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沁水盆地高煤阶煤层气水平井高效压裂技术及实践

High-efficiency fracturing technology and practice of high-rank coalbed methane horizontal wells in Qinshui Basin

  • 摘要: 沁水盆地是我国高煤阶煤层气的主要生产基地,年产量占全国煤层气产量的60%。高煤阶煤储层物性差,渗透率低,需要采用压裂改造的方式才能实现有效的渗流产出,采用水平井压裂开发成为高煤阶煤层气田最主要的开发方式。为了进一步增强沁水盆地高煤阶煤储层改造效果,实现水平井压裂技术升级,提高单井稳产气量和EUR,基于沁水盆地高煤阶煤储层的特点,分析了影响压裂改造的工程地质因素,从压裂段间距、射孔方式、压裂液体系、支撑剂组合、施工排量以及压裂施工工序6个方面进行了优化。结果表明:沁水盆地煤岩塑性强,压裂改造难度大,需要更高的排量才能产生有效的缝网;水平应力差系数小,水力压裂时产生复杂缝,压裂难度大,水平应力差系数越大,水力压裂相对容易产生长缝,裂缝易延伸;聚能定向射孔能够聚集孔眼射流能量,避免簇间干扰引起近井裂缝复杂,提高主缝穿透深度和远井裂缝有效改造体积;活性水+低伤害变黏压裂液的复合压裂液体系,造缝的同时润湿煤层表面形成保护层,低伤害变黏体系通过黏度变化实现降低施工摩擦阻力,提高支撑剂携带能力等功能,满足储层保护、体积改造和有效支撑的需求;“粉砂前置+细中砂分级支撑+自悬浮支撑剂辅助携砂”的多段组合加砂模式,可以实现各级裂缝有效支撑;依据煤岩破碎指数从低到高,将沁水盆地高阶煤划分为3个整体评价区间,其中易改造区(破碎指数<6)易压开,较低排量(7 m3/min左右)改造即可获得高产;可改造区(破碎指数7~13)需达到合理排量需求(7~15 m3/min左右),才能获得高产;难改造区(破碎指数≥14)需达到较高排量(15 m3/min以上),才有获得高产能力;前置液阶段采用“多台阶小步幅”的多级变排量施工方法,通过平衡主缝、分支缝的关系,形成了初始低净压力(5 MPa)、后期高净压力(>10 MPa)的控压造缝模式,减少近井地带储层压实、控制缝高不突破顶底板、构建体积缝网。“聚能定向大排量、逐级造缝、粉细砂组合、井组同步干扰作业”为核心的高煤阶煤储层压裂技术在沁水盆地规模化推广应用,水平井压裂缝长增加73%、缝网体积增加216%、单井日产量实现翻番,实现了沁水高煤阶煤层气田的高效开发,推广应用到全国高煤阶煤层气区块,为煤层气产业做大做强提供了技术支撑和可供借鉴的示范。

     

    Abstract: Qinshui Basin is the main production base of high-rank CBM in China, and its annual production accounts for 60% of the national CBM production. High-rank coal reservoirs have poor physical properties and low permeability, which require fracturing to achieve effective seepage production. Horizontal well fracturing has become the main development method of high-rank CBM fields. In order to further enhance the reconstruction effect of high-rank coal reservoirs in Qinshui Basin, upgrade the fracturing technology of horizontal wells, and improve the stable gas production and EUR of a single well, based on the characteristics of high-rank coal reservoirs in Qinshui Basin, the engineering geological factors affecting the fracturing reconstruction were analyzed, and the optimization was carried out from six aspects, including fracturing interval spacing, perforation mode, fracturing fluid system, proppant combination, construction displacement and fracturing construction process. The results show that the coal in Qinshui Basin has strong plasticity and is difficult to be fractured, so higher displacement is required to produce effective fracture network; The lower the horizontal stress contrast, the more complex fracture networks tend to form, which increases the difficulty of hydraulic fracturing. The larger the horizontal stress difference coefficient is, the longer the hydraulic fracturing is, and the fractures are easy to extend; Focused energy directional perforation can gather the jet energy of the hole, avoid the near well fracture complexity caused by cluster interference, and improve the penetration depth of the main fracture and the effective reconstruction volume of the far well fracture; The composite fracturing fluid system of active water+low damage variable viscosity fracturing fluid can wet the coal seam surface while making fractures to form a protective layer. The low-damage variable-viscosity fluid system reduces friction pressure during pumping and enhances proppant transport efficiency through controlled viscosity changes, thereby meeting the requirements of reservoir protection, volume fracturing, and effective proppant placement. The multi-stage combined sand adding mode of “silt front+fine and medium sand staged support+self suspended proppant assisted sand carrying” can achieve effective support of fractures at all levels; According to the crushing index of coal from low to high, the high-rank coal in Qinshui Basin is divided into three overall evaluation intervals, in which the easy stimulation area (crushing index<6) is easy to open, and the low displacement (about 7 m3/min) stimulation can obtain high yield; The stimulative area (crushing index 7−13) needs to meet the reasonable discharge demand (about 7−15 m3/min) in order to obtain high yield; In the difficult stimulation area (crushing index ≥14), it is necessary to reach a higher discharge capacity (above 15 m3/min) to obtain high yield; The multi-stage variable displacement construction method of “multi steps and small steps” is adopted in the preflush stage. By balancing the relationship between main joints and branch joints, a pressure controlled fracture making mode of initial low net pressure (5 MPa) and later high net pressure (>10 MPa) is formed, which reduces the compaction of reservoir near the well, controls the height of joints without breaking through the top and bottom plates, and constructs a volumetric fracture network. The large-scale application of high-rank coal reservoir fracturing technology with the core of “energy accumulation directional large displacement, step-by-step fracture forming, fine sand combination, and well group synchronous interference operation” in Qinshui Basin has increased the fracture length of horizontal well pressure by 73%, the fracture network volume by 216%, and doubled the daily output of a single well, realizing the efficient development of high-rank CBM fields in Qinshui, and has been popularized and applied to high-rank CBM blocks in China, providing technical support and referential demonstration for the expansion and strengthening of the CBM industry.

     

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