Study On The Influence Law Of CO₂ Water Alternating Gas On The Recovery Efficiency Of Low Permeability Reservoir

Injection of CO₂ water alternating gas technology is a relatively mature oil recovery technology,which is often used in the late stage of conventional development technology,and has been widely used in production practice.However,due to unreasonable parameters such as injection rate,slug size and different gas-water ratios,CO₂ breakthrough in production Wells was premature or the expected development effect could not be achieved.Based on this,this paper intends to carry out experiments on high temperature and high-pressure physical properties of CO₂ and formation oil,as well as experiments on water-gas alternate displacement,to clarify the influence rule of different parameters on development results in the process of CO₂ water alternating gas.On this basis,the optimized parameters of CO₂,water and air alternate injection were determined by numerical simulation technique combined with response surface algorithm（RSM）and particle swarm optimization（PSO）.Firstly,PVT experiments of formation oil and CO₂ in the study block were carried out,including a flash experiment of formation oil,constant composition expansion experiment and CO₂ injection expansion experiment.It was found that the viscosity of the formation oil in the target block was 11.00 m Pa·s,the density was 0.717 g/cm³,the dissolved gasoline ratio was243.17 cm³/cm³,and the saturation pressure was 20.9 MPa.Therefore,it is judged that this reservoir is a black oil reservoir,and CO₂ injection has a good expansion effect on the formation of oil,so CO₂ is suitable for oil displacement and energy replenishment development.Experiments of water flooding,continuous CO₂ flooding and CO₂ water alternating gas flooding were carried out for low permeability cores in the target block.Through the comparison of the three groups of experiments,it is found that injection of CO₂ water alternating gas has the best effect on the enhanced oil recovery of low permeability reservoir,which can increase by 10.71%after the water drive oil recovery of 32.52%.Through the oil displacement experiments with different injection rates,gas-water ratio and slug size,it was found that the optimal combination of schemes with the best water-gas displacement effect was a CO₂ injection rate of 0.036 ml/min,the gas-water ratio of 1,CO₂ slug size of 0.10 PV,and final recovery rate of 45.34%.To verify the reliability of the experimental results,the core-scale numerical simulation of CO₂ water alternating gas in a low permeability reservoir was carried out,and the influence of reservoir water saturation,CO₂ injection rate,gas-water ratio,CO₂ slug size and other parameters on the oil flooding effect was studied.It is found that the gas breakthrough time at the produced end of the core is later when CO₂ water alternating gas is in oil displacement,and the CO₂ displacement profile is better controlled.By comparing the simulation results of different numerical simulation schemes,it is found that when the core water saturation is12.25%,the injection rate is 0.036 ml/min,the gas-water ratio is 2.6,and the slug size is 0.133PV,the oil displacement effect is the best,and the recovery efficiency is 46.49%.Some optimization parameters are different from the laboratory experiment.The main reason is that the scale of the numerical simulation model is larger than that of the experimental core.The central composite design method（CCD）was used to design the optimal scheme of CO₂,water alternating gas parameters,and the response surface methodology（RSM）was used to study the effects of different combinations of five injection parameters（water saturation,injection rate,gas-water ratio,slug size and injection sequence）on reservoir recovery.The optimal parameter combination was found by taking recovery efficiency as the objective function:The water saturation was 12.3%,the gas-water ratio was 2.563,the injection rate was0.022 ml/min,the CO₂ slug size was 0.133 PV,the water slug was injected first,and the reservoir recovery was 45.337%,which was similar to the numerical simulation optimization of 46.49%.Then particle swarm optimization（PSO）is used to predict the objective function equation of the response surface methodology.The optimal recovery rate of the core is 46.868%,which is 1.531%higher than that of the response surface methodology and 0.378%higher than that of the numerical simulation.The optimal CO₂ water alternating gas displacement parameter combination obtained by particle swarm optimization:The water saturation was8.9%,the gas-water ratio was 2.68,the injection rate was 0.022 ml/min,the size of the CO₂slug was 0.1325 PV,and the water slug was injected first.Therefore,the optimal CO₂ water alternating gas displacement parameter combination was the particle swarm optimization scheme,which was suitable for the CO₂ water alternating gas development in the actual reservoir.