|Place of Conferral||北京|
|Keyword||水力压裂 固-液界面 全应力模型 应力奇异性 流体滞后区|
我国页岩气储量丰富, 已探明的页岩气储量居世界首位, 页岩气等非常规能源开发是国家重要培育方向. 但是国内页岩气藏储层呈低孔隙度、低渗透率的物性特征, 需要更有效的水力压裂以改善页岩气在岩石中的渗透性质, 开采难度远大于其他国家. 水力压裂技术难以在页岩中形成大规模的交联裂纹网络, 是我国页岩气开采中存在的基本难题之一. 水力压裂是流-固耦合作用下强非线性的固-液界面问题. 从力学角度出发, 如何控制裂纹扩展方式、合理设计压裂参数, 需要厘清裂纹扩展规律. 即使将岩石看作不可渗透的均匀介质, 在裂纹扩展中, 岩石断裂和流体流动相耦合, 仍然形成宏观多特征尺度问题. 水力压裂伴随的主导因素转变、应力奇异性的改变、流体滞后区的演化、三相接触线的移动和高地应力增加了问题的求解难度.
正是在此背景下, 本论文采用理论分析并辅助以数值计算的手段, 主要针对两个核心学术问题: 耦合全应力分量下水力压裂的裂纹扩展规律; 水力压裂过程中固-液界面的渐近性质、应力奇异性、流体滞后区等独特性质, 开展研究工作.
在固-液界面上耦合压裂液流动引起的对称压强与切应力, 建立了固-液界面耦合全应力的幂律流体水力压裂模型. 传统的水力压裂模型仅考虑流体压强作用下的流-固耦合, 然而固-液界面上的切应力奇异性强于压强, 并且以压裂液黏度的实验室测量值直接用于压裂设计的方法往往导致切应力被低估. 该模型提供更准确的水力压裂设计和模拟方案.
厘清了幂律流体压裂中主导机制和裂纹尖端应力奇异性的转变. 从时间角度出发, 通过标度分析得到幂律流体水力压裂模型的主导机制转变和相应的无量纲控制参数; 从空间角度出发, 使用渐近分析得到裂纹尖端应力场的奇异性, 进一步讨论和修正了裂纹扩展判据.
探索并发现了切应力主导机制. 切应力、压强、地应力和表面张力在裂纹尖端附近的竞争由反映各力的无量纲参数表现出来, 切应力主导机制出现在裂纹扩展的初期. 在此主导机制下, 通过渐近分析发现裂纹沿直线连续扩展的假设与裂纹扩展条件相悖, 水力压裂裂纹扩展出现失稳, 由此获得了失稳判据, 并且得到本工作发展的高精度数值计算的验证.
裂纹尖端流体滞后区的尺寸估计及其对水力压裂的作用. 流体滞后区和裂纹长度之比在地应力的作用下随着压裂过程逐渐减小, 提出了滞后区可忽略的特征裂纹长度, 并获得了滞后区含滞后区和无滞后区情况下的尺寸估计. 发展了一套不依赖于精确初始条件的水力压裂数值计算方案, 发现由无滞后区模型得到的切应力主导机制仍然适用于含流体滞后区的情况.
本论文系统地探索了固-液界面及裂纹尖端应力奇异性在水力压裂过程中的变化和机理, 为解决页岩气开采中存在的基本难题提供了理论基础, 并为实现控制裂纹扩展方式、合理设计压裂参数、最终提高实际生产提供理论指导.
Ranking first in the world, China is rich in the shale-gas reserves. The development of unconventional energy sources including shale gas is one of the national major projects. However, shale-gas reservoirs in China are of low porosity and permeability, making it much harder to exploit in China than in other countries. Effective hydraulic fracturing technology is required to improve the permeability of the shale gas in rocks. One of the basic problems in the development of shale-gas reservoirs is to form a large-scale crack network in shale by using the hydraulic fracturing technology. Hydraulic fracturing is a strongly non-linear solid-liquid-interface problem. In view of mechanics, Understanding the crack growth behaviors is essential to control the crack path and to reasonably design fracturing treatments. Even if the rock is modeled as an isotropic, homogeneous, impermeable and elastic full space, multi-scale problems are formed in light of the coupling between the fracture of rock and the flow of viscous fracturing fluid. Dominant-regime transitions, stress singularity alteration, the variation of the fluid-lag zone, moving contact line and high crustal stress increase the difficulty of solving this problem.
In this context, adopting theoretical analysis and numerical calculations, this dissertation focuses on two key academic issues: the crack growth behaviors of hydraulic fracturing with considering the full-stress coupling at the solid-liquid interfaces; the asymptotic properties of the solid-liquid interfaces, the stress singularities and fluid-lag zone near the crack tip.
A full-stress model is established to account for the combined effect of pressure and flow-induced shear stress at the solid-fluid interfaces. The fracturing fluid is modeled with a power-law rheology. Only pressure is considered at the solid-liquid interface in traditional hydraulic fracturing models. But it is proved the singularity of shear stress is stronger than that of the pressure, and the effect of shear stress is much underestimated when the laboratory-measured viscosity is used in the design of hydraulic fracturing. This model provides a more accurate scheme for the design and simulation of hydraulic fracturing.
It is clarified the dominant-regime transition and stress singularity alteration with a power-law fracturing fluid. From the perspective of time, dominant-regime transitions of the full-stress model and the corresponding dimensionless control parameters are obtained through the scale analysis. From the perspective of space, the stress singularity is obtained by using the asymptotic analysis, and the crack propagation criteria are further discussed and modified.
Shear-stress-dominant regime is discovered. The competition among shear stress, pressure, crustal stress, and surface tension near the crack tip is represented by the dimensionless parameters. The shear-stress-dominant regime appears in the initial stage of crack growth. In this regime, it is found, through the asymptotic analysis, that the assumption that the crack grows continuously and straightly is contrary to the crack propagation condition, and the growth of a hydraulic fracturing crack may be unstable. This phenomenon is validated and the instability criterion is obtained through the high-precision numerical scheme developed in this work.
The size of the fluid-lag zone at the crack tip is estimated and its effect on hydraulic fracturing is discussed. The ratio of the fluid-lag zone to the crack length decreases gradually in the fracturing process due to the in-situ stress, and a characteristic crack length is derived for the fluid-lag zone being negligible. The size is estimated under the condition for there being either a crustal stress or not. A numerical scheme for the full-stress model with a fluid-lag zone is developed, and the scheme does not rely on the initial conditions. With this numerical scheme, unstable crack growth is found indicating that the shear-stress-dominant regime is still applicable to the problem with a fluid-lag zone.
In this dissertation, the changes and mechanisms of the stress singularity and the solid-liquid interface near the crack tip are systematically explored during the hydraulic fracturing process. This study provides a theoretical basis for solving the basic problems in the shale-gas exploitation and controlling the crack propagation. Potential theoretical guidance is provided for the design of hydraulic fracturing treatments.
|沈文豪. 水力压裂中固-液界面及裂纹尖端应力奇异性研究[D]. 北京. 中国科学院大学,2018.|
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