Effects of Shale Pore Size and Connectivity on scCO₂ Enhanced Oil Recovery: A Molecular Dynamics Simulation Investigation

Abstract

The differences in pore width distributions and connectivity of shale reservoirs have significant influences on supercritical carbon dioxide (scCO 2 )-enhanced oil recovery (CO 2 EOR) in shale. Herein, the molecular dynamics simulation was adopted to investigate the microscopic mechanism of CO 2 EOR in the shale nanopores with different pore size width distributions and pore connectivity. The results show that the pore connectivity has significant effects on the oil displacement, and the recovery efficiency is ordered as: connected pore > double pore > single pore for the 3 nm pore, which are 91.32, 74.43, and 65.93%, respectively. Therefore, the increase in pore connectivity can significantly improve the recovery efficiency of the small pore of the connected pore system. For the shale reservoirs with different pore width distributions, the oil recovery rate of large pores is generally higher than that of small pores. In addition, the displacement of oil in the small pore of the double pore system is accelerated due to the pushing effect of the discharge fluid from the large pore. The results furnish a certain theoretical support for the research of the microscopic mechanism of CO 2 EOR in the shale pore with different pore width distributions and connectivity and the exploit of shale oil.

Critical Contribution of Water in Hybrid CO2/Water Enhanced Shale Oil Recovery: Insights from Molecular Dynamics

“…The injection rates were selected as 0.1, 0.2, 0.4, 0.6, and 1.0 nm ns –1 , respectively. As to quantitatively describe the influence of the hybrid CO 2 /water displacing agent in shale reservoirs on enhancement of asphaltene recovery, the asphaltene recovery efficiency R % was calculated, which was defined and calculated by the following equation R % = N out N total where R %, N out , and N total represent the recovery rate, the number of asphaltene molecules flowing out of the slit, and the total asphaltene molecule number, respectively, as shown in Figure (a). The recovery efficiency of the asphaltene molecules in the calcite slit was calculated separately for different water content displacing agents with a fixed injection rate of 0.2 nm ns –1 , and the results are shown in Figure (b).…”

“…Molecular dynamics (MD) simulations, in conjunction with experimental techniques, provide powerful methods to study the dynamics, structure, and energetic properties of nanoscale systems, including those encountered in shale oil reservoirs. ,,, MD simulations have been effectively utilized to model fluids in diverse reservoir rocks, such as clays, quartz, carbonates, and kerogen . However, most of the existing research studies involve pure water or gas phase , replacement fluid; little attention has been paid to hybrid fluids. Meanwhile, the representative oil phase components selected in the existing studies are mostly nonpolar hydrocarbons, whereas polar components, such as asphaltenes, which are also significant components in shale oil, , have been rarely involved.…”

“…11,12,14,38 MD simulations have been effectively utilized to model fluids in diverse reservoir rocks, such as clays, 39 quartz, 30 carbonates, 9 and kerogen. 11 However, most of the existing research studies involve pure water or gas phase 12,40 replacement fluid; little attention has been paid to hybrid fluids. Meanwhile, the representative oil phase components selected in the existing studies are mostly nonpolar hydrocarbons, whereas polar components, such as asphaltenes, which are also significant components in shale oil, 30,41 have been rarely involved.…”

Study on the Effects of Wettability and Pressure in Shale Matrix Nanopore Imbibition during Shut-in Process by Molecular Dynamics Simulations

“…Because of the asynchronous advancement of the imbibition front, the spontaneous imbibition process in the nanopore can be divided into two stages based on changes in displacement efficiency ( DE ) [ 41 ]: where N O represents the number of oil molecules originally in the pore; N t represents the number of oil molecules in the pore at time t .…”

“…The results show that the mineral composition of the Lucaogou Formation shale matrix is complex and diverse, but quartz has the highest relative content and plays an important role. Quartz is often used to simulate inorganic nanopores and is generally hydrophilic [ 41 , 60 , 61 , 62 ]. Furthermore, it is mentioned in physics of petroleum reservoirs that spontaneous imbibition can only occur in hydrophilic capillaries.…”