Fracture initiation and propagation behaviours of supercritical CO2 enhanced fracturing in layered shale of horizontal wells

Abstract

Globally, shale oil/gas has become an important alternative energy for conventional oil and gas. The potential advantages of supercritical CO₂ (ScCO₂) make it an ideal substitute for hydraulic fracturing, which is used for shale reservoir stimulation. However, its poor proppant carrying capacity and viscous fingering limit its application. This article proposes an experimental method for ScCO₂ enhanced fracturing, which is divided into three stages: first, ScCO₂ fracturing is used to form microcracks around the wellbore (at which point the rock is not fractured); Afterwards, maintain the pressure constant, stop the pump and soak the well, use CO₂ to react with the rock to soak the surrounding area, reduce the fracturing pressure, and increase the energy of the formation; Finally, hydraulic fracturing is used to induce fractures and increase their width and complexity. Research has shown that: (1) Compared with ScCO₂ fracturing, the fracture pressure of ScCO₂ enhanced fracturing is reduced by 13 %. (2) Compared with hydraulic fracturing and ScCO₂ fracturing, the fracture morphology of ScCO₂ enhanced fracturing is more complex, and can produce approximate “cross fractures”, indicating that ScCO₂ enhanced fracturing may break through the constraints of geostress and generate the complex fracture network desired on site. (3) The total length of ScCO₂ fracturing fractures is about 1.34 times that of hydraulic fracturing, and the total length of ScCO₂ enhanced fracturing fractures is about 3.48 times that of hydraulic fracturing, with more branching fractures appearing. (4) The fracture width of ScCO₂ enhanced fracturing is 1.93 times that of hydraulic fracturing and 4.10 times that of ScCO₂ fracturing. In summary, ScCO₂ enhanced fracturing has significant advantages in increasing fracture complexity, expanding fracture width, and reducing fracture pressure. The research results have demonstrated the technical feasibility of ScCO₂ enhanced fracturing, which is of great significance for shale gas development.