GAS MINI-PERMEAMETERS

Gas mini-permeameters are used to rapidly measure gas permeability on porous rock and sediment outcrops, and on core and block samples. Large numbers of measurements can be assembled to help characterize spatial heterogeneity as a function of deposition, diagenesis, and deformation. At sufficiently low moisture contents the gas permeability is related (linearly) to water permeability. The measurement is based on measuring the rate of gas flow through a porous media from a tip seal placed against the outcrop, or a hole drilled into the outcrop, for a known pressure drop. Provided that the flow rate is not too high (non-linear effects), and the pore dimensions are not too small (relative to mean free path; gas-slippage effect), gas flow through a porous media is governed by Darcy’s Law. For example, assuming an ideal gas, a homogeneous, isotropic porous media, and steady flow, the gas flow rate from a gas mini-permeameter can be calculated by solving an axi-symmetric version of Laplace’s equation for the appropriate boundary conditions. The solution is inversed to find the permeability. If the moisture content is or below the gravity drainage residual this value of permeability approximates the intrinsic permeability. Like an aquifer pump test the measurement is a spatial average of the material properties that compose the sampled volume. We examine a number of performance characteristics for this class of instrument, for both locally homogeneous and locally heterogeneous materials, and review a variety of applications from simple fluvial and volcanic deposits to faults and clastic dikes.