Apparent Toughness Anisotropy Induced by Roughness of in Situ Stress: A Mechanism that Hinders Vertical Growth of Hydraulic Fractures and Its Simplified Modeling

P. Fu, Ji-xiang Huang, R. Settgast, J. Morris, F. Ryerson
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引用次数: 8

Abstract

A hydraulic fracture's height growth is known to be affected by many factors that are related to the layered structure of sedimentary rocks. While these factors are often used to qualitatively explain why hydraulic fractures usually have well-bounded height growth, most of them cannot be directly and quantitatively characterized for a given reservoir to enable a priori prediction of fracture height growth. In this work, we study the role of the "roughness" of in situ stress profiles, namely alternating low- and high-stress among rock layers, in determining the tendency of a hydraulic fracture to propagate horizontally versus vertically. We found that a hydraulic fracture propagates horizontally in low-stress layers ahead of neighboring high-stress layers. Under such a configuration, a fracture mechanics principle dictates that the net pressure required for horizontal growth of high-stress layers within the current fracture height is significantly lower than that required for additional vertical growth across rock layers. Without explicit consideration of the rough stress profile, the system behaves as if the rock is tougher against vertical propagation than it is against horizontal fracture propagation. We developed a simple relationship between the apparent differential rock toughness and characteristics of the stress roughness that induce equivalent overall fracture shapes. This relationship enables existing hydraulic fracture models to represent the effects of rough in situ stress on fracture growth without directly representing the fine-resolution rough stress profiles.
地应力粗糙度诱导的表观韧性各向异性:阻碍水力裂缝垂直扩展的机制及其简化建模
水力裂缝的高度增长受到许多因素的影响,这些因素与沉积岩的层状结构有关。虽然这些因素通常被用来定性地解释为什么水力裂缝通常具有良好的高度增长,但对于给定的油藏,大多数因素无法直接和定量地表征,从而无法对裂缝高度增长进行先验预测。在这项工作中,我们研究了原位应力剖面的“粗糙度”,即岩层之间交替的低应力和高应力,在确定水力裂缝水平与垂直传播趋势中的作用。研究发现,水力裂缝在低应力层中水平扩展,超前于邻近的高应力层。在这种配置下,裂缝力学原理表明,在当前裂缝高度内,高应力层水平扩展所需的净压力明显低于跨越岩层的额外垂直扩展所需的净压力。如果没有明确考虑粗略的应力分布,系统的表现就好像岩石在垂直方向上比在水平方向上更坚硬。我们在表观差异岩石韧性和应力粗糙度特征之间建立了一个简单的关系,从而导致等效的整体断裂形状。这种关系使得现有的水力裂缝模型能够在不直接表示精细分辨率粗糙应力剖面的情况下,表征粗糙地应力对裂缝生长的影响。
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