Natural rock fractures can be tensile, shear, or mixed-mode while coalesced faults are characterized by shear. The smooth manufactured fractures that are conventionally studied in laboratories do not capture the hydromechanical properties of the rough-surfaced fractures that are ubiquitous in nature. Here, we employ triaxial direct-shear to investigate the hydromechanical properties of fractures that are created and maintained at in-situ stress conditions. Our results show that in-situ created fractures are rougher, leading to higher frictional strength and more tortuous transport properties than the values available in the literature. The in situ created fractures are very rough and heterogeneous with peak-to-peak amplitude that can be larger than 3 mm for a 25 mm length of fracture. In contrast, manufactured fractures often have less than 1 mm amplitude roughness. The difference in fracture roughness causes significant differences in hydromechanical properties. For example, the apparent frictional strength of in-situ created fractures is usually larger than 1 and shearing causes significant damage to asperities, which creates fines. In comparison, manufactured fractures exhibit strengths around 0.7 and produce relatively fewer fines. For fluid flow, the ratio of hydraulic aperture over mechanical aperture for in-situ created fractures is much smaller than 1 (e.g., 0.001), while the literature values are often larger (e.g., 0.01-1). This highlights the importance of considering the effect of higher surface roughness when modeling subsurface fracture mechanics and fluid flow.
Characterizing shear slip on natural fractures of unconventional reservoirs can contribute to induced seismicity predictions, fault stability analysis, stimulated fracture fluid conductivity estimates, and shale gas production predictions (Meng et al., 2021). Most previous research used manufactured fractures, e.g., saw-cut fractures or tensile fractures, to study the hydromechanical behavior of shear on fractures in unconventional reservoir rocks. These planar fractures were smooth, unlike natural fractures. Such simple fracture surface geometry makes the hydromechanical measurements more easily repeated and controlled. This work on planar fractures produced important fundamental findings such as frictional stability-permeability relationships (Fang et al. 2017), mechanical and transport properties relationships (Schwartz et al. 2019), and fracture friction influence on induced seismicity (An et al. 2020).