Multiple friction tests under low load conditions: application to planar fractures in a shale reservoir

Knowledge of the friction coefficient on fracture surfaces within tight reservoirs, geological storage systems, and the seals of conventional reservoirs, is increasingly important in the geomechanical analysis of fracturing treatment and the assessment of seal integration. We measured the static friction coefficient (µ) of 118 samples of bedding fractures and 52 samples of tectonic fractures taken from a borehole core from the depth interval of 3-4 km in the early Paleozoic Baltic Basin shale reservoir (North Poland). In total, 3372 measurements of µ were made. Based on repeated measurements under the same loading and wetting conditions, the repeatability of the data obtained was evaluated. A significant reduction of the µ value with increasing load was determined, due to the wetting of fracture surfaces. The gradient of µ value decrease caused by the increase in load was compared with predictions for realistic ranges of parameters estimated on the basis of Barton's empirical law. For groups of fractures with similar µ values, the trends are comparable with Barton's predictions and the gradients systematically decrease with decreasing µ. This allowed a rough extrapolation of values measured under very low-load reservoir conditions.