电−化学复合能脉冲致裂煤储层增渗机理

Mechanisms behind permeability enhancement of coal reservoirs induced by electro-chemical composite energy pulse fracturing

    摘要
  • 摘要:
    背景 电−化学复合能脉冲致裂技术(金属丝电爆炸引爆含能材料)可有效改造煤储层孔隙结构并提升煤层渗透性。阐明其对煤体孔裂隙改造尺度及瓦斯解吸规律的影响机制，可为该技术在煤层增渗领域的推广应用提供理论依据。
    方法 以山西潞安漳村矿煤样为研究对象，依托自主搭建的电−化学复合能脉冲致裂煤岩实验平台，开展了脉冲致裂煤体实验研究。采用高压压汞法、液氮吸附法和 CO2 吸附法，对致裂前后煤体孔隙结构特征进行了定量测试与表征，其中高压压汞法用于表征大孔(>50 nm)，液氮吸附法用于表征介孔(2~50 nm)，CO2 吸附法用于表征微孔(<2 nm)。进一步通过瓦斯吸附−解吸实验，系统研究了脉冲致裂作用前后煤体瓦斯解吸特性的变化规律，并利用扫描电子显微镜(SEM)对比分析了致裂前后煤基质中裂隙的发育形态与分布特征。
    结果 致裂后大孔孔容在特定尺度范围内表现出明显增加，其中孔径约为 140 000 nm的大孔孔容提高了 29.7%。介孔平均孔径整体呈现增大趋势，但其孔容降低了 34.8%，比表面积减少了 16.94%；微孔孔容亦出现一定程度的下降，这主要归因于脉冲致裂后介孔发生扩孔及向大孔的转化作用。在瓦斯吸附压力为 0.5、1.5 和 2.5 MPa条件下，脉冲致裂后煤样在 120 min内的甲烷累计解吸量及解吸速率均明显提高。扫描电子显微镜观察进一步揭示了脉冲致裂后煤体的典型冲蚀破坏特征，主要表现为煤基质剥离及其邻近区域孔隙与裂隙的发育，同时伴随新生锯齿状裂隙的形成以及孔−裂隙连通结构的显著增强。
    结论 电−化学复合能脉冲致裂作用于煤储层后，煤体内部发生了由微孔向介孔、介孔向大孔并进一步向裂隙演化的逐级转化过程，显著提升了孔−裂隙体系的连通性。为瓦斯的解吸、扩散与运移提供通道，从而有效提高了瓦斯解吸速率和解吸量。上述认识从煤孔隙结构演化与瓦斯解吸机理层面揭示了电−化学复合能脉冲致裂的增渗机制，为该技术在煤层增透领域的工程应用与推广提供了可靠的理论支撑。

     

    Abstract:
    Background Electro-chemical composite energy pulse fracturing (ECCE-PF) technology (i.e., the electrical explosion of metal wires igniting energetic materials) can effectively change the pore structure of coal reservoirs and enhance coal seam permeability. Elucidating the mechanisms behind the influence of this technology on both the scales of modified coal pores/fractures and methane desorption pattern can provide a theoretical basis for the wide application of this technology to coal seam permeability enhancement.
    Methods This study investigated coal samples from the Zhangcun Coal Mine in the Lu’an coal mining area, Shanxi Province. Using a self-developed ECCE-PF experimental platform, an experimental study was conducted on the pulse fracturing of coals. Specifically, the pore structures of coals pre- and post-fracturing were quantitatively analyzed and characterized using mercury injection capillary pressure (MICP), liquid nitrogen adsorption, and CO2 adsorption methods, which enable the characterization of macropores (> 50 nm), mesopores (2−50 nm), and micropores (<2 nm), respectively. Furthermore, this study systematically investigated the variation patterns of methane desorption behavior of coals pre- and post-fracturing using methane adsorption-desorption experiments, followed by a comparative analysis of the morphologies and spatial distribution of fractures within the coal matrix pre- and post-fracturing using scanning electron microscopy (SEM).
    Results Post-fracturing coal samples exhibited a pronounced increase in macropore volume within specific pore-size ranges, with a 29.7% increase observed in the pore volume of macropores with sizes of approximately 140 000 nm. For mesopores in coal samples, their average pore sizes generally showed an upward trend, while their pore volume and specific surface area decreased by 34.8% and 16.94%, respectively. Similarly, the pore volume of micropores in coal samples also declined slightly. These changes were primarily attributed to the enlargement of mesopores and their transformation into macropores post-fracturing. Under adsorption pressures of 0.5 MPa, 1.5 MPa, and 2.5 MPa, coal samples showed significantly enhanced cumulative methane desorption capacity and desorption rate within 120 min post-fracturing. SEM observations further revealed typical erosional damage characteristics of coals post-fracturing, as predominantly manifested by coal matrix stripping and the development of pores and fractures in adjacent zones, accompanied by both the formation of new serrated fractures and markedly enhanced pore-fracture connectivity.
    Conclusions The application of ECCE-PF technology to coal reservoirs triggers progressive transformation processes in coals, where micropores evolve into mesopores and mesopores transition to macropores and then to fractures. These processes significantly enhance the connectivity of the pore-fracture system, providing effective pathways for methane desorption, diffusion, and migration and, thereby, significantly enhancing both the methane desorption rate and cumulative desorption capacity. The insights of this study elucidate the permeability enhancement mechanisms of ECCE-PF from the perspective of pore structure evolution and methane desorption behavior, providing a robust theoretical basis for the engineering application and wide promotion of this technology in coal seam permeability enhancement.

     

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