Dependence of Fracture (Fault) Permeability Evolution on Surface Roughness

It is of critical importance in shale reservoir stimulation and enhanced geothermal stimulation to understand how fracture permeability evolves in association with induced seismicity. Previous studies have probed how fracture (fault) permeability evolves during shearing using fractures (faults) with smooth surfaces, sinusoidal surfaces, or real fracture surfaces. However, few previous studies have systematically evaluated the dependence of permeability on surface roughness due to the difficulty to produce and to replicate the desired magnitude of non-periodic roughness, which can be described by fractal dimension (D as a proxy for wavelength) and standard deviation of asperity height (std as a proxy for amplitude). We report shear-permeability experiments on 3D printed samples with reconstructed real surface roughness and subsequent DEM simulations to better understand permeability evolution of fractures (faults) in nature. From the experiments we observe that there are two mechanisms for permeability evolution depending on surface roughness. When the roughness is low to intermediate (D1.3 in this study), standard deviation of asperity height has a first-order influence on the magnitude of permeability enhancement; the larger the std, the greater the permeability enhancement. Counterintuitively, when the roughness is high (D=1.5 in this study), the magnitude of permeability enhancement is significantly lower. This reduced permeability enhancement may be attributed to: (1) a reduction in aperture, and (2) clogging with wear products that result from the comminution of the peaks of the rough surfaces. Furthermore, whether a certain asperity breaks can be determined by the value of its asperity height to wavelength ratio (H/L), and thus D is the first-order factor to determine how much clogging occurs at millimeter scale, since D significantly changes the value of H/L at this scale. Thus, for permeability of fresh initially-mated fractures (faults) to be significantly enhanced, the fracture (fault) surface should have a D of an intermediate magnitude (1.3, in this study) with a std of up to 0.48. Conversely, to stem permeability enhancement, the fracture (fault) should have a high D (1.5 in this study) with as high a std as possible.