Energized Fracturing with CO2: A Numerical Simulation of the Effects of Thermodynamic Properties of CO2

ABSTRACT Energized fracturing with CO2 is considered as an alternative method to conventional water-based hydraulic fracturing, which brings advantages in water conservation and environment protection as well as in hot enhanced geothermal systems (EGS). However, the varying thermodynamic and transport properties of CO2 could be problematic in energized fracturing. To investigate the effects of CO2 pressure-temperature-dependant properties on fracture propagation, an extensive numerical simulation is performed using a robust fracturing simulator developed on Complex Systems Modelling Platform (CSMP-HF). Based on the Span-Wagner equation of state, the thermodynamic and transport properties of CO2 in varied in-situ conditions (i.e., pressure and temperature) during energized fracturing process are calculated. The simulation results show that in-situ stress conditions affect the fracture growth speed under constant CO2 mass rate. The additional storage volume brought by injection system contribute significantly to the hydraulic fracturing speed. Higher propagation speed may result in dynamic fracture propagation and thus resulting in branching, a subject for further research. INTRODUCTION Hydraulic fracturing technology plays a key role in promoting efficient energy extraction. Not only hydraulic fractures can enhance the oil and gas production from unconventional reservoirs, but also de-risk coal mining by inducing and controlling the time of goaf events (Adachi et al., 2007; Hou et al., 2021; Jeffrey & Mills, 2000; Middleton et al., 2015). According to the background review commissioned by the Department of the Environment (Commonwealth of Australia, 2014), the estimate in 2012 of Australian economic demonstrated resources of CSG was 35,905 Petajoules, which is equivalent to nearly 10 times the total yearly energy use. Besides, for the future mining cave extensions to massive and deep orebodies in Australia, such as Northparkes mine (Webster et al., 2020), Ridgeway Deeps (Cuello & Newcombe, 2018), and Cadia East (Orrego et al., 2020), hydraulic fracturing is proposed as a profitable and sustainable preconditioning technique to manage and enhance the caving process (Catalan, 2015).