煤岩气全域支撑压裂的几个关键问题与思考

Several key issues and insights in full-scale proppant-support fracturing for deep coal rock gas formations

  • 摘要: 我国煤岩气资源丰富,水力压裂是有效开发煤岩气储层的关键工程技术。由于深层煤岩岩石力学特征、微构造、煤岩气赋存状态和产能控制因素等有别于中浅层煤储层且区块间储层差异大,当前压裂改造技术的适应性仍面临挑战,压裂改造技术创新是煤岩气高效开发的必由之路。首先结合煤岩气储层的地质特征,论述了由此带来的压裂改造技术难题。针对地质特征的特殊性和压裂技术难题,提出了“点解吸,线疏通,面促缝,体支撑”的基质孔缝—割理裂隙协同改造的高效开发理念,并根据其内涵梳理了以下关键问题:①压裂诱导基质孔隙结构和表面性质改造促解吸、渗吸置换以及吸附−游离多态甲烷协同高效供气;②激活割理裂隙,缩短气体扩散距离,疏导基质释放储量;③促进裂缝均匀扩展,“控近扩远”造“长缝网”,精准调控裂缝形态;④实现“多级缝宽与支撑剂粒径精准匹配、层理缝平面支撑、割理缝和主缝立体支撑”的煤岩储层压裂缝网全域支撑。结论认为:需要基于深层煤岩的赋存特征和生产特征,进一步研究裂缝参数和生产动态的关系,明确能够满足吸附气和游离气 “连续−协同”供气的裂缝参数,为裂缝调控和施工优化提供支撑。深入开展煤层水力裂缝网络扩展规律和裂缝扩展数值模拟技术研究,结合施工净压力双对数图版,实现煤岩气体积缝网的有效调控。采用前置高黏降滤失、造主缝,后置低黏造复杂缝实现“控近扩远”造“长缝网”。针对深层煤岩气压裂对裂缝网络体积和导流能力的更高要求,在有限砂液量下使裂缝体积最大化与流动能力最优化,是实现降本增效的有力途径。研发适用于深层煤岩的全域支撑压裂技术,实现“主缝+层理缝+割理缝”长效连通,提高裂缝有效支撑体积。通过多级裂缝、不同铺砂模式下的长效导流能力测试,优化不同支撑剂、纤维的组合模式和配比等。完善现有压裂液体系,探索减水、高砂比低成本压裂液和清洁促解吸剂。

     

    Abstract: Our country has abundant coal rock gas resources. Hydraulic fracturing is a key technology for effectively developing coal rock gas reservoirs. Due to the differences in the mechanical properties, microstructure, gas occurrence states, and productivity-controlling factors of deep coal rock compared with shallow and medium-depth coal reservoirs, as well as the significant property variation in reservoirs between blocks, the adaptability of current fracturing technologies still faces challenges. Innovation in reservoir stimulation technologies is essential for the efficient development of coal rock gas. The difficulties in reservoir stimulation brought by the geological characteristics of coal rock gas reservoirs are discussed first. To deal with the geological features and the challenges of fracturing technology, an efficient development concept of matrix pore-cleat/fracture simultaneous stimulation, summarized as “point desorption, line dredging, fracture geometry improvement, and propped bulk fracture network”, is proposed, and the following key issues are figured out based on its connotation: ① fracturing-induced matrix pore structure and pore surface property modification, enhanced gas desorption, imbibition displacement, and adsorbed-phase and free-phase methane collaborative and efficient gas supply; ② activate cleats/fractures, shorten gas diffusion distances, and facilitate matrix reserves releasing; ③ promote uniform fracture propagation, enhance fracture complexity, and precisely control fracture morphology; ④ full-scale proppant support for coal rock reservoirs that “precisely matches proppant particle size with multi-level fracture widths, supports bedding-plane fractures, and provides three-dimensional support for cleat and main fractures”. Results indicate that it is necessary to further study the relationship between fracture parameters and production dynamics based on the gas storage and production characteristics of deep coal rocks, in order to identify fracture parameters that can realize the adsorbed and free gas “continuous-cooperative” supply, and to provide support for fracture property control and treating design optimization. Conduct in-depth research on hydraulic fracture network propagation rules in coal rock reservoirs and fracture propagation numerical simulation technologies, and combine the net pressure log-log diagram during fracturing to effectively control the fracture network propagation behavior in coal rock gas reservoirs. Use high-viscosity and leakage-weakening fluid first to create the main fractures, then use low-viscosity fluid to create complex fractures, achieving “controlled near-wellbore fracture complexity and sufficiently extended fractures” to form a “long fracture network”. To deal with the requirement of high conductivity and larger volume for fracture networks in deep coal rock formations, maximizing fracture volume and optimizing flow capacity with limited proppant and fluid is an effective way to reduce costs and increase efficiency. Propose the full-scale proppant-support fracturing technology for deep coal rock reservoirs to achieve long-term connectivity of “main fractures + bedding planes + cleats” and increase the effective support volume of fractures. Optimize different proppants and fiber combinations through long-term fracture conductivity tests for multi-size fractures with different proppant placement patterns. Improve existing fracturing fluid systems, explore water-reducing, high-sand-ratio, low-cost fracturing fluids, and clean desorption-promoting agents.

     

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