Research On CO₂ Action Mechanism And Parameter Optimization In Pre Storage Fracturing Of Mahu Conglomerate Reservoir

Mahu conglomerate reservoir in Junggar basin has low effective permeability,which belongs to ultra-low permeability tight reservoir.The reservoir has the characteristics of deep burial,undeveloped natural fractures,strong heterogeneity,abnormal high pressure and high closure stress.The initial production after fracturing is high but decreasing rapidly.Field tests show that CO₂ injection before hydraulic fracturing can effectively improve oil recovery.However,the interaction between CO₂ and Mahu reservoir has not been systematically studied.Mechanism analysis and parameter optimization of Mahu conglomerate reservoir and CO₂ are carried out by experimental test and numerical simulation in this thesis.CO₂and reservoir rock experiments are carried out,including mineral composition test,CO₂dissolve mineral test,CO₂ influence core porosity and permeability test;CO₂ and crude oil experiments are carried out,including crude oil basic properties test,CO₂ and crude oil miscibility test,CO₂ replace crude oil test.Based on experimental data,the numerical simulation models of CO₂ pre storage fracturing injection,well shutting,flowback and production are established to conduct extensive numerical simulation research.Numerical simulation is based on production capacity to optimize construction parameters.It is found that Mahu reservoir contains more than 15%carbonate rock,and the carbonate rock can be dissolved by soaking in CO₂ aqueous solution.After soaking for 7days,the core permeability generally becomes more than twice that of the original.The minimum miscibility pressure between CO₂ and Mahu crude oil is 26.73 MPa.CO₂ can be miscible with crude oil in situ under formation conditions.CO₂ aqueous solution can replace crude oil in the core by physical and chemical action,and the replacement rate of crude oil reaches 40.95%in 7 days.The influencing factors and laws of CO₂ pre storage fracturing productivity are analyzed through numerical simulation.The stimulation mechanism of CO₂ pre storage includes increasing energy,miscibility,increasing porosity and permeability,replacement and viscosity reduction.Numerical simulation recommends single fracture injection of 40～60 tons liquid CO₂,well shutting 15～20 days,fracture half-length 150 m,fracture conductivity 20 um²·cm,continuous injection.If existing fracture is refractured,the development method of CO₂+fracturing fluid is recommended;If new fracture is refractured,the development method of fracturing fluid+CO₂+fracturing fluid is recommended.This study provides theoretical basis for CO₂pre storage fracturing in Mahu reservoir,and provides technical support for field scheme implementation.