煤双重孔隙压力动态变化对煤岩渗透率演化机理研究

Dynamic change of coal permeability plays an important role in prediction of gas production and mitigation of coal-gas outburst. Current theoretical permeability models are proposed based on the assumption that the coal fracture pressure is equal to the matrix pressure at any time. In fact, this assumption not only neglects influence of lag change of coal matrix pressure on the coal permeability, but also is likely to overestimate the coal permeability. Furthermore, the boundary condition of the uniaxial strain model is inconsistent with that of laboratory observation. This proposal will interpret the contribution of dual pore pressures to the coal permeability, and conduct a serials of isothermal gas adsorption tests for coal samples with different sizes. The experimental results are used to investigate the multi-scale changes for Langmuir adsorption parameters and diffusion pathway length of the coal with gas pressure, aiming to interpret the characteristic of pressure gradient distribution in the coal matrix. Secondly, both the coal deformation and permeability are continuously measured throughout gas seepage-adsorption-diffusion processes. The dynamic interaction between coal fracture and matrix is investigated considering changes of pore pressure in cleat and matrix. This proposal will analyze the coupling effects of gas mass convection and effective stress transfer on the cleat-matrix system, and interpret evolution mechanism of coal permeability induced by gas unsteady state transport in dual porosity of coal. This study will provide a multi-time, multi-scale, multi-physics method to prediction of the permeability evolution of coal bed.

研究煤渗透率变化规律对于预测煤层气产量至关重要。现有的渗透率模型均假设在气体运移过程中煤裂隙压力与基质压力瞬时相等。该假设不仅忽视基质孔隙压力滞后变化对煤渗透率影响，还会过高估计低渗煤层的渗透性能，特别是单轴应变假设不符合实验室测试条件。本课题首先从理论上阐明煤的双重孔隙压力变化是影响煤渗透率的关键因素。其次，研究不同粒径煤样的吸附气体性能差异，揭示煤吸附参数及吸附扩散路径随孔隙压力的多尺度变化规律，以及煤基质孔隙压力梯度空间分布特征；并根据气体渗流-吸附-扩散全过程煤变形及其渗透率的时间演化曲线，研究双重孔隙压力对煤裂隙-基质动态相互作用关系，揭示受气体质量交换与有效应力传递控制的煤裂隙-基质动态作用机制，阐明煤双重孔隙中气体非稳态流动对渗透率演化影响机理。本申请为研究低渗煤层渗透率提供一种“多时度-多尺度-多物理场”分析方法，对提高煤层气产量以及预测煤-瓦斯突出有重要研究意义。

多孔介质渗透性能是流体与介质孔隙结构相互作用的宏观表现。经典渗透率模型与测试方法均假设介质孔隙压力分布处于平衡态。然而，对于赋存多尺度孔隙-裂隙结构的煤层而言，降压开采煤层气或升压注入二氧化碳气体都会改变原始气-固平衡状态，生成多重异步变化的压力系统，进而使孔隙-裂隙结构发生多尺度变化，给精准预测煤储层渗透性能带来挑战。本项目采用理论分析、物理模拟与数值模拟相结合的方法，得到以下研究结论：.（1）构建了考虑流-固耦合的双重孔隙介质弹性力学模型。首先，通过对比氦气与氮气等温吸附过程中煤样体积应变，修正了吸附应变，证实了气体压力与吸附膨胀之间竞争控制煤基质体积变化，揭示了氦气和氮气扩散系数与气体压力正相关特性。其次，考虑了气体渗流-扩散-吸附过程对裂缝-基质变形的影响，定义了煤基质内膨胀系数量化基质变形对裂隙开度的贡献，建立了有效应力-吸附变形控制的双重孔隙介质弹性力学模型。.（2）揭示了不同吸附性气体非稳态流动导致煤体渗透率化规律。采用改进瞬态法观测了不同应力条件（恒定围压与恒定压差）下裂缝煤样内氦气和氮气渗透率变化过程。实验结果表明氦气和氮气渗透率随时间演化呈先上升后下降再回弹的趋势；在相同孔隙压力条件下氮气渗透率低于氦气的渗透率。测试结果表明内膨胀系数随着扩散与吸附过程逐渐减小。.4）基于气体分子的动力学特性，构建考虑气-固耦合的渗透率模型，研究发现当注气压力高于1MPa时，煤体气体流场分成粘性流态区与滑脱流态区。在稳态流动条件下，这两种流态区的渗透率与煤样的整体渗透率相等。讨论了气体滑移对气体流量的贡献度，分析等效渗透率与实测渗透率的误差。.5) 建立考虑吸附热的“变形-渗流-扩散”耦合计算模型，在固体变形-气体流动-吸附／解吸耦合关系中引入吸附热。对于吸附过程，注气压力越大，升温越显著。对于解吸过程，降压越大，降温幅度越大。吸附热引起的煤储层温度升高会产生连锁反应并会随着时间和尺度增大而放大。对比考虑吸附热模型与未考虑吸附热模型结果，恒压注二氧化碳累计注气量的差异随着时间推移逐渐增加。恒定注二氧化碳中注气压力的差异也随时间逐渐增加。忽略吸附热将高估注气效率。