Evolution Characteristics of Micro-Structures in Rocks Subjected to Liquid Nitrogen Cyclic Freezing

Cyclic liquid nitrogen fracturing, as an ultra-low temperature waterless fracturing, is a stimulation method with great technical potential and application prospect in unconventional hydrocarbon recovery, showing its merits in environmental protection, reduced formation damage and high efficiency. Rock damage under super-cold conditions is the basis and key for the application of liquid nitrogen in reservoir stimulation. In order to further explore the microscopic mechanism of cyclic liquid nitrogen fracturing, the laboratory experiments of sandstone and coal rocks subjected to liquid nitrogen cyclic freezing-thawing are carried out in this study. The evolution characteristics of microscopic pore structure and pore-throat connectivity of sandstone and coal rocks under liquid nitrogen cyclic freezing shock are investigated by treatment means of micro-CT scanning. In addition, the influence of pore water on the freezing effect is analyzed. The experimental results show that with the increase in the number of liquid nitrogen freeze-thaw cycles, the number and size of pores are on the rise, and the topological relationship between pores is enhanced. The damage degree and modes of rocks with various lithology differ due to differences in mineral composition and cement structure. Coal is more sensitive to liquid nitrogen cyclic freezing than sandstone. After liquid nitrogen freezing for 5 times, the porosity and fractal dimension of coal rock increased by 482.4% and 13.47% respectively, which were 5.5 and 4.6 times those of sandstone. Pore water can make the core produce a more complex and higher connectivity fracture network. In summary, the expansion and interconnection of pore structures are promoted by liquid-nitrogen cyclic freezing, whereby rock permeability is significantly enhanced and its mechanical properties are degraded. A deeper understanding of the effects of liquid nitrogen freezing shock on rock mechanics and seepage behaviors is strongly supported by these findings from a microscopic mechanism perspective.