深埋特厚煤层坚硬顶板多维分段水力压裂控冲减损技术

Multi dimensional segmented hydraulic fracturing impact ground pressure control and loss reduction technology for deep buried thick coal seam hard roof

  • 摘要: 水力压裂技术是煤矿顶板弱化及冲击地压防治的有效方法之一。以陕西某煤矿2305特厚煤层综放工作面为工程背景,采用理论分析、数值模拟、现场试验以及工程监测等手段对坚硬顶板区域弱化控制技术进行研究。基于“塑性绞线”理论构建基本顶“薄板结构”力学模型,提出了结合煤层采出过程中坚硬顶板初次破断临界状态积蓄的弯曲应变能及微震事件分布与响应特征确定水力压裂目标层位的方法,根据此方法确定2305工作面压裂目标层位为14.50 m厚粗砂岩;构建流固耦合模式下应变软化数值计算模型,在目标层位设计有无定向分段水力压裂数值模拟对比试验,并引入强度应力比参数对顶板局部稳定性进行分析,结果表明:定向长钻水力压裂有效破坏了基本顶的整体性并缩短了基本顶来压步距,初次来压步距降低了25.81%,周期来压步距降低了24.64%,减小了因顶板悬空面积过大而形成巨大动载及诱发冲击地压的可能性;根据2305工作面地质条件,设计定向长钻+常规浅孔的多维分段水力压裂施工方案,20号、21号、22号定向长钻孔压裂过程中分别出现30、35、23次3 MPa以上压降现象,定向分段水力压裂使顶板的整体性得到破坏,常规浅孔压裂过程中不同裂隙的拓展形式在压裂曲线上表现出双阶段稳定型及多阶段发育型的不同阶段性特征,顶板以及顶煤压裂效果显著;联合多种监测手段对2305工作面围岩活动进行监测,多维分段水力压裂技术的实施破坏了坚硬顶板的完整性,与未实施水力压裂的2303工作面相比,初次来压步距与周期来压步距分别降低24、12 m,降幅分别达33.33%、32.19%,有效减小了采面支架工作阻力,降低了大能量微震事件发生的可能性,为井下安全生产提供了保障。

     

    Abstract: Hydraulic fracturing technology is one of the effective methods for weakening the roof of coal mines and preventing rockburst. This article takes the 2305 thick coal seam fully mechanized caving working face of a coal mine in Shaanxi Province as the engineering background, and uses theoretical analysis, numerical simulation, on-site testing, and engineering monitoring methods to study the weakening control technology of the hard roof area. Based on the theory of “plastic stranded wire”, a mechanical model of the basic top “thin plate structure” is constructed, and a method is proposed to determine the hydraulic fracturing target layer by combining the accumulated bending strain energy and the distribution and response characteristics of high-energy microseismic events at the critical state of the first fracture of the hard top plate in the coal seam extraction process. According to this method, the fracturing target layer of 2305 working face is determined to be 14.50 m thick coarse sandstone; A numerical calculation model for strain softening under fluid structure coupling mode was constructed, and a comparative experiment was designed with and without directional segmented hydraulic fracturing in the target layer. The strength stress ratio parameter was introduced to analyze the local stability of the roof. The results showed that directional long drilling hydraulic fracturing effectively broke the integrity of the basic roof and shortened the step distance of the basic roof. The initial step distance was reduced by 25.81%, and the periodic step distance was reduced by 24.64%, reducing the possibility of forming huge dynamic loads and inducing impact ground pressure due to the large suspended area of the roof; Based on the geological conditions of the 2305 working face, a multi-dimensional segmented hydraulic fracturing construction plan combining directional long drilling and conventional shallow drilling was designed. During the fracturing process of directional long drilling holes No.20, No.21, and No.22, there were 30, 35, and 23 instances of pressure drop above 3 MPa, respectively. The directional segmented hydraulic fracturing caused damage to the integrity of the roof. During the conventional shallow drilling fracturing process, the expansion forms of different fractures showed different stage characteristics of two-stage stability and multi-stage development on the fracturing curve. The fracturing effect of the roof and top coal was significant; Multiple monitoring methods were used to monitor the surrounding rock activity of the 2305 working face. The implementation of multi-dimensional segmented hydraulic fracturing technology destroyed the integrity of the hard roof. Compared with the 2303 working face without hydraulic fracturing, the initial and periodic pressure step distances were reduced by 24 m and 12 m, respectively, with a reduction of 33.33% and 32.19%, effectively reducing the working resistance of the mining face support and reducing the possibility of high-energy microseismic events, providing a guarantee for underground safety production.

     

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