Experimental study of mechanics and seepage characteristics of sandstones after liquid-nitrogen stimulation

Because the conventional method of treating hydraulic fracturing uses a large amount of water, a waterless fracturing technology is needed urgently in those areas lacking sufficient water resources. This study experimentally investigates the mechanical and seepage characteristics of sandstones at four saturations (dry, water, oil, mixed water and oil) after being immersed in liquid nitrogen for a long period. After becoming fully saturated, the samples are tested for their acoustic velocities, elastic modulus and uniaxial compressive strength. Only the dry samples have an increase in all of these attributes; the rest of the samples decline in all of these attributes. Deformation measurements and microstructural studies are conducted to explain the mechanical observations. For the dry samples, shrinkage occurs without damage; for the remaining samples, there are incompatible deformations between rock skeletons and pore fluids that might lead to the generation of damage. Along with the changes in mechanical properties, the permeability of all sandstone samples declines. Based on the microstructural analysis and the Scanning Electronic Microscope (SEM) images, we suggest that the shrinkage of small pores (10 nm < radius < 700 nm) is the main reason for the permeability decrease of dry samples, the blocking of small pores and the closure of large pores (700 nm < radius < 10 μm) lead to the permeability decrease of water saturated, oil saturated and water-oil-saturated samples.