Changes in the pore structure of lignite after repeated cycles of liquid nitrogen freezing as determined by nitrogen adsorption and mercury intrusion

Liquid nitrogen (LN2) fracturing is a technology that can dramatically enhance the stimulation performances of high-temperature reservoirs, such as hot dry rock geothermal and deep/ultra-deep hydrocarbon reservoirs. The aim of the present study was to investigate the damage characteristics of high-temperature rocks subjected to LN2 thermal shock, which is a critical concern in the engineering application of LN2 fracturing. In our work, the rocks (granite, shale and sandstone) were slowly heated to different temperatures (25 °C, 150 °C and 260 °C) and maintained at the target temperatures for 10 h, followed by LN2 quenching. After thermal treatments, we tested the physical and mechanical properties of the rocks to evaluate their damages. Additionally, sensitivities of the three rocks to thermal shock were also compared and analyzed. According to our experiments, LN2 thermal shock can enhance the permeability of the rocks and deteriorate their mechanical properties significantly. Increasing rock temperature helps strengthen the effect of LN2 thermal shock, leading to more severe damage. Inter-granular cracking is the primary contribution to the rock damage in the LN2 cooling process. Compared with granite and shale, sandstone is less sensitive to LN2 thermal shock. The lower sensitivity of sandstone to thermal shock is mainly attributed to its larger pore spaces and weaker heterogeneity of mineral thermal expansion. The present paper can provide some guidance for the engineering application of LN2 fracturing technology.

Damage Analysis of High-Temperature Rocks Subjected to LN 2 Thermal Shock

Waterless fracturing technologies for unconventional reservoirs-opportunities for liquid nitrogen

To determine the effect of liquid nitrogen (LN2) cooling on the damage of heated rock, we conducted a series of physical and mechanical tests on Shandong granite samples. These granites were first slowly heated to the target temperatures (25~600 °C) and held for 10 h, followed by rapid cooling with a coolant. Three coolants were used and compared in our experiment: air, water and LN2. Physical properties and mechanical properties were tested after thermal treatments. Microstructural changes were also observed using scanning electron microscope and optical microscope. According to experimental results, permeability of the heated granites increases significantly after LN2-cooling, while density, P-wave velocity, strength and elastic modulus reduce. As heating temperature rises, changes in these properties become more pronounced. Compared to air-cooling and water-cooling, LN2-cooling induces greater changes in the physical and mechanical properties at any target temperature. This indicates that LN2-cooling can damage the heated rocks more remarkably than the other two cooling treatments. According to microscopic analysis, inter-granular cracking is the primary failure mode during thermal treatment, and most of the inter-granular cracks distribute at the boundaries of quartz. Our results in this paper are of great value for understanding the characteristics of thermal damage induced by rapid cooling.

Ran Li

Papers

Damage Analysis of High-Temperature Rocks Subjected to LN 2 Thermal Shock

Variations of Physical and Mechanical Properties of Heated Granite After Rapid Cooling with Liquid Nitrogen

A review of liquid nitrogen fracturing technology

Investigation on the damage of high-temperature shale subjected to liquid nitrogen cooling

Experiment on coal breaking with cryogenic nitrogen jet