The phase behavior of CO2 injection in shale reservoirs with nano-pores

Abstract

The main purpose of this paper is to study the solubility of CO₂ in oil and water phase under high temperature and pressure. Firstly, CO₂-crude oil PVT experiments were carried out to determine the physical parameters of the reservoir fluid in the study field in order to clarify the interaction mechanism of CO₂ with the crude oil. Secondly, the solubility of CO₂ in the reservoir fluid under different pores and the minimum mixed-phase pressure of the CO₂–crude oil system were calculated by the improved Peng–Robinson equation of state. In this paper, the effects of nano-pores limitation on CO₂ solubility were studied. The results show that pressure increase is favorable to CO₂ dissolution, the solubility increases with the increase of the oil–water ratio. CO₂ solubility decreases with temperature increase. The greater the mineralization of formation water, the lower the CO₂ solubility. Nanopore confinement causes the phase envelope to contract and the minimum mixed-phase pressure to decrease. When the pore radius is smaller, the restriction of the phase envelope is stronger. In this paper, the minimum mixing pressure of crude oil and carbon dioxide is reduced from 31.25 MPa at 50 nm to 21.25 MPa at 5 nm, thus it is beneficial for enhanced oil recovery (CO₂-EOR). Nanopore confinement favors CO₂ to enhance shale oil recovery. The results of this study are critical to evaluate the effect of CO₂ sequestration, solubility and phase behavior changes of CO₂ in shale reservoirs with nano-pores.