Experimental evidence for permeability minima at low-velocity gas flow through naturally formed porous media

The objective of this paper is to describe and discuss experiments that document the presence of minima in the permeability to nitrogen gas for a number of rock samples at fluid velocities of 0.3-0.8 x 10(-3) m/s, and mean pressures up to 0.4 MPa. The k-minima are interpreted as occurring in a transitional flow regime, in which, as fluid velocities decrease the contribution of viscous or Poiseuille flow is progressively exceeded and ultimately replaced by "molecular streaming" or Knudsen flow These are the first reported experiments known to the authors that are specifically designed to test the effects of low flow rates an the gas permeability of natural samples. The test samples are sandstones and limestones, displaying a broad range of porosities (0.12-0.30) and gas permeabilities (1.5-1600 x 10(-3) mu m(2)). For every sample, a k-minimum can be found at each of three mean pressures, but the three minima correspond to a single, "critical" fluid velocity. Fluid velocities as low and lower than those tested are common, if not predominant, under natural and induced subsurface conditions. The experimental results indicate that the widely accepted Klinkenberg correction for gas ''slippage,'' based an the mean pressure-dependence of slip flow is not applicable at these low fluid velocities. Instead, fluid velocity is proposed as a critical parameter controlling the onset of subviscous flow mechanisms.