Confined Fluid Phase Behavior and Its Influence on Unconventional Hydrocarbon Recovery

Abstract

Unconventional reservoirs are predominantly consisted of meso to nanoscale pores, which impose strong confinement effect to the encapsulated reservoir fluids and result in drastic deviations of confined fluid properties. Due to the lack of overall understanding of the nanoscale confinement, the phase behavior of confined fluids has not been well characterized. Furthermore, the influence of nanoscale confinement on the production and the ultimate recovery of unconventional reservoirs is not well predicted. The focus of this dissertation is twofold: firstly, to propose solid theoretical models to characterize the confined fluid phase behavior within nanopores; secondly, to investigate the influence of nanoscale confinement on the primary and enhanced oil recovery (EOR) production of unconventional reservoirs. Regarding the first objective, a modified Peng-Robinson equation of state (PR EOS) is proposed with incorporation of both molecule-wall interaction and geometric constraints to determine the critical property shift and the deviated phase transition boundaries of confined fluids. The capillary condensation pressure of both single- and multicomponent fluids confined within nanopores are computed by a modified Kelvin equation. For the second objective, an improved algorithm with application of the modified PR EOS is established to compute the minimum miscibility pressure (MMP) of unconventional reservoir fluids with different injected gases. The deviated properties of confined fluids are incorporated into the compositional simulation model to predict their effect on the unconventional hydrocarbon recovery. Both the theoretical models and improved algorithm are validated with either experimental or molecular simulation results. The modified PR EOS model is validated to be able to predict the confined fluid phase behavior at various pore sizes. Confinement effect imposes an overall shrinkage to both the P-T diagram and the two-phase region in a ternary diagram of CO2/hydrocarbon systems, benefiting the miscible gas EOR in unconventional reservoirs by increasing the possibility of achieving the first contact miscibility. The modified Kelvin equation is applicable to compute the suppressed capillary condensation pressure of single- and multicomponent fluids with overall relative deviations of 7.65% and 6.52%, respectively. The molecule-wall interaction potential has the most significant contribution to the improved accuracy. Moreover, comparison to the experimental results demonstrate that the improved multiple mixing cell (MMC) algorithm is a reliable method to determine the MMP of unconventional reservoir fluids with different injected gases. Nanoscale confinement results in the drastic suppression of MMP and the suppression rate increases with decreasing pore size. For 100% CO2 injection, the MMP suppression rate of Bakken oil and Eagle Ford oil at 10 nm are 6.22% and 13.01%, respectively. Compositional simulations demonstrate that the nanoscale confinement has obvious influence on the primary production and the ultimate recovery of gas huff-n-puff in unconventional reservoirs. The oil recovery factor of Eagle Ford well is increased by 12.20% at the end of the 13 years production with CH4 huff-n-puff. The performance of gas huff-n-puff EOR in unconventional reservoir is highly dependent on the composition of reservoir fluids and properties of reservoir formations. The results of this dissertation will deepen our understanding of the confined fluid phase behavior and provide reliable instructions for the unconventional hydrocarbon recovery. In addition, it will shed light on the characterization of confined fluid systems which would potentially be applied in some other nanoscale disciplines.