Roughness of hydraulic fractures: Importance of in-situ stress and tip processes

The surface roughness of hydraulic fractures reflects the fracturing proces s at the tip. We conducted an experimental study of hydraulic fracture prop agation and observed a characteristic roughness pattern. The roughness deve loped without sheer or torsional loading in the plane of the penny-shaped f racture and is determined by material properties and the externally applied stresses on the sample, which represent the effective in-situ stresses. We quantified the roughness and showed that it correlated well with a measure of the plastic zone size around the fracture tip. New explanations for fra cture surface roughness that develops under Mode I loading conditions are p resented.