改善煤层透气性是提高煤层瓦斯高效抽采的常用方法，利用液态CO₂为压裂介质冻融煤体是改善煤层透气性的方法之一。液态CO₂通过对煤体造成变形损伤改变煤层透气性，深入研究液态CO₂冻融煤体变形损伤特性，是揭示液态CO₂致裂增透煤体强化瓦斯抽采机制的基础。采用物理实验方法，基于自主研发的液态CO₂冻融煤体实验系统，开展液态CO₂冻融条件下煤体变形损伤特性实验，监测冻融过程中煤样表面温度、应力、应变及罐体压力参数，分析液态CO₂冻融对煤体应力、应变的影响规律及罐中CO₂相态特征，探究液态CO₂冻融过程中热应力、水−冰相变冻胀力及汽化膨胀力对煤体造成的变形损伤贡献程度，揭示液态CO₂冻融的三重复合应力对煤体造成变形损伤的机制。研究结果表明：液态CO₂冻融煤样体积应变呈现先下降后上升的“U”字型变化趋势，低温冻结阶段煤基质发生收缩变形，融化阶段煤基质收缩变形逐渐恢复，最终形成了不可恢复的变形，总体表现为煤基质收缩变形与应变恢复两个阶段。实验过程中CO₂相态呈现气态−液态(气−液共存)-气态的变化趋势，液态CO₂冻融过程包括进液、冻结、缓慢卸压及室温融化4个阶段，对应煤样变形特征表现为冻缩变形、冻缩+冻胀+吸附膨胀变形、变形恢复及受热膨胀变形。非密封干燥煤样、密封干燥煤样及非密封饱水煤样的最小应变值绝对值分别为10056.636 ×10⁻⁶、11480.186×10⁻⁶及7881.893×10⁻⁶，残余应变分别为270.191×10⁻⁶、154.869×10⁻⁶及2033.636×10⁻⁶，胀缩率分别为2.686%、1.349%及25.801%。液态CO₂冻融复合应力作用对煤样造成的总变形损伤量为2033.636×10⁻⁶，热应力、汽化膨胀力及水−冰相变冻胀力分别造成的变形损伤量为154.869×10⁻⁶、115.322×10⁻⁶及1763.445×10⁻⁶，分别占总变形损伤量的7.615%、5.671%与86.714%，水−冰相变冻胀力占据主导地位。随着煤样含水率增加，煤体最小应变值绝对值降低，残余应变增加，水冰相变冻胀力占比增加。研究结果从煤样变形的角度阐明了液态CO₂冻融煤体变形损伤机理，丰富了液态CO₂致裂增透煤体强化瓦斯抽采技术体系。

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.