Experimental Study on the Formation Mechanism of Complex Network During Supercritical CO2 Fracturing in Jimsar Oil Field
Authors: Li Lizhe , Hu Longqiao , Tang Weiyu , Hou Xiaoyu , Zhou Fujian , Wang Bo 
Published: August 2024

ABSTRACT:
Shale oil reserves are abundant and are a key area for future oil development. The supercritical CO2(SC-CO2) has the property of low viscosity, which can produce a complex fracture network in the reservoir under certain pressure and realize the commercial exploitation of shale oil. This work focuses on the shale reservoir of Jimsar. Based on a triaxial physical simulation device and Computed Tomography (CT) scan imaging, the different fracture expansion laws of layered shale during the fracturing process of the SC-CO2 and the slickwater are conducted. Furthermore, nuclear magnetic resonance (NMR) technology was introduced to conduct the matrix energization experiment to quantitatively characterize the recovery enhancement after fracturing. This study is valuable for understanding the fracture network formation using SC-CO2 in shale reservoirs. To summarize, SC-CO2 can realize lower fracture pressure and more complex fracture networks under certain circumstances while being able to effectively increase the permeability of the shale matrix. The oilfield applications of SC-CO2 are promising.

1. INTRODUCTION
China's unconventional oil and gas resources are abundant(Xu, Li, et al., 2023; Xu, Zhou, et al., 2023), which has become the development focus in the petroleum industry. Hydraulic fracturing technology (Lizhe et al., 2022) is an indispensable technology in unconventional oil and gas exploitation. However, conventional hydraulic fracturing technology may lead to huge water consumption. Clay minerals in reservoirs tend to swell when they encounter water, which may cause reservoir damage. Therefore, anhydrous fracturing techniques such as supercritical CO2 (SC-CO2) (Gupta and Bobier, 1998; Yuan et al., 2015) become an important technology to increase recovery.
SC-CO2 is characterized by low viscosity, low surface interfacial tension, high diffusivity, high density and strong solvency. SC-CO2 fracturing (Jia et al., 2019) can be classified into pre-storage fracturing, post-fracturing, dry fracturing, quasi-dry fracturing, and foam fracturing. Currently, SC-CO2 fracturing has been proven to be an efficient technology for shale reservoir development, demonstrating great development potential. Zou et al., 2018 conducted indoor experiments and proposed that SC-CO2 fracturing can increase the brittleness index and reduce the compressive strength of the rock, indicating a low breakdown pressure. SC-CO2 fracturing can delay the decline of reservoir pressure and provide flowback energy, which is conducive to reducing reservoir damage. After CO2 fracturing, the generated fracture network is complex, and the fracture roughness is large (Memon et al., 2022; Middleton et al., 2015). In addition, the fractures have greater permeability and effective hydraulic openings. It can easily communicate with natural fractures, so SC-CO2 fractures are more inclined to produce complex fracture morphology (Chen et al., 2015; Kizaki et al., 2012).