Impact of fluid property shift and capillarity on the recovery mechanisms of CO₂injection in tight oil reservoirs

Abstract

The phase equilibria with the confinement effect could shift in nano‐pores, which could have a great impact on the recovery mechanisms of CO2 injection in tight oil reservoirs; this has not been systematically studied. In this paper, the confinement effect with property shift and capillarity effect is introduced into the flash calculation of confined fluids. The Soave modification of the Redlich–Kwong equation of state is extended by the molecular‐wall collision parameter to describe the shifted pressure–volume–temperature properties of confined fluid, and the Young–Laplace equation is applied to evaluate the capillary pressure. This developed model could effectively be applied for phase equilibrium calculation in tight porous media because of the verification of experimental results. A binary mixture is investigated to study the different effect of capillary pressure and property shift on phase equilibria. Subsequently, a typical hydrocarbon fluid from Middle Bakken tight oil reservoirs is studied with CO2 injection. Results illustrate that the confinement effect could play an increasingly important part in the phase equilibrium state. The CO2 solubility and mass transfer driving force in tiny pores would be greater than those in large pores under the same conditions. The gas phase saturation would be smaller with the same compositions, which could extend the single‐phase region of fluid flow in porous media. Furthermore, bubble‐point pressure, the minimum miscible pressure of CO2/hydrocarbon, and the viscosity of tight oil dissolved with CO2 both decrease with the pore size, which has a good influence on tight oil recovery. In general, the confinement effect could effectively reinforce the recovery mechanisms of CO2 injection, which is conducive to the enhancement of tight oil recovery. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Review on Phase Behavior in Tight Porous Media and Microscopic Flow Mechanism of CO2 Huff-n-Puff in Tight Oil Reservoirs

“…Pang et al [47] The bubble point decreases due to the capillary pressure. Wu et al [48] The bubble point pressure decreases with the increase of pore size.…”

“…Wu et al [48] The MMP decreases with the increase of pore size. Zhu [49] Yu et al [50] The impurity gas increases the miscibility pressure.…”

“…Besides, the dew point increases in the upper dew point interval but decreases in the lower dew point interval (see Figure 8(b)). Wu et al [48] studied CO 2 injection in middle Bakken reservoir and found that bubble point pressure and minimum miscibility pressure (MMP) decreased with the increase of pore size. Zhu [49] found that the diffusion rate of oil and gas decreased while the pore size became smaller, and the inflection point pressure of displacement efficiency of n-decane was slightly larger than that of the MMP of the system in the PVT cylinder, which proved the effect of porous media on the MMP of n-decane.…”

Investigation of CO2/N2 injection in tight oil reservoirs with confinement effect

“…To validate this phase equilibrium calculation model, the calculated K ‐value of bulk fluids and confined fluids under certain conditions would be compared with the experimental measurement data, which has shown in Table . As is shown, the biggest relative deviation is 3.73%, which means this developed model could effectively predict the vapor‐liquid phase equilibrium of CO 2 /hydrocarbon systems in tight porous media.…”

“…The third term on the right of this equation represents the fluid molecular‐wall interactions proposed by Derouane . Besides, the capillary pressure caused by tiny pore size also should be considered, which could be obtained by:…”