Experimental study on deformation and damage characteristics of liquid CO₂ freeze-thaw coal

Improving the permeability of coal seam is a common method to improve the efficient extraction of coal seam gas. Using liquid CO₂ as fracturing medium to freeze-thaw coal is one of the methods to improve the permeability of coal seam. Liquid CO₂ changes the permeability of coal seam by causing deformation damage to coal body. In-depth study of the deformation damage characteristics of liquid CO₂ freeze-thaw coal body is the basis for revealing the mechanism of liquid CO₂ fracturing and permeability enhancement coal body to strengthen gas extraction. Based on the self-developed experimental system of liquid CO₂ freeze-thaw coal body, the experiment of deformation and damage characteristics of coal body under liquid CO₂ freeze-thaw conditions was carried out by means of physical experiment. The surface temperature, stress, strain and tank pressure parameters of coal body during freeze-thaw process were monitored. The influence of liquid CO₂ freeze-thaw on stress and strain of coal body and the phase characteristics of CO₂ in tank were analyzed. The contribution of thermal stress, water-ice phase change frost heave force and vaporization expansion force to deformation and damage of coal body during liquid CO₂ freeze-thaw process was explored, and the mechanism of deformation and damage of coal body caused by triple composite stress of liquid CO₂ freeze-thaw was revealed. The results show that: The volume strain of liquid CO₂ freeze-thaw coal shows a ‘U’ -shaped trend of decreasing first and then increasing. The coal matrix shrinkage deformation occurs in the low temperature freezing stage, and the shrinkage deformation of the coal matrix gradually recovers in the melting stage, and finally an irreversible deformation is formed. The overall performance is two stages of coal matrix shrinkage deformation and strain recovery. During the experiment, the phase state of CO₂ shows a trend of gas-liquid ( gas-liquid coexistence ) -gas state. The freezing and thawing process of liquid CO₂ includes four stages: liquid entry, freezing, slow pressure relief and room temperature thawing. The corresponding deformation characteristics of coal body are freeze-shrinkage deformation, freeze-shrinkage + frost heave + adsorption expansion deformation, deformation recovery and thermal expansion deformation. The absolute values of the minimum strain values of unsealed dry coal, sealed dry coal and unsealed saturated coal are 10056.636×10⁻⁶,11480.186×10⁻⁶ and 7881.893×10⁻⁶, respectively. The residual strains are 270.191×10⁻⁶, 154.869×10⁻⁶ and 2033.636×10⁻⁶, respectively. The expansion and contraction rates are 2.686 %, 1.349 % and 25.801 %, respectively. The total deformation damage caused by the combined stress of liquid CO₂ freeze-thaw is 2033.636×10⁻⁶. The deformation damage caused by thermal stress, vaporization expansion force and water-ice phase change frost heaving force is 154.869×10⁻⁶, 115.322×10⁻⁶ and 1763.445×10⁻⁶, respectively, accounting for 7.615 %, 5.671 % and 86.714 % of the total deformation damage, respectively. The water-ice phase change frost heaving force dominates. With the increase of water content of coal samples, the absolute value of the minimum strain value of coal decreases, the residual strain increases, and the proportion of water ice phase change frost heaving force increases. The research results clarify the deformation and damage mechanism of liquid CO₂ freeze-thaw coal from the perspective of coal deformation and enrich the technical system of liquid CO₂ cracking and anti-reflection coal enhanced gas extraction.