Visualization of Fracture Initiation Behavior During Liquid Nitrogen Fracturing in Geothermal Wells

Abstract

Hot dry rock geothermal energy exploitation can simultaneously satisfy the energy demand and environmental concerns. Creation of fractures by hydraulic fracturing is one of the most widely used methods to enhance the heat transfer efficiency in Enhanced Geothermal System (EGS). However, the occurrence of induced seismic events during massive hydraulic fracturing increased public concern. Besides, the economic and environmental burden of water-based fracturing fluid cannot be ignored. Using liquid nitrogen (LN2) fracturing as an alternative non-aqueous soft stimulation method for potential applications in EGS is proposed in this paper to solve the above issues. To elucidate the fracture initiation and propagation behavior of rocks by LN2 fracturing visually, samples made of a kind of transparent polymer named polymethyl methacrylate (PMMA) was used to represent the rock samples. The breakdown pressure and fracture morphologies generated by LN2 fracturing in various cyclic cooling parameters were compared with water fracturing. The results indicated that LN2 fracturing with cyclic cooling can significantly reduce the breakdown pressure as a comparison of water fracturing and LN2 fracturing without cyclic cooling. Moreover, LN2 fracturing with cyclic cooling was found easier to form complex fracture networks and enhance the stimulated reservoir volume (SRV). The reduction of breakdown pressure and formation of complex fracture networks are related to thermal stress and micro-fractures induced by cyclic cooling before LN2 fracturing. The experimental results are expected to provide a viable alternative for efficient and safe exploitations in EGS.