Effect of capillary pressure on fluid density and phase behavior in tight rocks and shales

Phase behavior is important in the calculation of hydrocarbons-in-place and in the flow of phases through the rocks. Pore sizes can be on the order of nanometers for shale and tight rock formations. Such small pores can affect the phase behavior of in situ oil and gas owing to increased capillary pressure. Not accounting for increased capillary pressure in small pores can lead to inaccurate estimates of ultimate recovery, and saturation pressures. In this paper, capillary pressure is coupled with phase equilibrium equations and the resulting system of nonlinear fugacity equations is solved to present a comprehensive examination of the effect of small pores on saturation pressures and fluid densities. Binary mixtures of methane with heavier hydrocarbons, and a real reservoir fluid from the Bakken shale are considered. The results show that understanding the impact of small pore throats on PVT properties explains the inconsistent GOR behavior, high flowing bottomhole pressures, and low gas flow rate observed in the tight Bakken formation. The small pores decrease bubble-point pressures and either decrease or increase dew-point pressures depending on which part of the two-phase envelope is examined. For the pore radius of 10 nanometers in the Bakken shale, the calculations show that there is more than a 900 psi reduction in the bubble-point pressure as the reservoir is depleted. Further, reduction of oil density due to small pores can impact the formation oil factor and ultimate reserve calculations. The results also show that the change in saturation pressures and fluid densities are very dependent on the values of the interfacial tension used in the calculations.