超临界CO2压裂中气体运移和裂纹扩展耦合作用机理研究

Shale gas is a major strategic energy source in our country. Its high-efficiency development is of great significance to satisfy the growing natural gas demand and to promote energy conservation and emission reduction. However, the shale gas is buried deeply with high ground stress and extremely low permeability, and fracturing technology is required for exploration. There are numerous advantages of supercritical carbon dioxide fracturing, e. g., low breakdown pressure and high fracture density. Supercritical carbon dioxide fracturing is a potential efficient stimulation technology. In the fracturing process, fluid migration and shale-crack propagation are strongly coupled, however, the coupling mechanism has not been formed. There are staged achievements for crack propagation and gas transport, separately, but the understanding of the coupling between them has not yet been studied. This project is aimed at the key issues of supercritical carbon dioxide fracturing: (1) Microscopic mechanism of crack propagation in carbon dioxide/shale gas environment; (2) Migration of carbon dioxide through cracks to low permeability reservoir during fracturing; (3) rapid prediction model for crack propagation in the engineering scale on the basis of the former two steps. It is expected that crack propagation behaviors, such as the dominant mechanism transformation and crack tip stress singularity change, are obtained, and the breakthroughs made from this project would promote the efficient development of shale gas in our country in the aspects of rapid design and optimization of supercritical CO2 fracturing in shale gas and reduction of fracturing fluid usage.

页岩气是我国重大战略性能源，其高效开发对满足日益增长的天然气资源需求和促进节能减排具有重要意义。然而我国页岩气埋深较深，地应力高，渗透性低，需要压裂技术进行开采。超临界CO2压裂拥有起裂压力小、压裂缝网密度大等优势，是潜在的页岩气高效开采技术。压裂过程中存在流体运移和页岩裂纹扩展现象，但尚未形成对二者强耦合作用的认识。本项目拟对超临界CO2压裂重点研究：(1)CO2/页岩气环境下裂纹扩展的微观机制；(2)压裂过程中CO2通过裂纹面向低渗储层的运移；(3)基于前两个研究内容的超临界CO2压裂工程尺度的裂纹扩展快速预判模型。预期通过本项目的开展，获得主导机制和转变、裂纹尖端应力奇异性改变等裂纹扩展规律，在页岩气超临界CO2压裂的快速设计和优化、降低压裂液的使用量、提高页岩气的开采效率等方面的基础研究取得若干突破性成果，推进我国页岩气高效开发。

页岩气作为非常规地质资源的重要组成部分，是我国重大战略性能源。压裂开采是页岩气开发的重要技术手段，其基本难题是如何在页岩中形成大规模的交联裂纹网络；吸附在纳米级孔隙中的页岩气解吸附并运移出页岩孔隙困难。已有的研究表明，超临界CO2压裂技术既在页岩气解吸附中，可以获得优于其他气体的效率，其在压裂过程中和多孔介质的相互作用是多学科交叉的关键前沿问题。.项目组围绕超临界CO2压裂中气体运移和裂纹扩展耦合作用，研究了流体在孔隙-裂隙-多孔介质运移以及驱动固体断裂形成裂纹扩展的规律。在微观机理研究的基础上建立工程尺度的超临界CO2压裂数学力学模型，根据该模型分析了裂纹尖端奇异性阶次并校核了几种常用的裂纹扩展模型；基于修正的谱方法构建裂纹尖端应力奇异性阶次、裂纹尖端位置的高精度捕捉数值方法，设计了稳定、高效的瞬态求解器方案；探索了非均质储层内裂纹扩展受屏蔽的现象，定量地给出裂纹扩展屏蔽条件，为压裂工艺参数设计达到多条裂纹齐头并进提供了定量的理论参考；基于物理界面建立了双重介质有限裂隙渗流模型，实现统一的渗流场处理孔隙-裂隙-多孔介质流动问题；建立工程尺度的裂纹扩展快速预判模型，获得裂纹扩展中各物理量的变化规律，在现场施工实时设计压裂工艺参数中具有重要参考价值。以上研究成果为提高页岩气等非常规地质资源采收率方面提供理论、计算和实验指导。