Research On Fracturing Generation Rules Of CO₂ Fracturing

In recent years,CO₂ fracturing technology has been widely used to develop tight oil and gas in China.CO₂ fracturing has many advantages over conventional hydraulic fracturing,such as less damage to the reservoir,high flowback efficiency after fracturing,etc.The fracture propagation and morphology during CO₂ fracturing can provide a reference for efficient fracturing design of tight reservoirs.Thus,the effects of temperature,lithostatic pressure and pore pressure on the mechanical properties of rock under the conditions of different fluids and supercritical CO₂ through the high-temperature and high-pressure triaxial rock mechanics test system were compared;the impact of CO₂ on the mineral composition,porosity,and other physical properties of rocks was clarified by CO₂ -immersed core test;the fracture propagation law during SC-CO₂ fracturing was studied by designing two kinds of physical simulation experiments;the influence of construction parameters and geological parameters on the fracture propagation during CO₂ fracturing was analyzed by software;and the characteristics and control mechanisms of CO₂ fracturing fracture expansion were revealed by well test interpretation after field fracturing in this study.The results show that the compressive strength and modulus of sandstone decrease and Poisson’s ratio increases in CO₂ environment;with increasing temperature,lithostatic pressure and pore pressure,both compressive strength and Young’s modulus tend to rise,but brittleness weakens.The dissolution of CO₂ increases the porosity and permeability of sandstone.CO₂ especially dissolves carbonate mineral components in sandstone.In the rocks with poor permeability,CO₂ fracturing results in larger fracture sizes and provides good communication between microfractures.The larger the flowback volume is,the longer and higher the fractures are.At the average sand ratio below 20%,the fracture half-length is negatively correlated with the average sand ratio and positively correlated with the average fracture width.The larger the amount of sand is,the wider the fractures are;the greater the formation permeability is,the greater the fracturing fluid loss is,leading to sand blockage and thus restricted propagation of fractures.The increase in formation temperature and pore pressure promotes the propagation of fractures.In addition,the well test interpretation after field fracturing shows that the fractures during CO₂ fracturing have strong flow conductivity and generally are less than 100 meters.