Experimental Investigation on Fracturing Behaviors after Liquid Nitrogen Pre-Injection in High-Temperature Sandstone

The fracturing process of sandstone is inherently complex due to its loose internal structure and deformation adaptability. Liquid nitrogen pre-injection has emerged as a promising approach to damage reservoir rocks, effectively reducing fracture pressure and establishing intricate fracture networks, thus offering a potential solution for reservoir reconstruction. To unravel the fundamental mechanisms governing sandstone fracturing behaviors following liquid nitrogen pre-injection, sandstone fracturing experiments were conducted under varying durations of liquid nitrogen injection, rock temperature, and in-situ stress conditions. The experiments showcased the evolution of injection pressure and fracture characteristics under different testing conditions, complemented by electron microscope analysis to elucidate the factors driving the complex fracture characteristics of sandstone. The findings revealed a significant decrease in fracture pressure after liquid nitrogen pre-injection, accompanied by a notable increase in the complexity of the fracture network and the roughness of the fracture surface. Moreover, prolonging the duration of liquid nitrogen injection and elevating reservoir temperature further contributed to reducing fracture pressure, consequently enhancing fracture complexity and surface roughness. Conversely, the application of confining pressure amplified fracture pressure while intensifying the degree of fracturing. Notably, the investigation highlighted the increased presence of microcracks in sandstone resulting from liquid nitrogen pre-injection, facilitating fluid diffusion during fracturing and yielding lower fracture pressures, thereby enhancing the effectiveness of sandstone reservoir reformation. The research results can provide theoretical guidance for geothermal reservoir reconstruction.