Application Of Modeling For Digital Complex Porous Media And Application In CO₂-Enhanced-Oil-Recovery Simulation

Carbon-dioxide-enhanced oil recovery(CO₂-EOR)is such technology that injects CO₂into oil reservoirs and utilizes its physical and chemical interactions with oil to achieve the purpose of both oil recovery enhancement and CO₂ effective sequestration,and has been widely applied into petroleum exploitation industries worldwide.The heterogeneity of reservoir rocks is one of the main factors that affect CO₂-EOR performance.Inside reservoir rocks,the diffusion between oil and CO₂,and multiphase flow are coupled with heterogeneous pore structure,which together controls CO₂-oil flooding process and multiphase distribution.At present,the pore-scale numerical simulation studies on CO₂/oil flooding behavior at miscible and near-miscible conditions are very few,which results in insufficient knowledge of the influence law and action mechanism of pore structure heterogeneity on CO₂/oil displacement processes and efficiency,influencing optimization design and economic analysis of different CO₂ injection schemes for CO₂-EOR.Pore-scale numerical simulations of CO₂/oil flooding behavior in complex porous media requires not only mathematical models that can reflect the physical nature of the displacement process but also pore geometry files for simulation domain construction.This paper first proposes a digital complex rock modeling method based on Computed tomography(CT)images or the measured data of particle size distribution and porosity.This method can produce a heterogeneous digital core which has almost the same pore and particle grain characteristics with the target porous media based on the simulation of grain growing,moving and shrinking processes.On this basis,this paper also developed a visualization modeling and analysis software Pore&Flow Analysis which has such advantage as multi-target,batch processing and good graphical interface,and can provide pore structure files for LBM and COMSOL simulators for pore scale fluid flow numerical simulations.Based on the phase-field method,this paper set up the CO₂/oil displacement mathematical models to describe the multiphase flow and diffusion coupling process in porous media under the immiscible,near-miscible and miscible conditions.Combined with the self-developed digital rock modeling software,this study simulated the CO₂/oil flow behavior at pore scale under immiscible,near-miscible and miscible conditions,and investigated the effects of the different CO₂injection rate,the diffusion coefficients of CO₂-oil and the heterogeneous pore structure on CO₂/oil flow behavior and oil recovery efficiency.The simulation results indicate that:1)Compared to immiscible flooding,near-miscible flooding can increase the CO₂ sweep area to some extent,but it is still inefficient to displace oil in small pore throats.2)For miscible flooding,although CO₂ still preferentially displaces oil through big throats,it may subsequently invade small pore throats.3)In order to substantially increase oil recovery efficiency,miscible flooding is the priority choice;however,the increase of CO₂ diffusivity has little effect on oil recovery enhancement.4)For immiscible and near-miscible flooding,CO₂ injection velocity needs to be optimized.High CO₂ injection velocity can speed up the oil recovery process while maintaining equivalent oil recovery efficiency for immiscible flooding,and low CO₂ injection velocity may be beneficial to further enhancing oil recovery efficiency under near-miscible conditions.