煤层气井水力压裂能量传递与均衡消突机理

The hydraulic fracturing of coalbed gas well is the main measure for the improvement and penetration of coal reservoir, and one of the main measures to eliminate the prominent danger in coal seam. The results show that the results are influenced by the physical properties of coal reservoir, the redistribution of stress field and the energy transfer and equilibrium of coal reservoir and etc. Previous studies have focused on the method, mechanism and influencing factors of coal reservoir fracturing, lack of hydraulic fracturing to coal reservoir energy accumulation and dissipation control and coal reservoir destruction mechanism, especially the ground fracturing corresponding to the underground area of the method and the effect of evaluation lack of effective theoretical support. This project selects No. 3 coal seam in Lu'an mining area of Qinshui Basin as the research object, adopts the method of laboratory experiment, numerical simulation and theoretical analysis, through the test experiment of coal body structure and rock mechanics characteristic after water leaching. The experiments of adsorption / desorption and desorption/ deformation of coal body after water leaching, the test and simulation of stress redistribution before and after fracturing, the numerical characterization of coal structure after fracturing, the model of energy accumulation and equalization transfer are established. The energy equilibrium model of coal reservoir under different fracturing scales is discussed. The physical and mechanical properties of coal body after coal seam fracturing are revealed, the essence of energy accumulation is related to equilibrium mode and outburst elimination index, and the mechanism of outburst suppression by hydraulic fracturing in coalbed methane wells is expounded. It provides theoretical guidance for underground coal and gas outburst prevention and metallurgy in the fracturing area of surface coalbed methane extraction well.

水力压裂即是煤储层改造、增透的主要措施，也是煤层消除突出危险性的主要措施之一。煤储层压裂改造后其消突效果受压裂后煤储层的物理性质、地应力场再分布及压裂过程中煤储层能量传递与均衡等因素影响。以往研究多集中在煤储层压裂增透方式、机理及其影响因素，缺乏水力压裂对煤储层能量聚积与耗散控制机理研究，尤其是压裂区对应井下区域的消突方式及消突效果判定缺乏有效的理论支撑。本项目选取潞安矿区3号煤层为研究对象，采用室内实验、数值模拟及理论分析相结合的方法，通过水浸后煤体结构及其岩石力学特性测试实验，水浸后煤体吸附/解吸与解吸/变形实验、压裂前后地应力再分布测试与模拟实验，建立压裂后能量聚积与均衡传递模型，探讨不同压裂尺度下煤储层能量均衡模型，揭示煤层压裂后煤体物理力学性质变化、能量聚积与均衡方式和消突指标关联实质，阐明煤层气井水力压裂消突机理，对煤层气井压裂区井下煤与瓦斯突出防治提供理论指导。

本项目研究目标是揭示煤层气井水力压裂对瓦斯的消突机理。在煤岩力学变形特征、裂缝扩展动态演化研究的基础上，探讨了煤储层能量扩散、传递与均衡消突机理，建立了水力压裂煤储层能量均衡的消突指标与压裂消突能量时效模型，为低渗煤层瓦斯综合治理提供理论基础。取得的主要认识如下：（1）通过地质分析、微震监测、数值模拟等方法，研究了压裂裂隙与构造裂隙之间的关系，揭示了压裂裂隙的扩展规律，建立了煤层气井裂缝扩展模型。一级裂缝方位总体平行于最大水平主应力方向，二级裂缝应沿外生裂隙进行延伸，三级裂缝应沿面割理和端割理进行扩展；在煤层与顶底板结合部位，由于顶底板与煤的弹性模量、泊松比相差较大，裂缝沿顶底板外生裂隙或顶底板与煤层之间的弱结合面顺层扩展，最终形成相互贯通的裂隙网络。（2）提出了煤储层水力压裂“4+1”边界，即铺砂区边界、裂缝扩展区边界、压裂液渗透区边界、瓦斯气涌区边界4个边界，以及在压裂排采后形成的1个最终解吸边界。进行了煤层气井压裂消突分析，认为煤层压裂影响区内是可以通过地面压裂煤层而达到消突目的或大大减弱煤与瓦斯突出的危险性。根据《防突规定》建立了压裂区抽采量与抽采时间的消突评价时效模型。（3）建立了煤层压裂排采和应力分布模型，分别从应力集中程度、井网布置、压裂区内外揭穿煤层过程等方面进行了煤与瓦斯突出危险性的分析。研究结果表明：菱形井网与矩形井网四周均出现应力集中，菱形井网最大主应力方向应力集中略高于矩形井网布置。界定了井网间消突区最大间隔宽度，计算出最优井网间距。分析了井下掘进巷道穿过压裂影响区不同位置的突出危险性，分析结果与井下瓦斯抽采效果吻合。（4）揭示了煤储层弹性能量失控与消突机理：压裂后煤体弹性潜能失控、煤层气体弹性能失控、煤层水体弹性能失控、煤体物理力学性质失控，引起煤体弹性余能减小、游离甲烷弹性能减小、煤层水体应力场均一化和煤体最小破坏能增大，人为措施实施后使煤层突出危险性减弱或消失。