裂隙页岩气藏多尺度扩散渗流理论研究

The combination of horizontal well drilling and staged hydraulic fracturing is the key technology for the volumetric treatment of shale gas reservoirs. Through this technology, the gas formation is broken into small blocks to form a fractured gas reservoir. The gas transport mechanism in the fractured reservoir decides the production decline curve and thus is the key issue for gas recovery enhancement as well as the production well life. However, this gas transport mechanism in the multi-scale fractured shale gas reservoirs is not clear so far. This proposal is to investigate the gas transport mechanism within a fractured shale gas formation through the extension of my previously proposed conceptual model that a fractured shale reservoir is composed of fracture network and shale matrix. The flow in the fracture network follows Non-Darcy law and gas transport in the matrix observes non-linear gas diffusions. A micro non-linear diffusion model will be proposed to develop the non-linear apparent diffusion coefficient for the gas transport in the matrix. This diffusion model can describe the Darcy flow in kerogen pore channels, molecular diffusion or convection, Knudsen flow in the nano-pore channels and transitional flow between as well as the surface diffusion in the Langmuir sorption layer, and the heterogeneous distribution of organic and inorganic matters in the matrix. In addition, a micro-pore model is to be reconstructed through micro-CT and SEM images and fractal analysis. The methane flow within this micro-pore model will be numerically simulated by molecular dynamics and lattice Boltzmann method and the apparent diffusion coefficient of the micro non-linear diffusion model will be validated. A diffusion time, which is the product of apparent diffusion coefficient and the characteristic dimension of fractured blocks, is introduced into the conceptual model to bridge the micro-flow in matrix and macro-flow in fracture network. The impact of flow consistency between macro Darcy flow and micro diffusion on gas well production decline curves from the fractured gas reservoir is investigated and key parameters to affect the production decline curve are explored. It is expected that the outcome of this research can establish a rough guideline for the design of horizontal well drilling and staged hydraulic fracturing.

体积压裂改造能提高页岩气藏采收率和延长抽采寿命，但改造后产能预测仍是世界难题，其裂隙页岩气藏中气体多尺度运移规律是亟待突破的核心科学问题。针对这气体多尺度运移特征，本研究提出一种裂隙气藏扩散渗流理论，考虑宏观裂隙中非达西流与微细观基质中非线性扩散；在有机质中考虑微细观气体自由扩散与对流、孔隙内分子扩散、吸附层表面扩散及吸附解吸等相互作用，在无机质中考虑吸附和渗透，从而建立适合页岩气纳米孔隙的细观非线性扩散模型；通过微细观实验，重构页岩微结构，并进行分子动力学或LBM数值模拟，探讨气体微观储藏和运移机理，验证细观扩散模型；将细观扩散模型嵌入裂隙气藏渗流理论，探讨产能和宏细观流协调性的关系和影响产能的关键因素。本项目研究成果将揭示裂隙页岩气藏中气体多尺度流动机理和宏细观流动协调性对采收率影响，为优化页岩气藏体积压裂改造设计、合理评价改造效果及产能预测等奠定理论基础。

页岩气藏是裂隙网络和储藏基质组成的非均匀裂隙页岩体，具有空间和时间多尺度特性，直接影响页岩气井的产能。本项目研究了裂隙页岩气藏的非常规渗流理论，取得如下四项成果：1，提出和完善多尺度渗流理论。通过对裂隙页岩体的多尺度和多流态特性分析，推导出气体扩散系数的解析表达式，求得三区压裂水平井的产气率半解析解；提出软-硬弹簧模型，描述裂隙和基质的非均匀变形和相互作用，建立不同区域的流固耦合方程；提出单元尺度的裂隙张量理论并建立等效的多尺度渗流-应力耦合连续介质模型，研究了裂隙弯曲、剪胀和法向开闭对页岩气生产速率的影响；设计参数优化程序，揭示页岩气藏多尺度渗流的协同机理，确定体积改造的关键参数。2，提出基质中气体储藏运移模型，描述气体微观流动和微观孔隙结构的相互作用。考虑页岩气微观储藏和流动的物理机理如气体吸附解吸、表面扩散、Knudsen扩散、滑移效应等，建立了页岩基质孔隙表观渗透率模型，研究了页岩气多层吸附的分形FHH模型和BET模型，并提出了对应的多层吸附表面扩散模型，修正了我们先前提出的表观渗透率。3，建立LBM数值计算模型，模拟气体在纳米级页岩基质微观孔隙结构中的流动，探讨页岩气在纳米级孔隙中的储藏、吸附解吸与扩散运移规律，从细观上探索纳米级孔隙壁表面吸附、吸附层扩散和对流、以及纳米孔隙中的扩散、对流机理对基质扩散能力的贡献，从而验证基质中气体储藏运移模型的正确性和可靠性。4，完成观察细观结构及其演化的物理试验，奠定数值模拟的试验基础。完成重庆页岩试样的基本物理特性和部分力学特性测试试验，包括页岩微尺度结构特征实验（XRD、SEM电镜扫描、压汞实验和液氮吸附实验）和层理页岩三点弯曲试验，开展小尺度页岩试样热破裂细观试验，测试微孔隙和裂隙的分布规律，探讨页岩热破裂过程中渗透率演化与热破裂机理研究。基于以上研究，发表SCI论文25篇，国际会议论文3篇，出版专著1部，培养博士生6人，硕士生3人。