Impact of pore proximity on transport of gas in nano-scale organic and inorganic pores is investigated by consideration of several effects, including the fractioning of different hydrocarbon species causing the composition of the non-adsorbed fluid phase to vary during transport through the reservoir, alteration of the hydrocarbon phase behavior and permeability by the pore proximity effects, and the influence of the adsorbed layer and non-Darcy flow effects in the non-adsorbed fluid. This is accomplished by developing a compositional model that accounts for all the phenomena necessary to quantify transport in organic and inorganic pores in shales and the permeability multiplier curves appropriate for non-adsorptive and adsorptive nanoporous media. The significance of the pore proximity and adsorptive phase correction to the critical properties and the apparent permeability on gas and condensate production over a wide range of pressure is demontrated. These additional corrections made to the phase behavior and PVT properties are shown to substantially influence the well production trends. These modifications indicate that the production of valuable components is underestimated in conventional simulation approaches and oil prouduction will last for a longer period in extremely tight formations. However, the heavier components in organic pores are more difficult to recover than dry gas methane from organic pores. Further, we show that effective gas permeability could be 10 times as much as effective oil permeability.
Although liquids-rich shale plays are economically producible, the existing conventional reservoir simulation technology fails to include many of the production phenomena encountered in liquids-rich plays. Xiong et al.(2012) pointed out the complex transport issues involved in the presence of adsorptive layer and non-Darcy flow conditions. The free and adsorptive transport model of Xiong et al.(2012) is only applicable to gas comprising of a single gas component. A multi-component transport model is necessary to correctly model the fluid system that consists of multiple gas species or phases.
Freeman et al.(2009) extended the mean free path of a single component gas to each component in a mixture of variable species. The non-Darcy flow effects and the total apparent gas permeability of the mixture are different in the presence of the interactions with other species. Further, the liquids reduce the pore space available for gas flow through the pores. The loss in pore space reduces the apparent permeability for gas, but not as much as in a conventional reservoir as the smaller pore size increases the non-Darcy term in the permeability expression. Moreover, because the oil is less mobile than gas in nano pores, the flowing fluid composition changes along the trasnsport path. In nanometer scale pores fluid phase diagrams are a function of pore size(Singh et al, 2011, Devegowda et al, 2012). Conventional PVT analysis assuming bulk fluid behavior is inadequate for pore sizes found in liquids-rich shale reservoirs, which range from a few to a hundred nanometers. This paper presents a comprehensive modeling approach to address the fluid phase behavior under confined condition and flowing compositional dynamics in consideration of adsorption, non-Darcy flow, and pore pore proximity effects.