilibrium calculation algorithm for evaluating phase properties in pores of variable sizes and a shale-gas pore-network model. We coupled them into a workflow to study the effect of shale gas condensate on the gas permeability on selected pore networks, considering an empirical criterion of condensate pore bridging and gas flow and transport mechanisms. The workflow makes use of Soave-Redlich-Kwong (SRK) EOS to model the fluid phase behavior, Zuo and Stenby's parachor based method to predict IFT and Pedersen's corresponding state model to predict viscosity by an iterative procedure underpinned by a modified negative flash algorithm. For any given pore network, this procedure is applied to calculate a full set of PVT properties for any confined reservoir fluid and to evaluate pore-bridging criterion for each and every pore element in the network before performing pore-network gas flow simulation. The pore network model is implemented to simulate the real gas flow in nanoscale porous media, taking into account the contributions from non-Darcy flow, adsorption, surface diffusion, and the formation of condensate bridges. Using an Eagle Ford gas condensate sample, this study shows that the decreasing pore size has led to an increase in condensate dropouts in a nanoscale single cylindrical pore. The condensate liquid has moderately higher PVT properties compared to the gas phase. The differences of those properties between condensate and gas phases become smaller with the decrement of pore size. This trend appears to be opposite to those of a non-confined fluid in which condensate drops out due to pressure depletion. It has implied that the fluid confinement effect causes less flow resistance than pressure deletion even if the amount of dropouts are the same. The roles of both the pore space confinement and topology were examined on representative models. The results from the simulations on uniform pore networks show gas adsorption and surface diffusion have opposite effects and result in a minor net negative impact to the apparent permeability even when pore radius is less than 50 nm. The simulation results on a regular pore network with randomly distributed pore size show there is only a limit impact on apparent permeability as a result of condensate bridging in small pores, but with an increase in tortuosity, this impact increases.