地层环境富有机质页岩吸附/解吸下的变形机制及渗透率演化规律研究

Adsorption and desorption characteristics and permeability evolution of organic-rich shale in the stratigraphic environment are key factors in determining production capacity of shale gas. The organic-rich shale of Lower Cambrian Niutitang formation and Lower Cambrian Longmaxi formation are taken as research subject. Adsorption and desorption characteristics and the rules and mechanisms of adsorption swelling and desorption shrinkage deformation of organic-rich shale in the stratigraphic environment are revealed by adsorption experiments carried out at different confining pressures and different gas pressures under constant temperature. Using non-adsorptive and adsorbed gas, shale permeability experiments are carried out under non-pre-adsorption and pre-adsorption equilibrium condition by means of instantaneous condition method and back-pressure static state method The relationship between effective stress and organic-rich shale permeability is analyzed. The mechanisms of adsorption and desorption effects on gas migration and permeability evolution of organic-rich shale are revealed. Appropriate porous medium model is abstracted based on experimental result of shale microstructure. Based on poroelasticity, fluid mechanics in porous medium, coupling theory and adsorption theory, organic-rich shale permeability mathematical model considering adsorption and desorption and the differences of gas pressure drop rate in pores with different sizes is established. Combining simulation results of this model and experiments, mutual authentication are realized. Using the model, the evolution rules and main controlling factor of organic-rich shale permeability are studied.

储层环境下富有机质页岩吸附/解吸特性及渗透率演化规律是制约页岩气开采产能的关键因素。以中国具有开发潜力的下寒武统牛蹄塘组及下志留统龙马溪组富有机质页岩作为研究对象，通过恒温环境不同围压及气体压力作用下的富有机质页岩吸附/解吸试验，揭示地层环境下富有机质页岩吸附/解吸特性及吸附膨胀/解吸收缩变形规律与机理；利用非吸附及吸附气体通过未预吸附及预吸附平衡下的页岩瞬态及背压稳态渗透率试验，获取有效应力与页岩渗透率的关系，揭示吸附与解吸作用对富有机质页岩气体运移及渗透率演化影响的机理；基于页岩微观结构观察结果，抽象出合适的物理模型，结合多孔介质弹性力学、渗流力学、耦合理论、吸附理论等，建立考虑吸附/解吸作用及不同孔隙结构内气体压力降落速率差异的富有机质页岩渗透率数学模型，将该模型的计算结果与实验相互验证，利用该模型研究开采引起的页岩渗透率演化规律与主控因素，为页岩气开采方案制定和优化提供理论支撑。

储层环境下富有机质页岩吸附/解吸与膨胀/收缩是影响页岩内部气体运移与渗透率演化的关键因素。以中国具有开发潜力的下寒武统牛蹄塘组富有机质页岩作为研究对象，测定了页岩的地化参数及孔隙结构，研究了地化参数对页岩孔隙结构及渗透率的影响。建立了致密页岩吸附试验过程中系统死体积和变形误差的分离方法，实现了致密富有机质页岩吸附/解吸与膨胀/收缩变形的准确获得。利用非吸附气体与吸附气体开展了恒温环境、不同围压及不同气体压力升、降下页岩内部气体流动试验研究，获得了页岩岩心孔隙度随应力及气体压力的演化规律，系统研究了页岩内不同气体的吸附量与吸附膨胀变形的动态演化规律及二者之间的关系，揭示了页岩吸附气体及膨胀变形对气体扩散及运移影响的内在机制。对比分析了非吸附气体及吸附气体引起的应变差异，研究了页岩变形的各向异性特征，并分析了吸附对页岩变形各向异性影响的内在机理。建立了稳瞬态联合测定致密页岩渗透率的测试系统及方法，开展了恒温条件、不同围压、不同气体压力下完整及含天然裂隙富有机质页岩的渗透率试验研究，获得了不同页岩渗透率与有效应力的关系，通过参数分析获得了压缩系数与有效应力的关系，揭示了页岩随有效应力演化的内在机制。基于上述试验研究和机理分析结果，针对页岩基质中有机质和无机质矿物气体吸附能力和力学性质差异对页岩内部气体流动的影响，结合多孔介质弹性力学、渗流力学、耦合理论、吸附理论等，建立了考虑吸附/解吸及有效应力作用的页岩渗透率数学模型。提出了一种微尺度的离散耦合模型，研究了页岩基质有机质和无机质矿物中力学变形与流动特性之间的耦合关系，计算了二者之间的传质和力学相互作用。通过COMSOL Multiphysics进行了模型求解，并通过耦合试验的结果对该模型进行了验证。分析了页岩气开采现场两类典型力学边界条件下，页岩渗透率和有效扩散系数、表面扩散系数的变化规律。项目研究成果为页岩气产能准确分析与预测提供了科学依据。