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薄至中厚煤层群矿区采动卸压煤层气抽采井优化设计

Optimal design of coalbed methane wells in mining area with thin to medium thick coal seam group

  • 摘要: 利用煤层气井对采煤过程中邻近煤层卸压煤层气抽采是一种高效的瓦斯综合治理与煤层气开发手段。查明采动覆岩应力−裂隙场演化特征,并据此开展煤层气井优化设计是提高卸压煤层气抽采效率的关键。以黔西山脚树煤矿221015工作面为工程场景和实例,基于地质−工程模型构建与工程数据分析,阐明了覆岩层内及层间应力演化特征,探讨了覆岩裂隙与增渗区分布形态,优化了煤层气井井位部署、井身结构设计及增产措施。研究表明:研究区煤系岩石力学性质频繁强弱交互的地质特征提高了卸压煤层气抽采效果。关键层逐次破断后,工作面上方5~20 m层段为贯通裂隙带,带内富集的大量游离态甲烷可跨层运移产出,为煤层气井主气源带。工作面上方20~45 m层段属弯曲下沉带内微裂隙发育带,具有近2 000 m3/d的单层煤产气潜力。该带卸压煤层气以层内运移为主,并积聚了裂隙带煤层气。压实区沿工作面走向的边界切线、压实区沿工作面走向的边界切线与回风巷间的中轴线、裂隙区边界线围成的区域是最有利的井位部署位置。采用高强度水泥固井与扩孔完井的“二开”井或采用筛管悬挂完井的“三开”井井筒稳定性好,可保障煤层气井长期抽采工作。优化方案为黔西地区煤矿采动区卸压煤层气抽采井日产气35 000 m3的突破提供了技术支撑,显著减弱了工作面瓦斯突出风险。基于多层叠置含煤层气系统的地质特点,“合层压裂排采+采动卸压抽采”的“一井多用”模式可作为“提产增效”的重点研究方向。

     

    Abstract: In mining areas, utilizing coalbed methane (CBM) wells to extract stress relief coalbed methane from adjacent coal seams is an efficient strategy. To enhance CBM extraction, it is vital to reveal the dynamics of stress and fracture fields evolution and optimization well locations and structure. Taking No. 221015 coalface in Shanjiaoshu coal mine as the study area. By integrating geo-engineering models and engineering data, the stress distribution within and between the overburden was elucidated, the distribution of fracture and the permeability increased zone in the overburden were elucidated, and the well location, structures and enhancement measures were designed. The research demonstrates that: The frequent alternation of lithological properties in the coal measure strata in the study area enhanced the mining influence on overburden. As the key strata progressively fail, the zone located 5−20 meters above the coalface were the fracture zones, in which a large amount of free methane can be migrated and produced across layers, serving as the primary gas source zone of CBM well. The 20−45 meters interval above the coalface belonged to the microfracture zone in the bending subsidence zone, possessing a gas production potential of nearly 2 000 m3/d from adjacent coal seams within a single layer. In this area, stress relief CBM primarily migrates intra-strata and accumulates methane originating from the fracture zone. Optimal well site locations was enclosed by tangential line of the compaction zone along the direction of the coalface and the center line between tangential line of the compaction zone and the return airway, and boundary lines around fractured zones. Meanwhile, the two - or three-stages structure wells with high strength cement cementing and using reaming or screen suspended completion have good stability, which can guarantee long-term extraction work of CBM well. The optimized extraction well scheme provides technical support for achieving a gas production breakthrough of 35000 m3/d from in mining areas of Qianxi region, significantly mitigating the risk of gas outbursts at the coalface. Based on the geological characteristics of multi-layered superimposed CBM systems, the “multi-usage” mode of the vertical well of “multi-layer fracturing and drainage + mining induced stress relief CBM extraction” can serve as a key development direction of “increasing production and efficiency”.

     

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