高温脉动气体作用下低渗透煤岩的体破裂机理与增透特性

Gas fracturing technology is an effective means of overcoming the inadaptation of hydraulic fracturing in low permeability coal seam to achieve volume reconstruction and efficient methane extraction. This project mainly researches on the mechanical mechanism and permeability improvement of high temperature pulsating gas fracturing in low permeability coal by using macroscopic and mesoscopic indoor experimental and theoretical analysis. Firstly, the high temperature pulsating gas fracturing experiments will be conducted under different fracturing treatment parameters; the tensile and shear crack evolution mode will be studied adopting moment tensor inversion of acoustic emission, and the fracture surfaces will be analyzed by fractal theory, and these two aspects jointly reveal the volume fracturing mechanism induced by coupling “temperature + pulse +gas pressure”. Secondly, the initiation fracture pressure, crack propagation direction and crack morphology under different fracturing parameters will be monitored by use of acoustic emission source location and Micro CT technology. According to the measurement of coal permeability during the whole fracturing process, the role of each fracturing parameter on increasing coal permeability will be investigated. Finally, based on the energy criterion of crack propagation and seepage theory, a mechanical model for high temperature pulsating gas fracturing will be built among fracturing parameters, crack evolution and coal permeability, and an effective fracturing design and permeability improvement prediction method will be put forward. Research findings can provide scientific basis for numerical simulation and engineering practice of gas fracturing in low permeability coal.

气体压裂技术是克服低渗煤层水力压裂不适应性、实现储层体积改造和瓦斯高效抽采的有效手段。本项目采用宏细观试验与理论分析相结合的方法，研究低渗透煤岩的高温脉动气压致裂机理及增透特性。首先，开展不同压裂参数下低渗透煤岩的高温脉动气压致裂试验，利用声发射矩张量反演方法研究压裂过程中的张拉裂纹与剪切裂纹演化模式，利用分形理论研究破裂面细观形貌的能量耗散与微结构效应，揭示“温度+脉动+气压”耦合作用的体破裂机理；其次，利用声发射源定位及显微CT技术研究不同压裂参数下煤岩破裂的起裂压力、裂纹扩展方位和裂纹形态，并通过不同压裂阶段的气体渗透性测试，探究高温脉动气压致裂煤岩的增透特性；最后，基于裂纹扩展能量准则和渗流理论，构建气体压裂参数—煤岩裂纹演化—渗透性变化相关联的高温脉动气体压裂力学模型，形成有效的煤岩体致裂设计与增透效果预测方法。研究结果将为低渗煤层脉动气压致裂的数值仿真与工程实践提供科学依据。

低渗透煤岩储层水力压裂改造存在水锁效应、水资源消耗量大、地下水体污染等诸多缺点，高温脉动气体压裂新技术有望克服这些问题，因此，本项目开展了系统的低渗透煤岩高温脉动气压致裂宏细观试验，研究了温度、气压、脉动作用对煤岩体的损伤致裂机理，获得了不同气体压裂条件下煤岩的损伤演化、宏观破裂和渗透性变化规律，探索了气体压裂的层理效应，建立了气体压裂的破裂压力计算模型，并开展了脉动气体压裂技术的现场工业性试验。结果表明：高温作用可显著提高甲烷分子的解吸能力和煤岩孔隙体积，降低其强度和刚度；脉动气压作用导致煤样体积的重复膨胀与收缩，将煤基质颗粒破坏，促进孔隙度和孔比表面积的增加，过渡孔、中孔和大孔的孔体积分别增加了25.85%、117.86%和105.07%，渗透率增加63.64%，脉动疲劳次数阈值约为100次；渗透率、气体粘度、增压速率等因素对煤岩破裂压力和渗透面积有较大影响；现场工业性试验中注气孔和考察孔的瓦斯纯抽采流量显著增加，脉动气体压裂对煤层的影响范围在9m以上，高效抽采时间达到了40天，改善效果显著。这些研究结果将为压裂方案优化设计及增透效果预测等提供科学依据，进而推动高温脉动气体致裂低渗透煤体新技术的进步。