含水煤岩超声波诱导甲烷解吸响应及其物理化学机制

Ultrasound is a new principle of stimulation technology to enhance coalbed methane recovery. In recent years, it has been widely researched but is still not able to transfer to a practical technology. One of the main bottlenecks is the unclear mechanism of enhancement recovery with ultrasound. Some preliminary studies have been done by the applicant including effectively separating the mechanical vibration effect and thermal effect of ultrasound by a self-design device, but the behavior and mechanism of methane desorption in coal with the individual effect of ultrasound need to be studied systematically. Aiming at this scientific issue, physical simulation experiment and large-scale molecular dynamics simulation will be conducted based on the preliminary work. For wet coals, the responses of capillary pressure variation, methane desorption behavior and their coupling relationship respectively on the mechanical vibration effect and thermal effect of ultrasound will be investigated. Combined with the effects of coal (with different ranks and moisture conditions) and methane (with different adsorption states) on ultrasound inducing gas desorption, the variation of coal-water interface property and the essence of methane desorption kinetics in wet coal with ultrasound can be proposed. Finally, the molecular dynamic mechanism of the interactions among coal, moisture and methane with ultrasound will be revealed. This work will contribute to ultrasound application and upgrade for enhanced coalbed methane recovery.

超声波诱导甲烷解吸是近年来致力于探讨的煤层气（瓦斯）强化抽采技术原理之一，但对其作用机制理解不深入成为限制该原理转化成实用技术的“瓶颈”之一。申请者前期利用自主研制的实验平台有效分离了超声波机械振动和热效应，但两种作用形式下煤体甲烷解吸行为及机制的研究尚待系统开展。本项目拟围绕这一科学问题，立足前期初步探索，采用物理模拟和分子动力学数值模拟方法，重点研究不同含水率煤岩毛细管力变化、甲烷解吸行为及二者耦合关系对超声波机械振动和热效应的响应规律，探讨吸附剂（煤级、含水状态）-吸附质（甲烷吸附状态）对超声波诱导解吸效应的影响，阐释超声波作用下煤-水界面物理化学性质演化及其影响甲烷解吸的动力学实质，揭示超声波诱导下煤基质-水-甲烷之间相互作用的微观分子动力学机制，为超声波强化抽采技术的研发应用和改进提供理论依据。

超声波激励有助于促进煤层气解吸，是提高煤层气单井产量的潜在技术，但对超声波的作用机制理解并不深入，波-水-煤三者耦合条件下的甲烷解吸/吸附过程及机理尚待研究。本项目基于自主研发的超声激励煤岩吸附/解吸试验平台，分别开展了超声波综合效应、温度效应和机械振动效应下煤体吸附甲烷行为，分析水分在超声作用过程中的影响，探讨超声波的作用机制。结果显示：（1）超声波综合效应具有促进甲烷解吸的作用，这主要源于超声波温度效应的影响，超声机械振动反而一定程度上促进甲烷吸附；（2）升温（30℃→70℃）虽然对超声传导具有抑制作用，但分子结构热运动增强却进一步突显了超声机械振动促进甲烷吸附的作用，这在无烟煤（小幅升温对波阻抗影响较小）中表现更为突出；（3）由于水分对超声传导具有促进作用，煤中水分强化了超声温度效应和机械振动效应，且对后者强化效果更好，使得超声综合效应促进含水煤岩解吸甲烷的影响有所降低；（4）超声波通过水介质直接作用于煤体会造成煤体孔隙体积增大，且100小时以内作用最为显著，高频超声作用效果优于低频，烟煤相比无烟煤具有更好的响应特征；（5）超声波通过水介质、甲烷吸附罐体作用于煤体，能量衰减至3.3%，不足以对吸附罐内煤体孔隙结构造成不可逆损伤，因而超声前后甲烷等温吸附曲线并无变化；（6）超声机械振动对煤中脂肪结构、含氧杂原子结构和芳香碳氢结构等具有游离端、杂原子的分子结构作用性更强，从而提高了煤分子结构秩理化程度，提高了煤表面自由能和吸附势，水分通过强化超声振动传导强化了上述效果；（7）超声机械振动以附加载荷形式作用于煤基质和甲烷吸附相是其促进甲烷吸附的本质，基于此构建了高拟合度的超声作用下甲烷吸附数学模型。