Oil Recovery Dynamics of Natural Gas Huff ‘n’ Puff in Unconventional Oil Reservoirs Considering the Effects of Nanopore Confinement and Its Proportion: A Mechanistic Study

Summary When reservoir fluids are confined by nanoscale pores, pronounced changes in fluid properties and phase behavior will occur. This is particularly significant for the natural gas huff ‘n’ puff (HNP) process as a means of enhanced oil recovery (EOR) technology in unconventional reservoirs. There have been considerable scientific contributions toward exploring the EOR mechanisms, yet almost none considered the effects of nanopore confinement and its proportion on the oil recovery dynamics. To bridge this gap, we developed an approach to calculate fluid phase equilibrium in nanopores by modifying the Rachford-Rice equation and Peng-Robinson equation of state (PR-EOS), completed by considering the shifts of fluid critical properties and oil/gas capillary pressure. Afterward, the effect of nanopore radius (rp) on the phase behavior between the injected natural gas and oil was thoroughly investigated. Compositional simulation was performed using a rigorously calibrated model based on typical properties of a tight reservoir to investigate the production response of natural gas HNP, including the effects of nanopore confinement and its proportion. We demonstrated that the critical pressure and temperature of fluid components decreased with the reduction in rp, especially for heavy constitunts. The saturation pressure, density, and viscosity of the oil in the presence of natural gas all declined linearly with 1/rp in the confined space. The suppression of fluid saturation pressure was indicative of an extended single-phase oil flow period during production. The cumulative oil production was approximately 12% higher if the confinement effect was considered in simulation. Moreover, the average reservoir pressure declined rapidly resulting from this effect, mainly caused by the intensified in-situ gas/oil interaction in nanopores. The results of this paper supplement earlier findings and may advance our understanding of nanopore confinement during natural gas HNP, which are useful for field-scale application of this technique.