Numerical simulations of proppant transportation in cryogenic fluids: Implications on liquid helium and liquid nitrogen fracturing for subsurface hydrogen storage

Abstract

The over dependence on fossil fuels for human energy requirements together with the resultant adverse environmental effects in the form of greenhouse gas discharges has redirected our attention towards renewable energy sources. Researchers have identified hydrogen, a non-carbonaceous energy source holding the capacity to substitute fossil fuels, as a viable fuel option that has the potential to be generated utilizing eco-friendly techniques. Studies have identified underground sites that are operational or have the potential to be used for hydrogen storage. Shale formations, characterized by their low permeability, offer promising opportunities for underground hydrogen storage. These formations require stimulation techniques to generate flow channels. Pumping frac-fluid into these formations at high pressure induces fractures in them and the engineered fractures are supported by the proppants injected along with the fluid. Propped fractures form a vital conduit in unconventional gas reservoirs, which are a potential medium for storage of hydrogen. Cryogenic fracturing is an innovative approach that aims to enhance and broaden the capabilities of conventional hydraulic fracturing processes. The transmission and settlement of proppants are influenced by certain critical aspects including their density, size and volume fraction and fluid velocity, which are investigated in this contribution. This study incorporates water and cryogenic fluids like liquid nitrogen (LN2) and liquid helium (LHe) as fracturing fluids. The primary objective of this effort is to analyze proppant transportation phenomenon and the key influential factors in LN2 and LHe. The proppant volume fraction contour, equilibrium dune level (EqDL) and dune height (EqDH) data are illustrated in this research for better understanding of proppant migration. It was observed that when the fracturing fluid is given a higher velocity, the proppant grains experience increased drag force, causing them to penetrate deeper into the fracture. The desired range of proppant volume fraction for improved grain transmission was determined as 0.45 to 0.50 and the critical proppant density that significantly affected the EqDL as 187.2839 lb/ft3.