Evaluation of Forchheimer equation coefficients for non-Darcy flow in deformable rough-walled fractures

•Three typical nonlinear flow behaviors were identified in deformable rock fractures.•Two empirical equations were proposed for Forchheimer's nonlinear coefficient.•The effect of fracture roughness on the non-Darcy flow behaviors was quantified.•A new criterion was presented for assessing the applicability of Darcy's law.

SummaryThis study focuses on experimental evaluation of the Forchheimer equation coefficients for non-Darcy flow in deformable rough-walled fractures. Water flow tests through twelve granite fracture samples with different roughness were conducted in a triaxial cell under confining stresses varying from 1.0 MPa to 30.0 MPa. A total of 2280 experimental data in the form of pressure gradient versus discharge were collected. Three representative types of nonlinear flow behaviors induced by inertial effect, fracture dilation and solid-water interaction, respectively, were observed. Regression analyses of the experimental data show that the Forchheimer equation adequately describes the non-Darcy flow behavior induced by significant inertial effect. Based on the experimental observations, two empirical equations were proposed for parametric expression of the Forchheimer's nonlinear coefficient, one as a power function of hydraulic aperture and the other dependent on both hydraulic aperture and peak asperity of the fracture surface. A new criterion was presented for assessing the applicability of Darcy's law, which relies on the ratio of discharge or pressure gradient predicted by the Forchheimer's law incorporated with the single-parameter equation to that predicted by the Darcy's law. A sensitivity analysis was performed using the double-parameter equation for examining the dependence of the Forchheimer's nonlinear coefficient on peak asperity, demonstrating the importance of incorporating the fracture roughness in the development of non-Darcy flow models. The experimental results and the proposed models are useful for understanding and numerical modeling of the nonlinear flow behaviors in fractured aquifers.