微波辐射下煤体电磁热流固耦合效应及其改性增透机制

As coalbeds in China are characterized by microporosity, strong adsorptivity and low permeability, stimulation methods have been widely proposed. Microwave heating is notable for high efficiency, strong penetrability and integrity. Thus, microwave heating has an exciting prospect in coal stimulation. However, the mechanism of microwave stimulation of coal still remains unclear. We have found that microwave heating can promote the development of pores and fractures of coal. Therefore, in this project, the multiphase porous feature and multi-field coupling feature of coal will be considered. In addition, electromagnetics, heat transfer, rock mechanics, seepage mechanics theories will be combined. First, a multiphase porous media model of coal in a microwave cavity will be established, and the electromagnetic evolution, mass and heat transfer in coal will be explored. Second, the effects of microwave on the composition, molecular, pore and fracture structure will be studied. At the same time, the damage mechanism of coal under microwave irradiation will be revealed by combining thermodynamics and fracture mechanics. Third, the impacts of coal structure variation subjected to microwave irradiation on the adsorptivity and permeability of coal will be analyzed. Finally, a scaled physical model and a numerical model will be established to investigate the evolution of thermal, stress and seepage field. In addition, the electromagnetic-thermal-hydraulic-mechanical coupling and permeability improvement effects of coal will be revealed. The results will lay theoretic foundations for microwave stimulation of low-permeable coalbeds.

针对我国煤层“微孔隙、强吸附、低渗透”的瓦斯赋存特征，煤层增透技术被广泛提出。以高效、强穿透、体积性加热著称的微波技术在煤层增透领域应用前景可观，但其增透机制不明。申请人前期研究发现，微波注热能够有效促进煤体孔裂隙发育，鉴于此，本项目以煤体多相多孔性和多场耦合性为切入点，从电磁学、传热学、岩石力学、渗流力学等角度进行学科交叉研究。拟建立微波谐振腔内煤的多相多孔介质模型，考察电磁场时空演化特征及煤体传热传质规律；研究微波对煤体物质组成、分子结构、孔隙结构及裂隙结构的影响，结合热力学和断裂力学揭示微波辐射下煤体损伤、破裂机理；分析微波作用下煤体物性结构演化对其吸附性、渗透性的控制作用；建立微波辐射受载含瓦斯煤层相似物理模型和数值模型，探究煤体温度场、应力场及渗流场的演化规律，揭示微波下煤体电磁热流固耦合效应及其增透机制。研究成果旨在为低透气性煤层微波增透奠定理论基础。

建立了微波场内煤的多相多孔介质模型，研究了煤的微波热力响应机制。发现微波辐射热效应包括介质损耗产热、水分蒸发散热、煤体内部对流换热及表面对流散热；高温下，液态水蒸发为水蒸汽并溢散到空气中，煤体温度的不均匀分布导致水分蒸发呈现异步性，从而引起高压蒸汽喷溢，继而造成煤体损伤。.利用红外光谱研究了微波对煤体分子结构的改造过程，利用扫描电镜观察了微波对煤体孔隙形态的作用效果，利用压汞和核磁共振揭示了微波对煤体孔隙分布的影响机制。结果表明，微波能够促使煤中脂肪烃分解并以挥发分的形式脱除，高温高压气体冲击会导致闭合孔打开并连通，孔容增大；煤发生芳构化反应及缩聚反应，芳香度提高，微、小孔减少而中、大孔增多，孔隙比表面积减小；此外，羟基、羧基等亲水性含氧基团的减少会导致煤体束缚水能力及吸附瓦斯能力的降低。.设计了循环实验以研究微波对煤体宏观裂隙场、温度场及波速场的影响机制。结果表明，微波能够有效致裂煤体，延长辐射时间有利于裂隙扩展，增大微波功率既有利于微裂隙发育，又有助于裂隙的相互贯通；另外，煤体非均质性越强，其在微波辐射下的破裂现象越显著；煤样含水率的增大也有助于微波致裂。.利用煤岩渗透率测试仪揭示了微波辐射对煤体渗透性的影响，利用瓦斯解吸仪研究了微波辐射下煤体瓦斯储运机制。结果表明，微波辐射后，干燥煤样渗透率升高，随着煤样含水率的增大，微波增透率呈指数型增长；微波辐射会导致煤体瓦斯解吸速度加快，总解吸量增大，说明微波辐射改善了煤体孔隙结构，使得瓦斯更容易从煤基质及孔隙系统中运移出来。.建立了微波辐射煤储层的电磁-热-流-固全耦合模型，利用该模型研究了微波辐射对煤层瓦斯储运的影响。结果表明，微波辐射下瓦斯解吸引起的基质收缩是导致煤体孔隙率、渗透率增大的关键因素；相对于常规抽采，微波注热条件下的累计瓦斯抽采量提高43.9%。