Study of High-Temperature HDR Fracture Features Under True-Triaxial Stress with Supercritical CO2 Fracturing
Authors: Fan Xuhao , Wang Haizhu , Liu Mingsheng , Wang Bin , Zhang Yunpeng , Mao Zelong , Zhang Yaochen , Stanchits Sergey , Cheremisin Alexey 
Published: August 2024

ABSTRACT:
The paper presents fracturing experiments on Hot Dry Rock using water and SC-CO2 (supercritical CO2) to obtain the characteristics of fracture morphologies under actual temperature and pressure conditions. These experiments utilized an in-house developed, real-time, high-temperature, high-pressure, true triaxial experimental system. The study investigates fracture propagation characteristics under various formation stresses and initial rock temperatures while quantitatively analyzing post-fracturing parameters such as fractal dimension, using tools like surface profilometers. The results demonstrate that the fracturing pressure with SC-CO2 is significantly lower than that of conventional hydraulic fracturing, with the fracture pressure of the rock reduced from 21.8 MPa to approximately 14.9 MPa, a decrease of 31.7%, and SC-CO2 fracturing results in more complex fracture morphologies and larger fractal dimensions of the fracture surfaces. Under the same conditions, compared to hydraulic fracturing, the increase in temperature results in a greater reduction in rock fracture pressure and a larger increase in fractal dimension with SC-CO2 fracturing. Additionally, high stress differentials impact fracture orientation and complexity. This research elucidates the fracture characteristics of Hot Dry Rock induced by SC-CO2, laying a foundation for understanding the complex fracture formation mechanisms in Hot Dry Rock SC-CO2 fracturing.

1. INTRODUCTION
With the continuous rapid development of the national economy, human demand for energy is increasing day by day. Conventional fossil fuels such as oil and coal are non-renewable resources, not only facing the problems of diminishing reserves and resource depletion, but also severe environmental pollution issues (Yijin, 2015). Therefore, the development and utilization of new alternative energy sources is urgent, and the vigorous development of clean renewable energy has become an important measure for countries around the world to cope with energy shortages and environmental pollution issues (Panwar et al., 2011; Pachauri et al., 2014). Geothermal energy, as an important clean and renewable energy source, has the characteristics of low carbon, environmental protection, stability, and efficiency. Compared with wind and solar energy, it is not affected by external factors such as seasons, climate, and day and night, making it a realistic and competitive new energy source (Chao et al., 2020). Hot Dry Rock (HDR) is a thermal rock body that contains no water or only a small amount of water (which cannot flow) and is an important component of geothermal energy resources (Tianfu et al., 2016). China is rich in HDR resources. According to data from the China Geological Survey, the resource amount of HDR buried at depths of 3 to 10 km is equivalent to 856 trillion tons of standard coal (Guiling et al., 2020). Therefore, accelerating the efficient development and utilization of HDR resources has significant political and economic significance in terms of meeting China's energy demand, optimizing the energy structure, and achieving carbon peak and carbon neutrality goals (Yaofeng, 2015; Zhu et al., 2015; Zhonghe et al., 2020; Breede and Dzebisashvili et al., 2013). Enhanced Geothermal Systems (EGS) are currently the main means of developing HDR (Anderson et al., 2019; Geothermal energy terms, 2018), which involves creating artificial reservoirs in high-temperature formations through methods such as hydraulic fracturing to form a network of fractures connecting injection wells and production wells, followed by circulating a working fluid for heat extraction, power generation, and comprehensive utilization. HDR reservoirs have characteristics such as hardness, density, and high geostress, requiring fracturing to enhance permeability and form effective heat exchange channels. However, traditional water-based fracturing methods and heat extraction technologies face challenges such as high fracturing pressure, poor enhancement effect, and low heat extraction efficiency, severely limiting the development and utilization of HDR (Zhigang et al., 2019). SC-CO2 fluid refers to CO2 fluid at temperatures and pressures exceeding its critical temperature (31.1°C) and critical pressure (7.38MPa), characterized by high density, high diffusivity, low viscosity, and low surface tension. Compared with conventional water-based fracturing fluids, SC-CO2 has strong fluidity and diffusivity, effectively reducing the fracturing pressure required for the reservoir and helping to form complex fracture networks, and its superiority in transforming low-permeability/dense reservoirs has been widely recognized (Haizhu et al., 2020; Haizhu et al.,2012; Junping et al., 2019; Middleton et al., 2015). Studies have shown that using SC-CO2 for fracturing HDR has the following technical advantages(Bing et al., 2021; Legarth et al., 2003; Honglian et al., 2022): (1) low viscosity and near-zero surface tension of SC-CO2, strong penetration ability, and the capability to connect more natural fractures; (2) extremely weak intermolecular forces and very low surface tension of SC-CO2, resulting in strong fluidity and facilitating SC-CO2 flow and diffusion in formations; (3) injection of CO2 into geothermal formations can promote water-rock reactions, increasing reservoir porosity and permeability, thereby enhancing heat extraction efficiency; (4) full utilization of greenhouse gas CO2 for fracturing, saving water resources, and facilitating CO2 geological sequestration. However, due to the high-temperature and high-stress environment of HDR reservoirs, the mechanical properties of rocks and CO2 flow parameters change, causing the CO2 fracturing mechanism in HDR reservoirs to differ from traditional fracture mechanics theories, making them unsuitable for guiding CO2 fracturing design and construction in HDR reservoirs.