Nonlinear flow in pores of rough fractured surfaces

Mutual contact disturbance of fracture surfaces under stress effects can affect gas storage and transportation. Studying the influence of effective stress on the contact ratio variation and nonlinear seepage characteristics of multi-roughness gradient fracture surfaces can provide theoretical support for reservoir seepage capacity evaluation and hydraulic fracturing technology (selection of fracturing fluids and proppants). In this study, seepage tests were conducted on fractures with various roughness degrees under different effective stresses using a seepage test platform, and the equivalent pore size of the fracture surfaces was calculated. The morphology of rock fracture surfaces was obtained by means of optical three-dimensional scanning technology, and the flow process was reproduced through numerical simulation. The results show that: (1) Both the increase in effective stress and gas flow rate enhance the nonlinearity of gas flow; high axial confining pressure leads to the generation of a high nonlinearity factor at low flow rates. (2) At low effective stress, the vortex structures along smooth fracture surfaces are characterized by high vortex density and low vorticity, whereas those on rough fracture surfaces exhibit high vorticity but low vortex density. These contrasting vorticity patterns highlight fundamentally different nonlinear flow responses. (3) Under elevated effective stress, smoother fractures tend to develop reduced hydraulic apertures, while rougher surfaces generate more extensive contact voids. The simulations indicate that these contact void regions exert a pronounced triggering effect on the development of vortices and vorticity.