Measuring low gas permeability of granites: a comparative assessment of quasi‑steady‑state and pulse‑test methods

Measuring gas permeability in low‑permeability geomaterials remains challenging and of practical importance for subsurface engineering and waste storage. We benchmark two widely used approaches, the quasi‑steady‑state (QSS) and pulse‑test methods, by performing matched experiments on two granite specimens at three mean pore‑pressure levels. Across these conditions, both methods return comparable apparent permeabilities, with no systematic bias between techniques. Klinkenberg's analysis of repeated measurements reveals pronounced gas slippage in Sample G2, from which the intrinsic permeability is obtained as Kint = 1.37 x 10−17 m2; Sample G1 shows negligible slippage over the same pressure range. The characteristic measurement time observed experimentally is consistently shorter than the value predicted by a single‑storage, homogeneous model. To explain this, we introduce a minimal two‑zone framework in which a low‑permeability domain (A1) coexists with a minor, higher‑permeability pathway (A2). This composite model reproduces both the measured permeabilities and the accelerated transients. The results delineate conditions under which QSS measurements can perform on par with pulse tests for ultra‑low permeabilities, and they provide a transparent procedure to report Kint when slip effects are non‑negligible.