甲烷/二氧化碳吸附压力导致煤基质各向异性变形机理研究

Adsorption-induced deformation of coal plays a significant role in the porous structure of coal. It usually treats coal sample as homogenous specimen rather than takes into account the deformation characteristics of anisotropic porous structure of coal affected by adsorptive gas pressure. This project, different from phenomenological analysis, aims to quantify correlation between gas adsorption volume and stress-strain of coal under isothermal adsorption and isobaric adsorption conditions, respectively, using multi-scale analysis method combined with non-destructive CT scanning. In the study, the micro-/meso-/macro-porous structure information under isothermal and isobar sorptive conditions are respectively obtained in terms of the datasets of images scanned from tested coal sample, which helps to interpret the three-dimension porous characteristics of coal before and after reaching adsorption equilibrium, as well as the equivalent representative volume element (RVE) of coal matrix after homogenization treatment. Secondly, both adsorption-induced deformation model and quantitative conversion between meso-scale and macro-scale for coal are formulated using the viewpoint of improved self-consistent model. Additionally, adsorption-induced swelling coefficient is also introduced as equivalent parameter in order to give the equilibrium adsorption gas volume for meso-porous structure for coal under isobaric conditions. Finally, multi-scale conversion for coal matrix under isobar circumstances is accordingly conducted in terms of correlation of stress-strain between meso-RVE and macro-volume, which enables equilibrium adsorption of coal with larger scale to be predicted. The goal is that the correlation between adsorption-induced deformation of coal matrix and adsorption pressure is better understood, in which the equibrium adsorption volume of multi-scale coal mass under isobar condition is also appropriately predicted. It is hope that our work will provide theoretical and experimental support to porous structure evolution of coal subjected to adsorption of gas.

煤吸附瓦斯导致基体变形是影响煤孔隙结构变化的主要因素。目前大多数研究是把测试煤样作为均质材料对待，而未能考虑到气体吸附压力对煤基质的多尺度各向异性变形影响。本项目借助无损伤CT扫描手段并结合多尺度分析方法。提取等温/等压吸附条件下煤的微-细-宏观结构图像信息，描述煤吸附气体前/后孔隙三维形貌演化特征，构建煤基质结构的等效均匀化代表性体积单元。利用改进的自洽模型从宏-细观尺度定量描述煤等温吸附膨胀变形特征。通过跨细/宏观尺度间煤的有效吸附膨胀参数的传递，表征出气体吸附压力恒定过程中煤微细结构的平衡吸附量。最后，根据宏-细观代表性单元体之间应力应变关系进行多尺度转换，预测等压吸附条件下煤样的平衡吸附气体量。期望能揭示煤吸附导致煤基质多尺度膨胀-收缩行为与吸附气压力相互作用的基本规律，评估等压吸附条件下多尺度煤体的平衡吸附量,为煤-气吸附作用下煤孔隙结构演化特征研究提供理论与测试依据。

煤层孔隙压力变化可以显著改变煤体有效应力与气体吸附/解吸特性，进而影响煤裂隙开度及其渗透率。相比煤裂隙系统中气体渗流与气体吸附/解吸过程，气体在煤基质内扩散非常缓慢。一旦裂隙压力发生变化，基质孔隙压力需要很长时间才能与裂隙压力平衡。在此过程中煤基质会发生膨胀或收缩，进而挤压或者扩张裂隙开度，影响煤体渗透率。因此，本项目分别考虑煤基质释放气体与吸收气体两类过程对煤基质变形演化影响，利用自主研制煤-气多物理场耦合测试系统开展以下研究工作。.(1) 研究热-力作用导致含气煤基质解吸气体的多物理过程.首先开展高温对含气原煤变形演化特征与对煤体力学性质影响研究。通过一系列实验研究发现温度升高不仅能导致煤体膨胀，还伴有吸附气体受热解吸-流动并诱发煤基质出现收缩现象。利用Arrhenius equation计算煤的原生吸附气体的解吸活化能，对32例样品结果统计表明吸附态二氧化碳解吸活化能明显高于吸附甲烷的解吸能。该结果不仅证实不同气体分子在煤基质表面存在竞争吸附，还为确定采空区煤层自燃、煤层气注热驱替开采提供关键参数。此外还探讨温度升高导致煤力学强度与弹性模量弱化机制，以及热-力耦合对煤体渗透率演化影响。其次，开展在热-力作用下含气原煤体释放吸附瓦斯过程研究。实验表明在进入热平衡状态的煤样在单轴压缩变形过程中显现复杂的固体变形-气体流动响应特征，具体表现为原生裂隙孔隙压密排出气体、煤开裂导致气体回流，以及煤骨架不均匀变形导致气体瞬时流动。.(2) 研究气体在裂隙渗流-基质扩散对煤体变形演化的影响，探讨煤变形演化对渗透率作用机制。测试结果表明在注入非吸附性气体过程中煤骨架经历从收缩到回弹，收缩量总是大于回弹量。这说明在煤裂隙与基质之间孔隙压力差的存在可以引起煤基质收缩变形，并随着气体持续扩散使压缩煤体出现部分回弹。当吸附性的氮气注入进入煤体后出现短时收缩，之后煤体膨胀并超过初始体积。这说明煤体积变化是吸附膨胀与力学压缩相互竞争的结果。针对煤基质气体扩散过程，本研究提出采用煤体局部应变表征由于气体扩散导致煤基质应变，利用双扩散模型得到氦气有效扩散系数与有效应力之间变化关系。结果表明氦气有效扩散系数与有效应力呈负相关，与孔隙压力呈正相关。这说明气体扩散系数并不是常数，而是与扩散路径或范围有紧密关系。以上结果为深入理解煤裂隙与基质相互作用对煤渗透率演化影响提供理论基础与实验技术支持。