Rock Damage Mechanism Under Supercritical Carbon Dioxide Shock Loading

With the advancement of the exploration and development of global oil and natural gas resources,unconventional oil and gas such as shale gas,coalbed methane,and shale oil show great resource potential and economic value.However,unconventional oil and gas often require fracturing to create a fracture network before commercial development.In recent years,my country has achieved certain results by learning from the"long horizontal well+staged fracturing"development model adopted in North America,but it has also brought a series of problems,such as large water consumption,reservoir damage,etc.It is urgent to develop anhydrous,environmentally friendly,low-cost,safe and transformative reservoir modification technology.Supercritical CO₂ gathering fracturing increases the pressure of CO₂ in the wellbore to above the fracture pressure of the reservoir through physical energy gathering,and then releases the high-pressure CO₂instantaneously to impact the reservoir,resulting in complex fractures that are not controlled by in-situ stress and have self-supporting characteristics network.This fracturing method is expected to become a new model for the efficient development of unconventional oil and gas resources.The energy release characteristics of supercritical CO₂,the response mechanism of rock under dynamic load,and the rock damage characteristics under supercritical CO₂impact load are the three core issues in the process of supercritical CO₂ energy fracturing.Series of studies:Based on the equation of state,the variation characteristics of CO₂ pressure and energy in the process of compression and expansion are studied.The results show that supercritical CO₂ has strong pressure retention and large energy release,which can meet the needs of large-scale fractures.The supercritical CO₂energy-forming fracturing hole pressure model has studied the effect of temperature and pressure on the hole pressure increase rate.The results show that with the increase of the initial temperature and pressure,the hole pressure increase rate increases,about1000 MPa/s;The dynamic strength change characteristics of rock under loading rate were tested.The relationship between the dynamic tensile and shear strength of different rocks with the loading rate was studied,and the dynamic properties of rock during supercritical CO₂ charged fracturing under different pressure and temperature were obtained.Strength;based on the finite element method,the load state of the rock during the propagation of the stress wave was studied,and the damage characteristics of the rock under different concentrated energy pressures were analyzed.The experimental system of supercritical CO₂ concentrated energy fracturing was developed,and the effects of in-situ stress and rock heterogeneity on the propagation characteristics of stress waves were experimentally studied;Quantitative characterization of the fracture surface was carried out,and the results showed that with the increase of the accumulated energy pressure,the pore size in the rock increased,the average width of the micro-fractures increased,the roughness of the fracture surface increased,and the self-supporting ability of the fractures increased.Through the research of the thesis,the damage mechanism of rock under supercritical CO₂ impact load is revealed,which provides theoretical support for the formation of supercritical CO₂ energy fracturing technology.