Study On Optimization Of CO₂ Flooding Prevention System For Gas Channeling In Ultra-low Permeability Reservoirs

At present,water injection development of ultra-low permeability reservoirs is difficult,and the use of CO₂ flooding at home and abroad has significant effects.However,due to the heterogeneity of the formation,CO₂ gas channeling is more serious.Therefore,it is necessary to carry out research on anti-gas channeling measures to slow down the gas channeling.Common measures to prevent gas channeling mainly include water-gas alternate technology,polymer gel plugging technology,foam plugging technology,organic amine precipitation plugging technology,foam gel composite plugging technology,etc.Among them,the water-gas alternate technology,polymer gel plugging technology,and foam plugging technology have all been used on site and achieved good results.Therefore,in this paper,the three plugging technologies of polymer gel,alternating water and air,and foam plugging are optimized and evaluated,and a suitable plugging system is screened out,and parameters are optimized.The common polymer gel is based on polyacrylamide as the main agent,and a cross-linking agent is added to react with it to form a gel with a certain strength.Common inorganic metal crosslinking agents and organic phenolic crosslinking agents are biologically toxic,and the temperature required for gel formation is relatively high,which cannot meet the current needs of environmentally friendly production.At present,modified starch and modified cellulose grafted acrylamide gel have good gel forming effect and less environmental pollution,so they are a good choice as a plugging agent.The oil reservoir in this paper belongs to the"three low"oil reservoirs,with high salinity(average 41670mg/L)and formation temperature of 47.2°C.Selecting modified cellulose(modified starch is easy to mold)grafted with copolyacrylamide to form a gel to block the gas channeling channel can solve the above problems,and at the same time avoid the problem of environmental pollution.This article selects modified cellulose graft copolymer gel plugging agent to investigate the optimization of each component of the gel system for gel formation,screen out a suitable formulation system,and evaluate its adaptability to the reservoir environment and blocking performance.The experimental results show that the optimal gel system formula is:0.3%ultrafine cellulose+4%acrylamide+0.1%crosslinking agent+0.05%initiator,the strength reaches 255.87Pa,the gel forming time is 14 hours,and the gel forming performance is good,Good adaptability to the environment.The plugging experiment shows that the gel cannot enter the homogeneous core,and it has a good plugging effect on the fractured core and has selective plugging performance,but the injection pressure is high(the core with a fracture width of 0.3mm,0.2PV gel injection,injection pressure Reach9.4MP).After plugging,the recovery rate is increased by more than 13%.According to the foam evaluation experiment,a suitable foaming agent and concentration were selected.The experiment showed that 0.3%anion-nonionic foaming agent(fatty alcohol polyoxyethylene ether sodium sulfate)has good foaming performance(foaming volume is 210m L,The half-life of the foam is 32.13min,and the strength reaches a medium-strength foam),and the formation environment has good tolerance.The core injection pressure difference is small,and the resistance factor is greater than 10,which can better seal the high-permeability pores and effectively expand the scope of coverage.The permeability range is 10,and the recovery rate is increased by12.26%after injection into the foam system;the permeability range is 25,and the recovery rate is increased by 9.41%after injection into the foam system.The results of the water-gas alternation experiment show that the effect of cone injection(gas-water ratio 1:5-1:1)is better than equal-proportion injection(gas-water ratio 1:1),and the comprehensive recovery degree is 1:1 compared with gas-water ratio.Increased by 9.09%.The permeability difference increases,and the channeling and oil displacement effect becomes worse.When the permeability range is 25,the comprehensive recovery stratification is 32.77%,and the recovery level is 19.97%lower than the level difference of 10.By comparing the plugging effects of three different plugging systems,it is found that the gel plugging effect is the best,but there is a problem of higher injection pressure;although water and gas alternation can improve gas channeling,it is greatly affected by the difference in permeability.The effect is limited;the foam system can effectively improve gas channeling,increase the degree of recovery,and has good injection performance.Therefore,this paper selects a foam plugging system to improve the gas channeling problem,and optimize the injection parameters to improve the plugging effect.Parameter optimization experiments show that when the 0.2PV foam system is injected and the gas-oil ratio is 500m³/m³,the recovery degree reaches73.18%,and the effect is the best.