Research and Practice of CO₂ Energized Volume Fracturing in Shale Oil Horizontal Wells of Ordos Basin

Abstract To solve the problems of high fluid flow resistance and lo energy-supplement efficiency in low-pressure shale oil reservoirs of Ordos Basin, a new idea of CO2 energized volume fracturing for this region was proposed. In this work, firstly, physical modeling experiments of fracturing with different injection media were carried out. During the experiments, fracture propagation dynamics were visualized and monitored online by using high-energy CT scanning, to find out the fracture propagation pattern and evaluate the feasibility of forming complex fracture network. Secondly, key parameters of CO2 injection were optimized through reservoir numerical simulations, to establish the model of regional CO2 energized volume fracturing. The research shows that the fracturing with CO2 as pad could create more complex fractures in Chang 7 shale oil reservoir; the fractures propagated along the weak plane of bedding and crossed beddings vertically to form a network of fractures; the micro-fractures were mainly shear fractures, which reduced the rock fracture pressure by 17.6%. The concept of energy supplement has changed from alternate energy supplement between stages in a single well to coordinated and integrated energy supplement between wells and stages through overall well-pad injection. The optimized energy-supplement model was whole well injection, which could achieve full coverage of fracture-controlled areas. The optimized injection rate of single stage was 4-6 m3/min, the optimized injection volume of liquid CO2 per stage was 300 m3. In Qingcheng Oilfield, a regional CO2 energized volume fracturing demonstration well-pad has been built, where the pressure retention degrees of test wells have increased by 2.1 times on average, and the initial oil production of individual wells has risen from 19.6 tons/day to 23.3 tons/day, proving the technique can enhance oil production significantly. The research results set an example for the efficient development of other shale reservoirs of the same type.