Liquid CO2 fracturing: Effect of fluid permeation on the breakdown pressure and cracking behavior

Abstract

Liquid CO2 fracturing is a promising alternative to hydraulic fracturing since it can circumvent problems stemming from the use of water. One of the most significant differences between liquid CO2 and hydraulic fracturing processes is that liquid CO2 permeates into matrix pores very rapidly due to its low viscosity. Here we study how this rapid permeation of liquid CO2 impacts a range of features during the course of the fracturing process, with a focus on the breakdown pressure and cracking behavior. We first conduct a series of laboratory fracturing experiments that inject liquid CO2, water, and oil into nominally identical mortar specimens with various pressurization rates. We quantitatively measure the volumes of fluids permeated into the specimens and investigate how these permeated volumes are related to breakdown and fracture initiation pressures and pressurization efficiency. The morphology of the fractures generated by different types of fluids is also examined using 3D X-ray computed tomographic imaging. Subsequently, the cracking processes due to injection of liquid CO2 and water are further investigated by numerical simulations employing a phase-field approach to fracture in porous media. Simulation results show that rapid permeation of liquid CO2 gives rise to a substantial pore pressure buildup and distributed microcracks prior to the major fracture propagation stage. The experimental and numerical results commonly indicate that significant fluid permeation during liquid CO2 fracturing is a primary reason for its lower breakdown pressure and more distributed fractures compared with hydraulic fracturing.