[论文解读] Three-Dimensional Numerical Modeling of Shear Stimulation of Naturally Fractured Rock Formations

Eren Ucar, Inga Berre+1 more|arXiv (Cornell University)|Sep 6, 2017

Hydraulic Fracturing and Reservoir Analysis被引用 2

一句话总结

本研究提出了一种3D数值模型，耦合流体流动、裂缝变形和岩石基质弹性，以模拟天然裂缝性地热储层中的剪切刺激。该模型揭示，裂缝与基质之间的流体交换显著影响渗透率演化和地震活动，对优化刺激策略同时最小化诱发地震具有重要意义。

ABSTRACT

Shear dilation based hydraulic stimulations enable exploitation of geothermal energy from reservoirs with inadequate initial permeability. While contributing to enhancing the reservoir's permeability, hydraulic stimulation processes may lead to undesired seismic activity. Here, we present a three dimensional numerical model aiming to increase understanding of this mechanism and its consequences. The fractured reservoir is modeled as a network of explicitly represented large scale fractures immersed in a permeable rock matrix. The numerical formulation is constructed by coupling three physical processes: fluid flow, fracture deformation, and rock matrix deformation. For flow simulations, the discrete fracture matrix model is used, which allows the fluid transport from high permeable conductive fractures to the rock matrix and vice versa. The mechanical behavior of the fractures is modeled using a hyperbolic model with reversible and irreversible deformations. Linear elasticity is assumed for the mechanical deformation and stress alteration of the rock matrix. Fractures are modeled as lower dimensional surfaces embodied in the domain, subjected to specific governing equations for their deformation along the tangential and normal directions. Both the fluid flow and momentum balance equations are approximated by finite volume discretizations. The new numerical model is demonstrated considering a three dimensional fractured formation with a network of 20 explicitly represented fractures. The effects of fluid exchange between fractures and rock matrix on the permeability evolution and the generated seismicity are examined for test cases resembling realistic reservoir conditions.

研究动机与目标

• 开发一种3D数值模型，以捕捉天然裂缝性储层中流体流动、裂缝变形和岩石基质力学的耦合行为。
• 研究高渗透率裂缝与周围岩石基质之间流体交换如何影响水力刺激过程中渗透率的演化。
• 评估在低渗透率裂缝性构造中剪切刺激过程产生的地震活动。
• 为优化刺激策略提供洞见，以增强渗透率同时最小化诱发地震风险。

提出的方法

• 将裂缝性储层建模为嵌入在渗透性岩石基质中的显式3D裂缝网络。
• 采用离散裂缝-基质模型模拟流体流动，实现裂缝与基质之间的物质传输。
• 采用双曲定律模拟裂缝变形，考虑可逆与不可逆的剪切和法向位移。
• 岩石基质变形由线性弹性定律控制，域内应力变化计算保持一致。
• 动量方程和流体流动方程采用有限体积法进行离散化，以实现精确的数值求解。
• 该模型应用于包含20个显式建模裂缝的3D域，模拟真实储层条件下的行为。

实验结果

研究问题

• RQ1 裂缝与岩石基质之间的流体交换在剪切刺激过程中如何影响渗透率的时空演化？
• RQ2 裂缝变形机制（可逆与不可逆）在控制渗透率增强方面发挥何种作用？
• RQ3 岩石基质中的应力变化如何影响刺激过程中地震事件的发生概率与分布？
• RQ4 裂缝网络几何形态与连通性对流体流动和诱发地震有何影响？

主要发现

• 裂缝与岩石基质之间的流体交换显著改变了刺激过程中渗透率的时空演化特征。
• 裂缝的不可逆剪切变形对永久性渗透率增强的贡献大于可逆弹性变形。
• 由于流体注入引起的岩石基质中应力变化可能触发地震事件，尤其在裂缝交汇处和高应力梯度区域。
• 该模型预测，增强的流体交换可促进更均匀的渗透率增长，减少局部应力集中。
• 模拟结果表明，裂缝网络的连通性显著影响诱发地震的强度与分布。
• 流体流动、裂缝力学与岩石基质弹性的耦合对于准确预测刺激结果至关重要。