One of the parameters that govern the interaction between hydraulic fractures (HF) and natural fractures (NF) is the coefficient of friction between the two NF-surfaces (Gu and Weng, 2010). Therefore, the characterization of natural fractures is important when analyzing this type of interaction in both numerical and experimental analyses. In this study, we conducted a set of Direct Shear Tests on Opalinus Clayshale specimens employing a shear box to obtain the coefficient of friction and cohesion of the sliding surfaces. Two conditions were tested: vertical and horizontal bedding planes, i.e. perpendicular and parallel to the fracture surface. Each condition was analyzed at different polishing levels of the natural fracture's surfaces, using sandpapers with average particle diameters ranging from 201 μm (N80) to 25.8 μm (N600). Also, two horizontal displacement rates were applied: slow and fast. We observed that the coefficient of friction in Opalinus Clayshale varies between 0.1 and 0.65 depending on the degree of polishing. The displacement rate practically does not affect the coefficient of friction but the bedding planes' direction does. Specimens with the same polishing grade have a higher coefficient of friction for the vertical bedding planes. Moreover, in this configuration, higher surface wear is observed in the layers of the Opalinus Clayshale with higher resistance (light layers). This can be explained because lower resistance layers (darker ones) have more material removed during the polishing process, exposing the higher resistance layers.

1. Introduction

The interaction between hydraulic fractures and preexisting natural fractures is important when analyzing the fracture networks generated in a reservoir. This problem is of particular interest for reservoirs that have stress regimes different than the normal (i.e., strike-slip and reverse regimes) in unconventional oil and gas operations (García et al., 2013). In these cases, hydraulic fractures tend to propagate in a direction different from the vertical, which is not desirable from an operational point of view (less interaction with the reservoir) (García et al., 2013).

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