The Format Criterion Of Doctoral Dissertation Of CUP

Supercritical CO₂(SC-CO₂)fracturing is a promising non-aqueous fracturing treatment,which can increase oil and gas production,minimize formation damage and protect the environment.Meanwhile,it faces some problems including low efficiency of carrying sand,high standard of treatment and high cost.SC-CO₂ fracturing is still in the stage of laboratory test and theoretical research.The previous studies had some limitations,for example,less considering the coupled thermo-hydro-mechanical-chemical(THMC)processes and the CO₂ phase change during the fracture propagation,resulting in the ambiguity in mechanism of SC-CO₂ fracturing.To this end,this paper uses methods of indoor experiments,theoretical model and numerical simulation to investigate the mechanism of SC-CO₂ fracturing and related dominated factors.On the above basis,the main conclusions are as follows:(1)SC-CO₂ fracturing is prone to create multi-fractures.From comparison of SC-CO₂ and hydraulic fracturing,it is found that the energy release rate and cumulative energy is 1-2 orders of magnitude higher than that of hydraulic fracturing,showing that SC-CO₂ fracturing can supply enough energy to create multi-fractures.With the increasing stress difference from 2MPa to 6MPa,the average incline angle increases from26°to 60°,not normal to the minimum principal stress.(2)Thermal stress induced by SC-CO₂ promotes the fracture initiation and propagation.The breakdown pressure of SC-CO₂ fracturing decreases with the specimen temperature.Thermal stress can induce branching or crossed fractures.(3)Dynamic fracture propagation can occur due to shock load and thermal stress during CO₂ phase change.While the unstable fracture initiates,the CO₂ phase is also occurring to accelerate the fracture velocity with 2 orders higher than that of hydraulic fracture.(4)The induced stress and weakening fracture toughness are main mechanisms during coupled thermo-hydro-mechanical-chemical(THMC)processes between SC-CO₂and rock.The mechanisms for SC-CO₂ fracturing are including:thermal stress and pore pressure decrease effective stress,zero surface tension decreases the critical fracture net pressure for microfracture propagation,SC-CO₂ adsorption lowers the fracture critical stress and SC-CO₂ phase change causes dynamic fracture propagation.(5)Both the SC-CO₂ viscosity and stress difference are key parameters for stimulating the naturally fractured reservoir.The fracture toughness-leakoff dominates SC-CO₂fracture propagation due to high leak-off and low fracture net pressure,which results in short fracture length and narrow fracture width.But SC-CO₂ tends to activate the natural fractures to create numerous branches.