Equivalent gas aperture of rock fracture: Theoretical model and experimental validation

Fractures are the main channels for fluid and gas flow in rock masses. Hydraulic aperture, which is derived for an incompressible fluid, is commonly employed to characterize flow properties of rock fractures. However, hydraulic aperture may not be suitable for describing the gas flow behavior through rock fracture when considering gas compressibility and slippage. Therefore, a new concept of gas aperture is proposed to characterize gas flow properties of rock fractures, and its definition and determination procedure are provided. A number of gas flow tests on granite fractures with flat or rough surfaces are conducted under varying confining and gas pressures. The results show that hydraulic apertures can underestimate gas apertures by 5–30 % when the ratio of the gas pressure difference between inlet and outlet to the mean gas pressure exceeds 0.25. Finally, an empirical formula is developed to describe the relationship between mechanical and gas apertures. This study advances understanding of gas flow behavior in rock fractures through the novel gas aperture concept, overcoming the limitations of hydraulic aperture in gas flow scenarios and enabling enhanced predictive modeling of gas transport in fractured rock masses.