Abstract
Hydraulic fracturing is a necessary technique for shale gas exploitation. In order to have efficient stimulation treatment, a complex fracture network has to be developed, whereas with rich bedding planes and natural fractures, the mechanism of forming a fracture network is not fully understood and it is so tricky to predict propagation and initiation of hydraulic fracture. Therefore, in this paper, considering the strong anisotropy of shale reservoir, numerical simulation has been conducted to analyze fracture propagation and initiation on the basis of finite element and damage mechanics. Simulation results indicate that hydraulic fracture is not merely controlled by in situ stress due to strong anisotropy in shale. With plenty of bedding planes, hydraulic fracture tends to have initiation and propagation along the bedding plane. In particular, this influence becomes stronger with low strength and high development density of bedding planes. Additionally, in combination with natural fracture and bedding plane, the initiation point is usually on a natural fracture plane, causing relatively small breakdown pressure. In the process of fracture propagation, hydraulic fracture connects with natural fractures and bedding planes, forming dendritic bifurcation and more complicated paths. Numerical simulation proves that bedding plane and natural fracture are vital factors of hydraulic fracture. Compared to natural fracture, the bedding plane has a stronger impact on hydraulic fracture propagation. For the initiation of hydraulic fracture, natural fracture is the major effecting factor. The outcome of this study is able to offer theoretical guidance for hydraulic fracturing in shale.
Highlights
Shale gas is potential unconventional gas in the world
This research gives a new approach for analyzing hydraulic fracture in shale, and its findings can offer a reference for hydraulic fracturing in shale reservoirs
In this research, according to the characteristic of shale reservoir, influences of the bedding plane and natural fractures on the hydraulic fracture around injection wellbore are analyzed
Summary
Shale gas is potential unconventional gas in the world. The success of shale gas development in America makes shale gas become a hotspot in petroleum engineering [1, 2]. Blanton [33] and Teufel and Clark [34] all found that the interaction between hydraulic fractures and natural fractures is associated with stress state and approach angle and pointed out that the weak plane with small friction coefficient or low cohesion is likely to modify the propagation direction. These conclusions are proved by Zhou et al [35] who analyzed the impact of weak plane strength on hydraulic fracture by building layered medium with paper sheets and cement block. This research gives a new approach for analyzing hydraulic fracture in shale, and its findings can offer a reference for hydraulic fracturing in shale reservoirs
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