Experimental study of the effect of liquid nitrogen cooling on rock pore structure

As liquid nitrogen brings about thermal damage to rock when it comes into contact with a reservoir, it can be used as a fracturing fluid under proper engineering conditions. To investigate the effects of liquid nitrogen cooling on rock pore structure, sandstone, marble, and shale samples were cooled with liquid nitrogen under dried and saturated conditions, respectively. The samples were examined before and after treatment using scanning electron microscopy and nuclear magnetic resonance. The results show that there are three main changes in the rock pore structure when the samples were cooled by liquid nitrogen: (i) a reduction in the number and volume of the pores, (ii) an expansion of the micro-fissures (micro-pores), and (iii) an increase in the pore scale. More specifically, the pore structure of dry sandstone showed a reduction in the number and volume of pores; dry and saturated marble and shale presented expansion of their micro-fissures (micro-pores); and the pore scale was increased in the saturated sandstone (to the extent that macro-cracks were observable in the surface). The changes in the rock pore structures were mainly caused by thermal stress and frost force, and the characteristics of the variations were influenced by the type of rock and water content. Liquid nitrogen cooling increased the fracture degree inside the rocks, especially for shale samples. Cracks appeared along the joints, which were effective in producing micro-cracks on the walls of major fractures. This therefore increased the stimulation reservoir volume during the fracturing process.