Study On Enhancing Oil Recovery In Tight Reservoirs After Water Flooding By CO₂ Flooding And Carbonized Water Flooding

Tight sandstone reservoirs are characterized by strong heterogeneity,small size of pore throat and poor mobility of fluid in pores,resulting in low water flooding recovery.To solve this problem,this paper takes tight sandstone of Yanchang 6 reservoir as the research object,firstly establishes the classification and evaluation criteria of reservoir micro-pore structure,and then carries out indoor physical simulation experiments of"water+CO₂"flooding and"water+carbonated water"flooding.Combined with nuclear magnetic resonance testing technology,the change of oil recovery rate of tight core after water flooding in the process of CO₂ flooding and carbonized water flooding and the degree of crude oil utilization in pores were evaluated.Finally,the influence degree of reservoir physical property,pressure and displacement medium on the recovery rate of"water+CO₂"flooding and"water+carbonated water"flooding in tight sandstone reservoirs was evaluated.The results show that:（1）Based on the characteristics of micro-pore structure,the Chang6 reservoir in the study area can be divided into three types:ClassⅠ,classⅡand ClassⅢ.From classⅠto classⅢ,physical properties,pore structure characteristics,pore throat radius,displacement pressure and median pressure gradually increased.（2）The results of"water+CO₂"flooding experiment show that:Compared with water flooding,CO₂ flooding the degree of crude oil production in large pores and small pores increased by 25.94%and 32.21%respectively,thus improving the core recovery after water flooding.The final average total recovery reached 65.76%,63.87%and 59.49%,respectively.（3）The experimental results of"water+carbonated water"flooding show that carbonized water increases the degree of crude oil utilization in large pores by 7.63%compared with water flooding,and the degree of crude oil utilization in small pores by 28.61%compared with water flooding,making the final total recovery of the three types of cores reach 53.43%,45.56%and 37.72%,respectively.（4）With the same displacement conditions,the reservoir with better physical properties has less flow resistance in the process of oil displacement,and can use the oil in the pores to a greater extent,and the final recovery is higher.（5）With the continuous increase of CO₂ flooding injection pressure,the improvement range of core crude oil recovery is also increasing.When CO₂ is in the near miscible state,the improvement range of oil recovery is the largest,which can increase the recovery rate of core oil by 32.48%on the basis of water flooding.Compared with water flooding,the application degree of crude oil in large pore in core is increased by 7.79%,34.64%and 35.21%,respectively,and the application degree of crude oil in small pore in core is increased by 22.77%,33.55%and 40.61%,respectively,under the three conditions.（6）At the same injection pressure,"water+CO₂"flooding can increase the core recovery by 29.92%on the basis of water flooding,and"water+carbonated water"flooding can increase the core recovery by 11.88%on the basis of water flooding.The effect of CO₂ flooding is better than that of carbonized water flooding.This is because CO₂ uses the remaining oil in the pores better than carbonized water.