Effects of Gas Types on Crack Propagation and Energy Evolution Characteristics in Low-Permeability Rocks

为研究不同气体(氮气、二氧化碳、空气)对低渗岩石裂纹扩展及能量演化特征的影响，揭示气体压裂机理并评价其工程应用潜力，文章基于叠加原理建立低渗岩石气体压裂力学模型，利用MatDEM离散元软件构建带钻孔的二维岩石数值模型，结合宏微观参数转换方法和范德瓦尔斯方程调整气体参数，模拟围岩压力与恒定气体压力耦合作用下的裂纹扩展、孔隙压力分布及能量演化过程。结果表明，氮气压裂产生的总裂纹数量最多，张拉裂纹占比超99%，其孔隙压力扩散范围最小但裂隙连通性最佳；二氧化碳压裂孔隙压力扩散范围最大，但裂纹数量仅为氮气的50%；空气压裂效果最弱，能量生成速率显著低于氮气。能量演化显示氮气压裂耗散能为二氧化碳的5倍，表明其驱动更剧烈的岩石结构重组。气体黏度差异通过流体损失速率主导压裂效果，高黏度氮气因低流体损失率优先在微小裂隙内积聚能量，促进密集裂纹网络形成。该研究为低渗储层气体压裂的工艺优化提供了理论依据。To investigate the influence of different gases (nitrogen, CO₂, and air) on crack propagation and energy evolution in low-permeability rocks, and to elucidate the mechanisms of gas fracturing while evaluating its engineering potential, this study establishes a mechanical model of gas fracturing based on the superposition principle. A two-dimensional numerical rock model with a central borehole was constructed using MatDEM discrete element software. By integrating macro-micro parameter conversion methods and adjusting gas properties via the van der Waals equation, the crack propagation, pore pressure distribution, and energy evolution under coupled confining pressure and constant gas injection pressure were simulated. The results demonstrate that nitrogen fracturing generates the highest total crack count, with tensile cracks accounting for over 99% of the total. Despite exhibiting the smallest pore pressure diffusion range, nitrogen achieves optimal crack connectivity. CO₂ fracturing shows the widest pore pressure diffusion range but produces only 50% of the cracks observed in nitrogen fracturing. Air fracturing yields the least effective results, with a significantly lower heat generation rate compared to nitrogen. Energy evolution analysis reveals that the dissipated energy during nitrogen fracturing is five times that of CO₂, indicating its capacity to drive more intensive rock structural reorganization. The viscosity differences among gases govern fracturing efficiency through fluid loss rates. High-viscosity nitrogen accumulates energy preferentially within microcracks due to its low fluid loss rate, thereby promoting the formation of dense fracture networks. This study provides theoretical insights for optimizing gas fracturing processes in low-permeability reservoirs.