Fracture spacing behavior in layered rocks subjected to different driving forces: a numerical study based on fracture infilling process

Natural layered rocks subjected to layer-parallel extension typically develop an array of opening-mode fractures with a remarkably regular spacing. This spacing often scales with layer thickness, and it decreases as extension increases until fracture saturation is reached. To increase the understanding of how these opening-mode fractures form in layered rocks, a series of 2D numerical simulations are performed to investigate the infilling process of fractures subjected to different driving forces. Numerical results illustrate that any one of the following could be considered as a driving force behind the propagation of infilling fractures: thermal effect, internal fluid pressure, direct extension loading, or pure compressive loading. Fracture spacing initially decreases with loading process, and at a certain ratio of fracture spacing to layer thickness, no new fractures nucleate (saturated). Both an increase in the opening of the infilled fractures and interface delamination are observed as mechanisms that accommodate additional strain. Interface debonding stops the transition of stress from the neighboring layers to the embedded central layer, which may preclude further infilling of new fractures. Whatever the driving force is, a large overburden stress and a large elastic contrast between the stiff and soft layers (referred to as a central or fractured layer and the top and bottom layers) are key factors favoring the development of tensile stress around the infilled fractures in the models. Fracture spacing is expected to decrease with increasing overburden stress. Numerical results highlight the fracturing process developed in heterogeneous and layered sedimentary rocks which provides supplementary information on the stress distribution and failure-induced stress redistribution., It also shows, in detail, the propagation of the fracture zone and the interaction of the fractures, which are impossible to observe in field and are difficult to consider with static stress analysis approaches.