Supercritical CO2, as a noted waterless fracturing fluid, is considered to have substantial potential for unconventional oil and natural gas exploration. The distribution of proppants in fractures is vital in both treatment design and post-treatment evaluation, but research on proppant sedimentation in supercritical CO2 fluids is limited. In this paper, we use computational fluid dynamics (CFD) to simulate supercritical CO2 slurry flow in a vertical fracture. The movement characteristics of proppant in supercritical CO2 and slickwater are compared. Sensitivity analysis is performed on parameters affecting the proppant placement in the fracture, including proppant density, injection temperature, proppant concentration, and proppant diameter. The results show that the supercritical CO2 is easy to settle in the fracture to form a proppant dune because the viscosity and density are lower than the slickwater. Low-density proppant will make the dunes more uniformly distributed in the fractures. The effect of temperature should be fully considered in the design of supercritical CO2 slurry pumping. There are differences in proppant transport mechanisms at different proppant concentrations and proppant diameters. The findings are beneficial for better understanding the transport mechanism of proppant in supercritical CO2 and optimizing pumping parameters in supercritical CO2 field fracking.

1. INTRODUCTION

Hydraulic fracturing technology is the most noticeable technological breakthrough made by the petroleum industry in recent decades. It has successfully achieved the commercial exploitation of unconventional oil and gas resources and promoted the prosperity of the world's energy, and is currently the main form of unconventional gas extraction. However, hydraulic fracturing often requires significant water resources. A typical shale gas well should have 6800 to 38,000 m3 of water injected into a deep tight gas reservoir during fracturing (Gallegos et al., 2015). This is obviously an unbearable pressure on scarce water resources and ecologically fragile areas. In addition, oil and gas and various chemical additives are mixed in the waste fluid returned after fracturing, which has potential environmental impact. 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. It has gradually become an important part of waterless fracturing technology.

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