Rock Pore Structure Damage Due to Freeze During Liquid Nitrogen Fracturing

Liquid nitrogen has been successfully used as a fracturing fluid in petroleum engineering. When liquid nitrogen is pumped into a well, the physical conditions of the reservoir rocks will be altered. In these experiments, pore structure damage in a sandstone and a coal due to freezing with liquid nitrogen was measured by nuclear magnetic resonance. The changes both in amplitude of the T (2) distribution curve and in maximum T (2) value were analysed. The results showed that the action of freezing with liquid nitrogen produced damage to the pore structure and caused the development of micro-pores or micro-fissures. The change in amplitude of T (2) distribution curves showed that the new pores or fissures in coal specimens were larger than those in sandstone. This indicated that coal specimens were subjected to more serious damage to their pore structure because of their inherent natural fractures and weaker grain cementation. For sandstone, the change in amplitude of T (2) distribution curve peaks revealed that the damage increased with increasing water saturation. By contrasting the pore structure damage characteristics of coal and sandstone, a tentative conclusion that liquid nitrogen fracturing was more suitable for use in coal bed methane extraction than in sandstone reservoirs was drawn.