Supercritical CO2 Shock Fracturing on Coal: Experimental Investigation on Fracture Morphology and Pressure Characteristics
Authors: Cong Richao , Yang Ruiyue , Wang Haizhu , Li Gensheng , Xia Zhihao , Hong Chunyang , Wen Haitao , Chen Jianxiang 
Published: September 2022

ABSTRACT:
For the efficient, economical and environmentally-friendly development of coalbed methane (CBM), a novel idea of using supercritical carbon dioxide (SC-CO2) shock fracturing to enhance CBM was proposed. To test the feasibility of this technique in the lab scale, experimental studies were conducted based on the self-designed true triaxial SC-CO2 shock fracturing experimental system. The influences of in-situ stress and initial releasing pressure on the morphology of coal fractures were studied. In addition, the pressure evolution characteristics and fracture morphologies were compared with conventional SC-CO2 fracturing and water fracturing. The results showed that the multiple fractures and crushed zones were generated in SC-CO2 shock fracturing under the combination effects of stress wave and high-pressure SC-CO2. The fracture tortuosity and complexity of SC-CO2 shock fracturing were higher than water fracturing and SC-CO2 fracturing. With the initial releasing pressure increasing, the bottom hole peak pressure gradually increases. There exists an optimal initial releasing pressure to obtain the maximum formation conductivity for a specific coal. The main fractures were approximately perpendicular to the direction of the minimum horizontal principal stress. This research confirms the advantages of SC-CO2 shock fracturing on coals and is expected to lay the foundation for its field application.

1. INTRODUCTION
Coal, as the primary energy source, has played a significant role in China. As an associated product of coal mine production, coalbed methane (CBM) is not only a kind of relatively clean energy, but also a kind of greenhouse gas (Xue et al., 2021). The greenhouse effect index of CBM is 30 times that of carbon dioxide (CO2) (Zou et al., 2015). Therefore, accelerating the exploitation and utilization of CBM resources can increase energy supply, reduce coal mine gas outburst accidents and greenhouse gas emissions simultaneously.
Due to the low porosity and permeability in coal reservoirs of China, the single well production is relatively low. Thus, it is difficult to achieve economic development (Li et al., 2017; Liu et al., 2019). Hydraulic fracturing is widely used to improve the formation conductivity of unconventional oil and gas reservoirs. However, there exists several challenges in conventional hydraulic fracturing. First, the water-based fracturing fluid entering the reservoirs could cause the expansion of clay minerals, which blocks the pore throats and reduces the formation permeability (Xue et al., 2021). Second, large volume of water consumption due to massive hydraulic fracturing cannot be ignored (EPA, 2016; Kondash et al., 2018). Third, chemical additives in water-based fracturing fluids could cause environmental pollution while increasing disposal costs (He et al., 2017; McLaughlin et al., 2016; Thomas et al., 2019). Finally, coal has the characteristics of low Young"s modulus, high Poisson" ratio and strong plasticity relatively, which results in a tendency to create the wide fractures with low fracture length (Lau et al., 2017).