Lattice Boltzmann Multiphase Flow Simulations for CO<sub>2</sub>-Enhanced Oil Recovery Using Various Modes of Immiscible CO<sub>2</sub> Injection
Yongqi Wang, Zhiqiang Fan, Qilin Wang, Ziqiu Xue, Dayong Wang
Abstract
We conducted lattice Boltzmann simulations to investigate the efficiency of various immiscible CO 2 injection modes in enhancing oil recovery and CO 2 sequestration during CO 2 -enhanced oil recovery (CO 2 -EOR) in a heterogeneous porous medium saturated with residual oil and water. The objective was to gain insights into the promotion mechanisms associated with these injection modes. The results obtained from the oil–water–CO 2 three-phase flow model, along with the corresponding computer codes, demonstrated excellent agreement with the results of theoretical analyses of classical examples and exhibited a good match with measured results from CO 2 flooding experiments. Further simulations indicate the following: (1) Under the pure-CO 2 injection mode, the heterogeneous pore structure facilitated the formation of preferential flow paths for CO 2, enabling the displacement of residual oil and sequestration of CO 2 . However, it was observed that increasing the injection rate beyond a certain threshold had limited impact on expanding the sweep area of CO 2, thereby contributing minimally to the displacement effect on residual oil and the overall oil recovery efficiency. (2) For water-alternating-gas (WAG) injection, which involved alternating cycles of water and gas injection, the injected water infiltrated specific pores along the pre-established CO 2 flow path, displacing the inner residual oil and effectively expanding the sweep area for subsequent CO 2 injection. The performance of WAG injection reached its upper limit after two cycles, with additional cycles offering negligible improvements in enhancing oil recovery and CO 2 sequestration efficiency. (3) Throughout the whole injection process, certain residual oil droplets remained undisplaced. These droplets were typically located in the pores with narrow entrances, which presented significant capillary resistance, hindering the entry of the outer fluids. Additionally, there were pore spaces where residual oil droplets were finally replaced by water rather than CO 2, which needs to be considered when evaluating the capacity of CO 2 sequestration.