Experimental Study on Fracture Propagation During Supercritical CO2 Fracturing in Layered Tight Sandstone Reservoirs

Abstract

ABSTRACT Supercritical CO2 (Sc-CO2) has a special physical property of ultra-low viscosity (0.02∼0.16 mPa·s) so that it can create branching hydraulic fractures in brittle rocks. However, the major controlling factors of hydraulic fracture propagation during Sc-CO2 fracturing in layered tight sandstones remain unclear. In this study, laboratory fracturing experiments were conducted to systematically investigate the major controlling factors of fracture propagation driven by Sc-CO2. Experimental results show that the samples with lower strength bedding planes tend to form more complex fracture geometry during Sc-CO2 fracturing. Even under a high vertical stress difference of 8 MPa, Sc-CO2 can still open some low strength bedding planes and create multiple hydraulic fractures. With the decrease of horizontal stress difference, the number of fractures formed by Sc-CO2 fracturing in low-strength samples increases. Compared with conventional water-based fracturing, Sc-CO2 fracturing can create multi-branch fractures. In the case of Sc-CO2 fracturing on samples with high strength bedding planes, increasing the injection rate can effectively increase the complexity of fractures. Based on the results of laboratory fracturing experiments, the major controlling factors of fracture propagation during Sc-CO2 fracturing are summarized as bedding plane strength > vertical stress difference > horizontal stress difference > fracturing fluid viscosity > injection rate. INTRODUCTION According to the International Energy Agency data, the global CO2 emission in 2019 is about 33.3 billion tons, and its contribution to the worldwide greenhouse effect has exceeded 60%. CO2 capture, utilization, and storage technology (CCUS) is crucial to reduce greenhouse gases effectively. It is the primary technology to realize carbon neutralization for our humanity [1,2]. CO2 fracturing, CO2 displacing, and CO2 huff-puff are valuable technologies for geological carbon neutralization and enhancing oil recovery in the petroleum industry. Since the 1980s, CO2 fracturing has been widely applied to stimulate the oil and gas reservoirs and coal seams characterized by water sensitivity and low formation pressure [3]. In the recent decade, CO2 fracturing has been exploratively applied to the stimulation of shale oil/gas reservoirs [4,5]. Most of the previous CO2 fracturing treatments have achieved a sound effect of increasing oil/gas production. It can be attributed to the advantages of CO2 fracturing, including creating complex hydraulic fractures, reducing formation damage and oil viscosity [6-9]. Moreover, CO2 fracturing can also save water resources and benefit CO2 geological capture [10]. The abovementioned advantages of non-aqueous CO2 fracturing could minimize the drawbacks of water-based fracturing fluids, such as slickwater, x-linked guar, and gel, which are widely used to stimulate unconventional oil/gas reservoirs.