页岩储层三重孔隙模型及吸附介质有效应力准则研究

发布时间：2018-07-17 01:57

【摘要】：近年来,随着常规能源的日益枯竭和碳排放的增加,页岩气等非常规能源的开发成为世界能源研究的焦点之一。目前对于储层变形与气体运移机制的研究尚不成熟,这极大地限制了页岩气开采技术的发展。本文基于多孔弹性力学和渗流力学等理论,建立了页岩三重孔隙结构下包含页岩变形、干酪根基质分子解吸与扩散、无机质系统粘性流、以及裂隙系统粘性流的多物理场耦合模型。此外,建立了多孔吸附介质的有效应力准则。主要取得以下研究成果。(1)提出多孔含裂隙页岩储层的三重孔隙力学模型。根据气体在页岩中的运移及吸附储存特征,建立了页岩储层的三重孔隙度数学模型以及各自系统的扩散和渗透率数学模型,并建立了三重孔隙结构下有效应力及吸附膨胀引起的变形控制方程、干酪根基质系统的气体扩散控制方程、无机质系统以及裂隙系统的流动控制方程。控制方程中包含了页岩变形、气体渗流、气体吸附等多物理场耦合作用。(2)揭示了由于三重介质导流能力的差异而引起的压力场演化不同步现象:裂隙快于无机质,并快于干酪根。研究结果表明产气过程中气体的交换类型属于干酪根-无机质-裂隙的“串联”流动模式:气体由浓度高(压力较大)的有机孔向无机质系统扩散,再由无机质系统向裂隙系统(压力较小)流动;但当干酪根基质的扩散系数相对较大时,其压力演化几乎与无机质同步,且均慢于裂隙系统的压力演化,此时气体的交换类型服从“并联”模式:裂隙系统中的气体同时来源于干酪根和无机质系统。(3)通过与现场水平井产气率的匹配,验证了该模型的数值模拟结果。敏感性分析结果表明,裂隙渗透率对初始产气率的影响至关重要,而干酪根基质的TOC含量和扩散系数是保证页岩气持续高效开采的重要参数。(4)通过引入吸附膨胀模量的概念,建立了考虑吸附膨胀效应下多孔吸附介质的有效应力准则,将多孔吸附介质的非线性变形问题简化成无孔非吸附介质的线弹性变形问题,并通过煤体的无约束实验和单轴应变实验证明了有效应力系数的统一性。(5)吸附膨胀模量是衡量多孔吸附介质吸附变形难易程度的指标。吸附膨胀模量越小,相同孔压情况下的吸附膨胀越明显。实验证明低孔压条件下(7 MPa),煤体对甲烷或二氧化碳的吸附膨胀模量比固体颗粒体积模量小一个数量级,这侧面反映了低孔压储层产气过程中的吸附效应对变形或渗透率的影响占主导地位。
[Abstract]:In recent years, with the depletion of conventional energy and the increase of carbon emissions, the development of unconventional energy such as shale gas has become one of the focuses of energy research in the world. At present, the study of reservoir deformation and gas migration mechanism is still immature, which greatly limits the development of shale gas production technology. Based on the theory of porous elasticity and percolation mechanics, this paper has established shale deformation, molecular desorption and diffusion of kerogen matrix, viscous flow in inorganic system under shale triple pore structure. And the multi-physical field coupling model of viscous flow in fracture system. In addition, the effective stress criterion for porous adsorption media is established. The main achievements are as follows: (1) the triple pore mechanics model of porous fractured shale reservoir is proposed. According to the characteristics of gas migration and adsorption and storage in shale, the mathematical model of shale reservoir's triple porosity and the mathematical model of diffusion and permeability of their respective systems are established. The governing equations of effective stress and adsorption expansion under triple pore structure, gas diffusion control equation of kerogen matrix system, flow control equation of inorganic system and fracture system are established. The governing equation includes the coupling of shale deformation, gas percolation and gas adsorption. (2) it is revealed that the evolution of the pressure field is out of sync due to the difference in the conductivity of the triplet medium: the fracture is faster than the inorganic mass. And faster than kerogen. The results show that the type of gas exchange in the process of gas production belongs to the "series" flow pattern of kerogen, inorganic substance and fissure: the gas diffuses from organic pores with high concentration (high pressure) to inorganic system. However, when the diffusion coefficient of cheese is relatively large, the pressure evolution is almost synchronized with inorganic matter, and both of them are slower than the pressure evolution of fracture system. At this time, the gas exchange type follows the "parallel" model: the gas in the fracture system comes from both kerogen and inorganic system. (3) the numerical simulation results of the model are verified by matching the gas production rate of the field horizontal well. The sensitivity analysis results show that the influence of fracture permeability on the initial gas production rate is very important, and the TOC content and diffusion coefficient of the kerogen are important parameters to ensure the shale gas sustainable and efficient production. (4) the concept of adsorption modulus of expansion is introduced. In this paper, the effective stress criterion of porous adsorption medium considering the effect of adsorption and expansion is established, and the nonlinear deformation problem of porous adsorption medium is simplified as the linear elastic deformation problem of porous non-adsorbed medium. The unity of effective stress coefficient is proved by unconstrained and uniaxial strain tests. (5) the adsorption modulus of expansion is an index to measure the difficulty of adsorption deformation in porous adsorption medium. The smaller the modulus of adsorption expansion is, the more obvious the adsorption expansion is at the same pore pressure. The experimental results show that at low pore pressure (7 MPA), the adsorption modulus of expansion of methane or carbon dioxide on coal is one order of magnitude smaller than that of solid particles. This side reflects the dominant effect of adsorption on deformation or permeability during gas production in low pore pressure reservoirs.
【学位授予单位】：中国矿业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TE31

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