A Systematic Investigation Into The Control Of Roughness On The Flow Properties Of 3d-Printed Fractures

Heterogeneous fracture aperture distribution, dictated by surface roughness, mechanical rock and fracture properties, and effective stress, limits the predictive capabilities of many reservoir-scale models that commonly assume smooth fracture walls. Numerous experimental studies have probed key hydromechanical responses in single fractures; however, many are constrained by difficulties associated with sample preparation and quantitative roughness characterization. Here, we systematically examine the effect of roughness on fluid flow properties by 3D printing seven self-affine fractures, each with controlled roughness distributions akin to those observed in nature. Photogrammetric microscopy was employed to validate the 3D topology of each printed fracture surface, enabling quantification using traditional roughness metrics, namely the Joint Roughness Coefficient (JRC). Core-flooding experiments performed on each fracture across eight incremental confining pressure increases (11-25 bar), shows smoother fractures (JRC < 5.5) exhibit minor permeability variation, whilst rougher fractures (JRC > 7) show as much as a 219% permeability increase. Micro-computed tomography imaging of the roughest fracture under varying effective stresses (5-13.8 bar), coupled with inspection into the degree of similarity between fracture closure behavior in 3D-printed and natural rock fractures, highlight the capabilities of 3D-printed materials to act as useful analogs to natural rocks. Comparison of experimental data to existing empirical aperture-permeability models demonstrates that fracture contact area is a better permeability predictor than roughness when the mechanical aperture is below similar to 20 mu m. Such findings are relevant for models incorporating the effects of heterogeneous aperture structures and applied stress to predict fracture flow in the deep subsurface.Plain Language Summary Stark permeability contrasts between fractures and surrounding rock make them a critical feature governing heat and mass transport in the Earth's subsurface, while also playing an integral role in many natural processes and subsurface engineering applications. A fractures aperture (void space) determines its fluid transport capabilities, making this crucial information needed to inform numerical models seeking to predict flow rates across many scales (centimeters to kilometers). For simplicity, common fracture representations equate the effective (hydraulic) and geometric (mechanical) apertures (i.e., perfectly smooth walls). In nature, all rock fractures display rough walls to various degrees, limiting the predictive capabilities of smooth-walled models. Here, we performed fluid flow experiments on 3D-printed fractures with controlled surface roughness to investigate the effect on fluid transport. We find that roughness enables the persistence of void space, which enhances fluid flow. High-resolution imaging shows that below similar to 20 mu m, the influence of fracture contact area on permeability is greater than roughness. Such findings are relevant for fracture flow models attempting to incorporate the effect of heterogeneous apertures and applied stress on fluid flow properties in fractures, which is important in the deep subsurface where applied stresses force fluid to navigate discrete contact points between opposing fracture walls.