Fracture Initiation and Propagation Characteristics of SC-CO2 Fracturing in Hot Dry Rock: A Coupled Thermal-Hydraulic-Mechanical-Damage Numerical Model Based on Phase Field Method

ABSTRACT: Hydraulic fracturing is a pivotal technique for harnessing geothermal energy from HDR. Conventional hydraulic fracturing encounters formidable challenges, including high fracture initiation pressure and simplistic fracture geometry. In contrast, SC-CO2 fracturing represents a novel and promising stimulation method capable of mitigating these challenges. However, limited work has been conducted on studying the fracture initiation and propagation of SC-CO2 fracturing. To have a clear understanding of how geological engineering parameters affect SC-CO2 fracturing, numerical simulations were conducted under some key parameters (horizontal stress difference, permeability, elastic modulus, etc.) based on the THM-D phase-field model. Furthermore, using breakdown pressure as the evaluation criterion, the primary control factors were determined. Results showed that the breakdown pressure in SC-CO2 fracturing is dominated by pore water pressure, thermal stress, and SC-CO2. Thermal stress and SC-CO2 could induce multi-directional fractures and reduce breakdown pressure. Decreasing the horizontal stress differences, permeability, injection temperature, and fluid injection rate is conductive to inducing more complex fracture morphology. The growth in coefficient of thermal expansion and elastic modulus promotes the number of the fractures. Horizontal stress difference and injection temperature play a more important role in fracture initiation. The study aims to uncover the potential of SC-CO2 fracturing in HDR. 1. INTRODUCTION Hot Dry Rock (HDR) is a kind of geothermal resource where no water or only a small amount of fluid exists inside the rock and the average temperature of HDR is typically above 180°C. HDR geothermal resources represent a stable, efficient, and clean renewable energy source unaffected by seasonal variations, applicable across power generation, heating, agriculture, and other sectors (Zhang et al., 2018; Zhou et al., 2020). Enhanced Geothermal Systems (EGS) play a pivotal role in harnessing HDR, employing hydraulic fracturing to create artificial fractures within the HDR reservoir, establishing high-conductivity pathways, and enabling stable and efficient heat extraction. However, conventional hydraulic fracturing techniques encounter challenges such as limited fracture propagation, reservoir hydrothermal mineralization reactions, and pipeline blockages. Consequently, Supercritical Carbon Dioxide (SC-CO2) emerges as a promising, environmentally friendly, and efficient fracturing medium (Xu et al., 2022; Li et al., 2021; Chen et al., 2021). SC-CO2 not only enhances reservoir transformation outcomes but also facilitates carbon utilization and storage during the fracturing process.