Numerical Simulation of Supercritical CO2 Impact Flow Field Under Different Perforation Azimuth Angles
Authors: Liu Mingsheng , Wang Haizhu , Zheng Yong , Wang Bin , Sun Lianhe , Shi Mingliang , Fan Xuhao , Ni Jun 
Published: September 2023

ABSTRACT
A new idea of supercritical carbon dioxide (SC-CO2) shock fracturing was proposed to develop unconventional oil and gas resources efficiently and environmentally friendly. Four 3D physical models with different perforation azimuth angles modeled the internal flow field of perforated wellbore in SC-CO2 shock fracturing. The CO2 real-gas-model in NIST database coupled the property parameters with the flow field. Numerical simulation obtained the pressure, temperature and phase parameters spatiotemporal distribution laws. The results show that SC-CO2 impact pressure and temperature vary temporally in three stages and the rapid depressurization stage dissipate most of the pressure energy (42.27% on average). Pressure drop rate calculation shows that SC-CO2 shock fracturing is dynamic unloading, faster than hydraulic fracturing but slower than chemical explosive fracturing. The phase interface position is determined by combining the temperature and pressure spatiotemporal distribution inside the perforation with the CO2 critical temperature-pressure iso-lines. The research results provide reference for the design of SC-CO2 shock fracturing.

INTRODUCTION
Oil and gas are important strategic resources related to economic and social development. The reserves of unconventional oil and gas resources such as shale oil and gas, tight oil and coalbed methane are abundant, and they are becoming new targets for oil and gas exploration and development in recent years(Hongjun et al., 2016). However, commercial development of unconventional oil and gas can only be achieved through fracturing. Since the beginning of 21th century, North America has achieved shale gas revolution and energy independence by using large-scale horizontal well hydraulic fracturing technology. It has promoted the prosperity of the world's energy, and is currently the main form of unconventional gas extraction. However, the complexity of hydraulic fracturing fracture is low, and the fracturing effect needs to be further improved. And the expansion of clay minerals in the reservoir will block the flow channel and cause reservoir damage(Shen et al., 2018; Khan et al., 2021), which will affect the production and recovery of oil and gas wells. In addition, hydraulic fracturing may cause earthquakes(Holland, 2013; Skoumal et al., 2018), and the backflow fluid will pollute the environment(Lester et al., 2015; Mohammad-Pajooh et al., 2018), which is difficult and costly to deal with(Vengosh et al., 2014; Zheng et al., 2020). In recent years, with the development of carbon capture, utilization and storage (CCUS) technologies, CO2 fracturing technology has attracted increasing attention for its good prospects in improving oil and gas recovery, reducing carbon emissions, safety and environmental protection(Wang et al., 2019). It has gradually become an important part of waterless fracturing technology. Specially, the liquid CO2 injected into formation will transition into supercritical state when the temperature and pressure exceeding 304.25K and 7.38 MPa(Gupta & Bobier, 1998). As a kind of environmentally-friendly waterless fluid, supercritical CO2(SC-CO2) has unique physical properties with not only low viscosity and low surface tension of gas, but also high density and high solubility of liquid(Wang et al., 2012). Due to the unique physical properties of SC-CO2, it exhibits significant advantages when applied to fracturing. Firstly, the excellent compatibility between SC-CO2 and reservoirs fundamentally avoids reservoir damage phenomena such as water blockage and wettability reversal, thereby protecting the reservoir(Zhou et al., 2019). Secondly, SC-CO2 can deteriorate the mechanical properties of rocks(Wu et al., 2022), increase the effective stress of the formation, and thereby reduce the initiation pressure for fracturing(Wang & Sepehrnoori et al., 2018; Yang et al., 2022). In addition, SC-CO2 has extremely low surface tension and strong diffusion ability, which enables it to enter micro-pores and micro-fractures that cannot be penetrated by water-based fracturing fluids, forming a complex network of fractures(Hou et al., 2021; Zhang et al., 2017). Lastly, CO2 that enters the reservoir can displace adsorbed methane, increasing the natural gas production and achieving CO2 geological storage(Qin et al., 2021; Shi et al., 2019). Based on field applications in North America and China, it has been demonstrated that the enhanced production effect of SC-CO2 fracturing is superior to hydraulic fracturing(Ribeiro et al., 2022; Siwei et al., 2019). However, the consequences of poor proppant carrying capacity and transportation efficiency(Wang & Wang et al., 2018; Zheng et al., 2022) can include sand blockages and wellhead overpressure, which limits the commercial viability of the technology. To improve the proppant-carrying capacity of SC-CO2, researchers have conducted extensive studies on the development of viscosity-enhancing agents(Liu et al., 2020; Zhao et al., 2021).But the target of low usage, environmentally and significant viscosity enhancement has not yet been achieved with the current viscosity-enhancing agents. Therefore, in addition to using viscosity-enhancing agents for fracturing fluid modification, exploring a method that can solve the above-mentioned shortcomings of SC-CO2 fracturing has become a research hotspot. And SC-CO2 shock fracturing is expected to be one of the future development directions(Cong et al., 2022).