超临界CO2压裂深部煤体的显微裂缝分布特征及其控制机理

Microscopic fracture distribution characteristics and control mechanism of deep coal body fractured by supercritical CO2 fracturing

  • 摘要: 深部煤层埋深大,煤岩结构致密,裂隙相对不发育,成为制约深部煤层气高效开采的重要因素。水力压裂主要在煤层中形成较宽大的裂缝,而超临界CO2 (ScCO2)由于其分子结构较小,黏度低,能够渗透到煤体微纳米尺度的孔裂隙内,从而产生较复杂的显微裂缝网络,有效增加气体运移空间。结合显微CT、声发射仪、形貌扫描仪研究了ScCO2压裂延安区块深部煤体后显微裂缝的分布特征及扩展规律,获得了不同应力约束条件下煤中压裂裂缝的分布特征,分析了煤体结构、地应力、ScCO2特性等因素对显微裂缝起裂与扩展的控制机理,定量表征了显微裂缝的形貌特征及其非均质性,揭示了显微裂缝扩展与分布的控制机理。研究结果表明,1~3号样的垂向应力差系数(Kv)均为0.4,4号样的Kv为0.6时,1~3号样的破裂压力均在25 MPa附近,而4号样的破裂压力仅为16 MPa,即Kv增加有利于煤体破裂。ScCO2压裂后煤体中显微裂缝发育,其形态复杂且连通性增强,有利于气体运移。ScCO2进入煤体,沿着较大孔裂隙通道逐步进入显微尺度的孔裂隙,随着注入气体量增大,ScCO2对煤体的挤压强度不断提高,煤体先发生显微破裂,随着ScCO2聚集压力增大,煤中裂缝被撑开发生整体破裂。由于ScCO2能够进入不同尺度的煤体内且其分布较均匀,因此形成的裂缝分布范围较广。1~4号样裂缝面的粗糙度分形维数分别为1.915、1.828、1.814和1.797,表明煤体的破裂压力越大,裂缝面的分形维数越高,裂缝的非均质性越强。煤体结构、地应力和ScCO2性质是影响深部煤体压裂裂缝展布的主要因素。研究结果为ScCO2压裂改造深部煤体渗透率提供了指导。

     

    Abstract: Deep coal seams are commonly characterized by high burial depth, dense coal-rock structures and relatively undeveloped natural fractures, which together restrict permeability enhancement and efficient coalbed methane (CBM) extraction. Under such conditions effectiveness of conventional hydraulic fracturing is constrained by its tendency to generate predominantly planar fractures. In contrast, supercritical CO2 (ScCO2), owing to its small molecular size, low viscosity, and strong penetration capability can interact with coal micro-and nano scale pores and fractures in coal, thereby promoting the development of more complex fracture networks which facilitate additional pathways for gas permeation and migration. This study integrates micro-computed tomography (micro-CT), acoustic emission instrument and morphology scanning to investigate the spatial distribution and propagation behavior of micro-fractures in deep coal from the Yan’an block following ScCO2 fracturing. Fracture spatial distribution characteristics under different stress constraints are systematically examined. The controlling mechanism of fracture initiation and propagation associated with coal structure, in-situ stress conditions and ScCO2 characteristics in fracture initiation and propagation were analyzed. In addition, fracture morphology and heterogeneity are quantitatively characterized providing insight into governing mechanisms of fracture propagation and spatial distribution. The results indicate that the vertical stress difference coefficient (Kv) exerts a strong control on fracture initiation pressure. When Kv =0.4 (sample 1-3), fracture pressures are approximately 25 MPa, whereas at is Kv=0.6 (sample 4), the fracture pressure decreased to 16 MPa, indicating the increased Kv facilitates to the fracture initiation. Following ScCO2 fracturing, extensive micro-fractures develop in coal, with complex geometries and enhanced connectivity, which is conducive to gas migration. ScCO2 preferentially accesses coal through relatively large pore–fracture channels and subsequently penetrates micro - nano scale pores and fractures. With increasing injected gas volume, the compressive effect of ScCO2 on the coal intensifies, leading initially to micro-fracturing; as ScCO2 accumulation pressure increases, fractures are progressively opened and coalesce into macroscopic failure. Owing to the ability of ScCO2 to penetrate coal across multiple scales and its relatively uniform distribution, the resulting fractures exhibit a wide spatial distribution.The fractal dimension of fracture surface roughness for samples 1-4 are 1.915, 1.828, 1.814 and 1.797 respectively. This indicates that the higher fracture pressures are associated with increased fracture surface complexity and stronger heterogeneity. The coal structure, in-situ stresses and ScCO2 properties are the primary factors controlling fracture development and distribution in deep coal. These findings provide guidance for permeability enhancement for deep coal seams and coalbed methane reservoir transformation using ScCO2 fracturing.

     

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