富有机质页岩延性提高对水力压裂裂缝网络发育的影响

Chang Huang, Shengli Chen
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引用次数: 0

摘要

尽管人们普遍认为页岩延展性的增加会阻碍水力裂缝的扩展,并在关闭注入时促进已形成裂缝的愈合,但目前还没有很好的定量解释富有机质页岩中水力压裂的困难。本研究旨在利用先进的基于xfem的模拟器,定量研究延展性增加对增产储层体积(SRV)的影响。为了实现这一目标,一个改进的内聚区模型已经集成到内部的全耦合孔隙弹性XFEM框架中。该研究继续通过扩展数值框架的功能来模拟多个相互作用的裂缝。该框架在开发过程中使用了面向对象的编程范式,通过创建更多的裂缝段实例,可以很容易地扩展到包含多裂缝网络。因此,具有任意数量相交分支的主水力裂缝可以建模。通过改变四个相关的材料参数,将进行一系列的参数研究,以探讨延性增加对诱导SRV的影响。修正黏聚区模型本质上是一种牵引-分离规律(TSL),其特征为4个参数:初始抗拉强度Tini、极限抗拉强度Tkrg、临界分离Dc和最终裂纹分离Dmax。它可以灵活地模拟不同的裂缝张开情况,更真实地模拟增加的页岩延性。采用最新的半解析解对全耦合多孔弹性XFEM框架进行了全面验证。数值计算结果表明,在裂纹内聚能和抗拉强度相同的情况下,TSL的形状会影响主水力裂缝的扩展和裂缝网络的演化。由此推断,塑性不仅受黏聚裂纹能和抗拉强度的控制,进一步说明了新提出的黏聚区模型的必要性。在所有4个TSL参数中,初始抗拉强度的大小对断裂长度和SRV的影响最大,而初始抗拉强度的大小控制着内聚裂纹何时开始扩展。这项研究的新奇之处在于两个方面。首先,新修正的黏结带模型能更真实地反映页岩延性的增加。其次,先进的XFEM框架可以模拟裂缝网络,可以研究延性增加对整个SRV的影响,而不是单个裂缝。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
The Impacts of Increased Ductility in Organic-Rich Shale on Fracture Network Growth by Hydraulic Fracturing
The difficulty of hydraulic fracturing in organic-rich shale caused by the increased ductility has not been well interpreted quantitatively, although it is well perceived that the increased shale ductility can impede the propagation of hydraulic fractures and enhance the healing of created fractures upon injection shutdown. This study aims to quantitatively study the impacts of increased ductility on the stimulated reservoir volume (SRV) using an advanced XFEM-based simulator. To achieve this goal, a modified cohesive zone model has been integrated into an in-house fully coupled poroelastic XFEM framework. The study continues by extending the functionality of the numerical framework to simulating multiple interacting fractures. The utilization of the object-oriented programming paradigm in the development of the framework makes it an easy extension to include the multi-fracture network by creating more instances of crack segments. A main hydraulic fracture with an arbitrary number of intersected branches can thus be modeled. A series of parametric studies will be conducted to investigate the impacts of increased ductility on the induced SRV by varying four involved material parameters individually. The modified cohesive zone model, which is essentially a traction-separation law (TSL), is characterized by four parameters: the initial tensile strength Tini, ultimate tensile strength Tkrg, the critical separation Dc, and the final crack separation Dmax. It can flexibly model different crack opening scenarios and simulate more realistically the increased shale ductility. The fully coupled poroelastic XFEM framework has been comprehensively verified against the latest semi-analytical solutions on the four well-known propagation regimes. The numerical results show that the shape of TSL does affect the main hydraulic fracture growth as well as the evolvement of the fracture network, given the same cohesive crack energy and tensile strength. It infers that ductility is not only controlled by cohesive crack energy and tensile strength, which further indicates the necessity of the newly proposed cohesive zone model. The magnitude of the initial tensile strength, controlling when the cohesive crack starts propagating, is found to have the greatest impacts on the fracture length, and SRV, among all four TSL parameters. The novelty of this study is two-fold. First, the newly modified cohesive zone model can more realistically represent the increased shale ductility. Second, the advanced XFEM framework that enables the simulation of a fracture network can study the impacts of increased ductility on the whole SRV but not a single crack.
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