Advantages of supercritical CO2 compound fracturing in shale on fracture geometry, complexity and width

Supercritical carbon dioxide (SC-CO2) fracturing has a great prospect for shale gas development, but the poor proppant carrying capacity hinders its effective application. In this paper, SC-CO2 compound fracturing method was proposed, which uses SC-CO2 as the pad fluid to generate complex fracture network and guar gum fluid as the carrier fluid to support and extend the fracture. Relevant tri-axial SC-CO2 compound fracturing experiments with shale specimens were designed to compare with SC-CO2 fracturing and guar gum fracturing. It was found that the breakdown pressure of SC-CO2 fracturing was on average 20.24% lower than that of guar gum fracturing. Both SC-CO2 fracturing and SC-CO2 compound fracturing could generate more complex fracture geometry than guar gum fracturing, especially in specimens with vertical bedding planes where SC-CO2 compound fracturing formed a complex three-dimensional fracture network. A dimensionless index fc was proposed to quantitatively evaluate the complexity of induced fractures. Results show that the fracture complexity of SC-CO2 compound fracturing was on average 20.64% larger than SC-CO2 fracturing, indicating that SC-CO2 induced fractures can be extended by guar gum fracturing. Digital microscope found that SC-CO2 induced fracture width was less than 0.01 mm while fracture width induced by SC-CO2 compound fracturing was 0.04 mm-0.12 mm, manifesting that the guar gum fracturing stage can increase the width by approximately an order of magnitude. The results prove the feasibility of compound fracturing technology, which is of great significance to shale gas development.