页岩气藏缝网系统内CO2/CH4双组分扩散流动机理与表征研究

Shale gas reservoir has obvious characteristics of low porosity and permeability. After fracturing, natural energy leads to low recovery. Enhanced gas recovery by CO2 injection is a potential substitute technology. At present, the injection, storage and enhanced gas recovery capacity of CO2 is still not evaluated scientifically and accurately, which restricts its field application. A clear understanding of the diffusion flow mechanism of CO2/CH4 binary components in fracture network system is the key issue to solve the above problems. In this project, the transport mechanism of CO2/CH4 in single micropore/nanopore is studied based on the molecular dynamics simulation. The interaction principle between each component and confined space is revealed. The transport principle of adsorbed phase and bulk phase fluid is clarified; then considering micro-scale effect, the pore network model constrained by mineral phase is reconstructed and the binary components pore-scale flow model is developed. Based on the pore-scale flow simulation and core displacement mechanism experiment, the prediction technology of apparent permeability is formed, which provides key parameters for matrix flow characterization. Finally, the coupling flow characteristics between matrix, natural fracture and hydraulic fracture are studied, and the binary components seepage characterization model in fracture network system based on projection embedded discrete fracture method (Projection-based EDFM) is formed. The new method will overcome the defect of EDFM non-physical simulation, and the non-linear seepage mechanism of fluid is analyzed. The study provides theoretical guidance for shale gas production performance prediction and development plan designing.

页岩气藏低孔、低渗特征明显，压裂后依靠天然能量开发采收率低，注CO2强化开采是具有潜力的接替技术。而目前CO2的注入、储集、提采能力等仍无法科学准确评价，制约了其现场应用。清楚掌握缝网系统内CO2/CH4双组分扩散流动机理是解决上述问题的关键。本课题基于分子动力学模拟研究微纳米单一孔隙内CO2/CH4扩散流动机制，揭示各组分与受限空间的相互作用原理，阐明吸附相和体相流体传输规律；考虑微尺度效应，构建矿物质相约束下的孔隙网络模型，通过两组分孔隙级流动模拟及岩心驱替机理实验，形成表观渗透率的预测技术，为宏观基质流动表征提供关键参数；在此基础上，研究基质-天然裂缝-水力裂缝间的耦合传输特征，形成基于投影嵌入式离散裂缝方法的裂缝网络系统内双组分渗流表征模型，新方法将克服传统EDFM非物理模拟的缺陷，并据此探究双组分非线性渗流机理。为页岩气生产动态预测、开发方案制定提供理论指导。

页岩气藏低孔、低渗特征明显，需要通过水平井钻井和水力压裂技术进行开采，将CO2注入衰竭页岩气藏可以实现CO2有效封存。但由于复杂裂缝网络系统中流体流动空间多样，对基质中的扩散流动机制缺乏清晰的认识，基质-裂缝耦合渗流模型无法建立，页岩气井CO2/CH4两组分流动动态无法预测。本项目聚焦CO2/CH4在基质中的传输行为和CO2/CH4两组分在复杂裂缝网络内的扩散流动表征等问题开展研究，分析了页岩孔隙中气体的多种传输机制，构建了两组分混合条件下的体相流体、吸附气体扩散流动表征模型，提出了两组分渗流问题的求解策略，探讨了CO2/CH4两组分流动特征，明确了基质流动关键参数的影响机制。基于微地震划分了复杂缝网形态，建立了不同缝网形态的表征方法，明确了水力裂缝-次级裂缝-基质的空间多尺度渗流耦合机制，通过扩展投影嵌入式离散裂缝模型，形成了不同缝网系统的投影嵌入式离散裂缝模型数值模拟方法。基于矿场尺度下CO2/CH4两组分渗流机制，构建了基质和裂缝耦合的宏观流动模型，形成了矿场尺度CO2/CH4两组分数值模拟方法，分析了复杂缝网条件下页岩气藏衰竭开采特征和CO2封存能力。通过上述成果的建立，完善了页岩储层缝网系统内复杂流动的基础理论体系，研究成果可以用于页岩气藏CO2可注入性评价和强化开采技术的可行性分析，同时，也能够为其他非常规油气资源压裂开发中的复杂流动建模数模问题的解决提供借鉴。

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