Study On Fracture Initiation And Propagation Of Carbon Dioxide Fracturing

The efficient development of unconventional oil and gas reservoirs is of great significance to energy security strategy of China.Unconventional oil and gas reservoirs have poor physical properties,which usually need to be combined with large-scale fracturing to achieve commercial development.As a new environment-friendly fracturing technology,CO₂fracturing has broad application prospects in unconventional reservoir fracturing.The study of fracture initiation and propagation under the influence of CO₂ invasion is one of the basic problems of CO₂ fracturing technology.Firstly,based on the maximum tangential strain criterion,the compound stress intensity factor of CO₂ fracturing is derived.Aiming at the perforated completion wellbore,considering the physical properties of CO₂ and wellbore pressurization rate,the semi analytical initiation model of CO₂fracturing is established and solved numerically.The rationality of the model is verified by the laboratory test data of CO₂ fracturing,The parameter sensitivity analysis of crack initiation pressure and crack initiation angle was carried out.The results show that the initiation pressure of CO₂ fracturing is 23.8%-25.1%lower than that of conventional hydraulic fracturing.The effect of stress anisotropy and perforation length on fracture initiation pressure and angle is very significant.When the perforation azimuth is 0°At the same time,the crack initiation pressure is the minimum.The fracture initiation pressure increases with the increase of pressurization rate,and increases slowly,but decreases with the increase of pressure in the perforated section.Then,combined with the physical property model of CO₂,the thermal fluid solid coupling fracture propagation model of CO₂ fracturing is established.The displacement discontinuity method is used to solve the model,and compared with the two-dimensional KGD analytical model.Based on this,the sensitivity analysis of the influencing factors of fracture propagation of CO₂ fracturing is carried out.It is found that compared with traditional hydraulic fracturing,the fracture width of CO₂fracturing is smaller,but the fracture length is longer.Finally,the thermal fluid solid coupling model of CO₂fracture propagation is established,and the influencing factors of CO₂ fracture propagation are analyzed.The results show that with the increase of perforation azimuth,the fracture deflection increases and the fracture width near the wellbore decreases.With the increase of horizontal in-situ stress difference,the fracture deflection increases and the fracture width near the wellbore decreases.The change of flow rate and formation temperature has little effect on the fracture steering.The fracture width increases with the increase of CO₂ flow rate and decreases with the increase of formation temperature.The injection pressure is positively related to the minimum horizontal principal stress.In this paper,the laws of initiation and propagation of CO₂ fracturing are systematically studied.The research results improve the relevant basic theory and help to promote the development of practical technology.