Abstract
In layered materials, the deformation style, orientation, confinement, and 3D connectivity of natural fractures is generally impacted by changes in sedimentary facies and alternations in mechanical properties. In this study we address this effect and perform a numerical sensitivity analysis. Mechanical properties, confining pressures, and interfacial frictions are varied for a three-layered model, to investigate and quantify the relation between contrasting material properties, the principal horizontal stresses and fracture behaviour (i.e. deformation style and orientation). Firstly, the results show that tensile stresses develop in the stiffer layers due to the contrasting elastic parameters. The magnitude of these stresses is dependent on the ratio between the elastic parameters of stiffer and softer layers (i.e. Estiff/Esoft and νstiff/νsoft). There are no horizontal tensile stresses, when applying a compressive horizontal confining pressure (approx. 1/5 of the applied vertical stress). Implementing an interfacial friction lower than 0.2 will result in decoupling of the layers, resulting in slip on the layer boundaries and no tensile stresses within the stiffer layers. Further, the acquired numerical results are in good agreement with previously conducted laboratory work. Finally, we discuss whether the presented results can be used for better relating contrasting mechanical properties to potential fracture deformation styles and orientations in layered outcrops or subsurface reservoirs.
Highlights
As the available high permeable reservoirs for petroleum and geothermal exploitation are declining, exploration companies generally turn to less permeable, unconventional reservoirs
Laboratory experiments performed by Douma et al (2019) and nu merical experiments performed by Guo et al (2017) showed that in layered materials, fractures initiate within the weaker brittle layers, showing mode II behaviour. These studies showed that these mode II fractures propagated in to the stiffer brittle layers showing mode I behaviour. This observation can in part explain why mode I failure occurs under σh=Brittle Tensile Strength (BTS) ratios lower than 1.0, since following from Linear Elastic Fracture Mechanics (LEFM) (Irwin, 1957) tensile stresses are localized in close proximity to crack tips
Our results indicate that the magnitude of the modelled tensile stress is dependent on the difference between the implemented mechanical parameters
Summary
As the available high permeable reservoirs for petroleum and geothermal exploitation are declining, exploration companies generally turn to less permeable, unconventional reservoirs. Apart from contrasting mechanical parameters, laboratory and nu merical studies showed that the interfacial friction and applied confining pressures have a high impact on the horizontal stress distri bution and fracture behaviour. The above stated field, laboratory, and numerical studies highlight that the relation between the mechanical stratigraphy, observed fracture behaviour and horizontal stress changes is well documented. These studies have yet not addressed the range of conditions at which this different stress and fracture behaviour occurs. Fracture deforma tion style, orientation and confinement will be referred to as fracture behaviour
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