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中煤阶煤层气储层压裂地质特征对气井产能控制机理

Mechanism of fracturing geological characteristics controlling gas well productivity in medium-rank coalbed methane reservoirs

  • 摘要: 中煤阶煤层气是我国深层煤层气勘探开发的核心目标,开展该变质程度煤储层压裂地质特征对气井产能的影响规律及控制机理的分析,有助于深化对深层煤层气储层压裂改造、甜点区评价及资源潜力评估的认识,为国内深层中煤阶煤层气高效开发与工艺优化提供针对性科学参考。以甘肃窑街煤层气工程为研究载体,聚焦中煤阶煤层气压裂地质特征与气井产能的耦合控制机理,结合系统煤岩测试、真三轴压裂物模试验以及现场生产动态监测数据,系统探讨其对气井试气效果的影响。结果表明:① 甘肃窑街矿区中煤阶煤岩有机质含量高,煤质好,煤岩生烃能力强,利于煤层气富集成藏。与页岩相比,中煤阶煤岩的弹性模量、硬度、断裂韧性相对较低,水力压裂易形成宽缝,而在压裂复杂缝网形成方面相对困难;② 中煤阶煤层天然裂缝系统包括大裂隙、气胀节理、内生裂隙及构造牵引裂缝,天然裂缝级配关系优异,是气藏流体产出的理想通道,这制约压裂缝网的形成。煤层中下部煤体结构更破碎,发育有顺层煤粉源集合体;③ 真三轴条件下压裂物模试验表明,中煤阶煤岩天然组构包括层理、天然裂缝等压裂激活难度较小,在较低的流体压力下即可撑开形成人工裂缝,与页岩相比,中煤阶煤层气压裂造缝机理较为独特;④ 制约中煤阶煤层气井产能的关键改造因素是压裂裂缝的级配关系以及主裂缝缝宽。其中,不同级别裂缝级配关系影响储层整体渗流效率,而主裂缝缝宽决定裂缝通道的持久导流能力,尤其是煤粉颗粒通过的能力,“宽缝压裂”可能是深层中煤阶煤层气压裂改造的技术趋势;⑤ 中煤阶煤层气储层天然裂缝发育,压裂液滤失严重,砂堵频发,保障压裂注入高排量,是煤层气压裂的关键问题。采取大排量循环注入程序,既保障主裂缝长度,又能通过往复式憋压,有利于造一定复杂程度的缝网,最终形成“控近扩远”的造缝效果。

     

    Abstract: Medium-rank coalbed methane is the core target for deep coalbed methane exploration and development in China. Conducting research on the influence laws and control mechanisms of fracturing geological characteristics of coal reservoirs at this metamorphic degree on gas well productivity helps deepen the understanding of deep coalbed methane reservoir fracturing reconstruction, sweet spot evaluation, and resource potential assessment, providing targeted scientific references for the efficient development and technological optimization of deep medium-rank coalbed methane in China. Taking the Gansu Yaojie coalbed methane project as the research carrier, The coupled control mechanism of fracturing geological characteristics of medium-rank coalbeds and gas well productivity is analyzed. Combined with systematic coal rock testing, true triaxial fracturing physical simulation experiments, and on-site production dynamic monitoring data, it systematically explores their impacts on gas well testing effects. The results show that: ①The medium-rank coal in the Gansu Yaojie mining area is characterized by high organic matter content, good coal quality, and strong hydrocarbon generation capacity, which is conducive to coalbed methane accumulation. Compared with shale, the medium-rank coal has relatively lower elastic modulus, hardness, and fracture toughness, making it easy to form wide fractures during hydraulic fracturing but relatively difficult to develop complex fracture networks; ② The natural fracture system of medium-rank coalbeds includes large fractures, gas-expansion joints, endogenous fractures, and structural traction fractures. The excellent grading relationship of natural fractures serves as an ideal channel for reservoir fluid production and plays a key role in the formation of hydraulic fracturing networks. The coal structure in the middle and lower parts of the coalbed is more fragmented, with developed bedding-parallel coal dust source aggregates; ③ True triaxial fracturing physical simulation experiments indicate that natural fabrics (such as bedding and natural fractures) in medium-rank coal are relatively easy to activate during hydraulic fracturing, and can be opened to form artificial fractures under low fluid pressure. Compared with shale, the fracturing fracture-formation mechanism of medium-rank coalbed methane is relatively unique; ④ The key reconstruction factors restricting the productivity of medium-rank coalbed methane wells are the grading relationship of fracturing fractures and the width of main fractures. Among them, the grading relationship of fractures at different levels affects the overall seepage efficiency of the reservoir, while the width of main fractures determines the long-term conductivity of fracture channels, especially the passage capacity of coal dust particles. “Wide-fracture fracturing” may become the technical trend for deep medium-rank coalbed methane fracturing reconstruction; ⑤ Medium-rank coalbed methane reservoirs are well-developed with natural fractures, leading to severe fracturing fluid loss and frequent sand plugging. Ensuring high-displacement injection during fracturing is a key challenge for coalbed methane fracturing. Adopting a large-displacement cyclic injection procedure not only guarantees the length of the main fractures but also facilitates the formation of fracture networks with a certain degree of complexity through reciprocating pressure build-up, ultimately achieving the fracture-formation effect of “ controlling near-wellbore and expanding far-field”.

     

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