Pore Scale Simulation Of Surfactant-polymer Flooding Using Lattice Boltzmann Method

Most of the eastern oil fields in China have entered the stage of high water cut development.Polymer-surfactant flooding is an important technology for further enhanced oil recovery in high water cut reservoirs.However,the flow mechanism of polymer-surfactant flooding process is complicated,and there is no microscopic simulation method that considers the above characteristics comprehensively,leading to the unclear understanding of the microscopic seepage mechanism,which seriously restricts the application and development of the polymersurfactant flooding in oil fields.Therefore,the current study has important practical significance.At the same time,multi-component physicochemical percolation is ubiquitous in many fields such as nature,industry and biomedicine,and the research is of universal significance.Firstly,based on the characteristics of multi-component multiphase of polymer-surfactant flooding,the Shan-Chen multiphase model of lattice Boltzmann method(LBM)is established.Based on this,the LBM multi-component multi-phase expansion method is studied.It provides a direction for large density ratio and large viscosity LBM simulation,and provides a basis for the expansion of surfactants and polymer components in the model.Secondly,LBM method for interfacial flows with surfactants is established.We introduce a diple to characterize the amphiphilic structure of the surfactant,which successfully reflects the reduction of interfacial tension mechanism and emulsification of the surfactant.Then we establish a surfactant adsorption boundary on the wall,and propose a new relationship between surfactant concentration and wetting condition.By this way,the adsorption characteristics and wettability alteration mechanism is considered in the microscopic simulation method for the first time.The surfactant effects on droplet deformation,breakup,collision and coalescence in shear flow is further studied.Then,based on the non-Newtonian characteristics and adsorption effects of polymer system,the power law fluid LBM model and Maxwell viscoelastic LBM model are established,and the related comparison verification and application research are carried out.Based on MS-P method and extended RSG method,we deduce a capillary entry pressure analytical formula for the pore with irregular cross-sections,and clarifies the influence of the adsorption on the capillary force,and further expands the model into the rough surface capillary.Finally,based on the above characteristics,a polymer-srufactant lattice Boltzmann method is established,and the mechanism of the viscous fingering and the re-starting mechanism of the remaining oil are studied.The main results of the thesis are as follows,the results of LBM multi-component multiphase expansion method show that the construction of multi-component multiphase flow model with large viscosity ratio is mainly related to the attractive force inside components and repulsive force between different components.Simulation results of surfactant-containing multiphase flow show that surfactant can increase the deformation of oil droplets,promote the breakup of oil droplets,inhibit the coalescence during oil droplet collision,and decrease the critical pressure gradient during the emulsion droplet passing through the throat.The simulation results based on the power law fluid LBM model show that the viscosity of the pseudoplastic fluid in the porous medium exhibits a high viscosity at the center of the pore and a low viscosity near the solid wall boundary,but the expansive fluid exhibits an opposite distribution.Meanwhile apparent permeability of the power law fluids shows pressure gradient dependence.The simulation results based on the viscoelastic LBM model show that the elasticity makes the fluid has a tendency to sweep deeper and improve the oil displacement efficiency.The capillary entry pressure formula of the irregular capillary shows that the threshold radius of the capillary entry pressure is the weighted harmonic mean of the maximum inner circle radius and the area equivalent radius.The analysis result based on the rough capillary entry pressure formula indicates the entry pressure of rough capillary and the wetting phase saturation are both greater than the smooth capillary under the same conditions.The research on the mechanism of viscous fingering shows that the larger the capillary number,the higher the viscosity ratio,and the poorer of the wetting conditions lead to more obvious fingering phenomenon.The shear thinning property of the polymer can promote the viscous finger and the elastic modulus increase can suppress the viscous fingering.The gravity mainly affects the distribution of the longitudinal displacement interface,and the larger the ratio of the Bond number to the capillary number,the smaller the water saturation at the breakthrough time.The starting mechanism of remaining oil in polymer-surfactant flooding mainly include expanding sweep volume,low interfacial tension mechanism,wettability altertion mechanism,viscoelastic mechanism,emulsification mechanism and “self-assembly mechanism”.The starting pressure gradient of the remaining oil on the wall drops by more than 90%,influenced by the wetting reversal and low interfacial tension mechanism.The residual oil patterns of the polymer-surfactant flooding are more dispersed,which mainly includes the oil film on the rock surface,the continuous oil zone in the pores,the remaining oil at the blind end,the Jiamin effect trapping oil droplets,snapped-off columnar oil in the throat and hidden oil on the down-gradient side of walls.