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Numerical Modeling of Fracture Network Evolution in Organic‐Rich Shale With Rapid Internal Fluid Generation
Journal of Geophysical Research: Solid Earth ( IF 3.9 ) Pub Date : 2020-06-03 , DOI: 10.1029/2020jb019445
Ole Rabbel 1 , Karen Mair 1 , Olivier Galland 1 , Carina Grühser 2 , Tobias Meier 2
Affiliation  

When low‐permeability and organic‐rich rocks such as shale experience sufficient heating, chemical reactions including shale dehydration and maturation of organic matter lead to internal fluid generation. This may cause substantial pore fluid overpressure and fracturing. In the vicinity of igneous intrusions emplaced in organic‐rich shales, temperatures of several hundred degrees accelerate these processes and lead to intense fracturing. The resulting fracture network provides hydraulic pathways, which allow fluid expulsion and affect hydrothermal fluid flow patterns. However, the evolution of these complex fracture networks and controls on geometry and connectivity are poorly understood. Here, we perform a numerical modeling study based on the extended finite element method to investigate coupled hydromechanical fracture network evolution due to fast internal fluid generation. We quantify the evolution of different initial fracture networks under varying external stresses by analyzing parameters including fracture length, opening, connectivity, and propagation angles. The results indicate a three‐phase process including (1) individual growth, (2) interaction, and (3) expulsion phase. Magnitude of external stress anisotropy and degree of fracture alignment with the largest principal stress correlate with increased fracture opening. We additionally find that although the external stress field controls the overall fracture orientation distribution, local stress interactions may cause significant deviations of fracture paths and control the coalescence characteristics of fractures. Establishing high connectivity in cases with horizontally aligned initial fractures requires stress anisotropy with σ V  > σ H , while the initial orientation distribution is critical for connectivity if stresses are nearly isotropic.

中文翻译:

快速内部流体生成的富含有机质页岩裂缝网络演化的数值模拟

当低渗透率和富含有机物的岩石(如页岩)受到足够的加热时,包括页岩脱水和有机物成熟在内的化学反应会导致内部流体的产生。这可能会导致大量孔隙流体超压和破裂。在富含有机物的页岩中的火成岩侵入附近,几百度的温度加速了这些过程,并导致了强烈的压裂。最终的裂缝网络提供了液压通道,从而允许流体排出并影响热液流体流动方式。然而,人们对这些复杂的裂缝网络的演化以及对几何形状和连通性的控制了解甚少。这里,我们基于扩展有限元方法进行了数值建模研究,以研究由于内部流体快速生成而引起的耦合流体力学裂缝网络演化。我们通过分析包括裂缝长度,开度,连通性和传播角度在内的参数来量化在变化的外部应力下不同初始裂缝网络的演化。结果表明,该过程分为三个阶段,其中包括(1)个人成长,(2)相互作用和(3)驱逐阶段。外应力各向异性的大小和最大主应力的裂缝排列程度与裂缝开度增加有关。我们还发现,尽管外部应力场控制着整体裂缝取向分布,局部应力相互作用可能会导致裂缝路径的明显偏离,并控制裂缝的合并特征。在水平排列的初始裂缝中建立高连通性需要应力各向异性σ V  >  σ ħ,而初始取向分布为连接至关重要的,如果应力接近各向同性。
更新日期:2020-07-05
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