Shale matrix permeability is important in evaluating the economic viability of unconventional reservoirs. Inconsistent permeability measurements reported from pressure decay analysis on crushed rock samples is forcing a re-examination of core plugs measurements. Various methods have been developed to measure permeability on core plugs which include steady state and transient methods. Because of the use of different techniques to measure permeability by different laboratories it becomes necessary to compare the resulting values. An experimental apparatus has been developed which has the capability to measure permeability using steady state and several transient methods.
To compare some permeability values, appropriate gas corrections have to be applied. Gas corrections depend on the flow regime prevalent during the measurement. The transient flow regime dominates at low pressures when the pore sizes are small. In the transition flow regime, gas permeability measurements cannot be corrected with a simple Klinkenberg correction but require second order permeability correction such as the double slip correction. An experimental investigation to understand the flow regime at different pore pressures was carried out on two Wolfcamp shale samples. Steady state permeability of these samples was measured using helium and nitrogen at a constant effective pressure of 3000 psi over different pore pressure ranges from 100 psi to 3000 psi; the permeabilities were corrected using the Klinkenberg and the double slip correction. It was assumed that whichever correction resulted in similar permeabilities for both gases over the pore pressure range, would be the appropriate correction for that range. It was found that for pore pressures less than 250 psi permeabilities required second order corrections, indicating a transition flow regime at these pore pressures in shales. However at pore pressures greater than 2000 psi permeabilities required negligible corrections. It is therefore recommended to use pore pressures greater than 2000 psi for gas permeability measurements in shale.
Permeability of five horizontal Wolfcamp shale samples were measured using steady state and transient methods at an effective pressure of 3000 psi using nitrogen gas. Three measurements were applied for each method at pore pressures less than 250 psi and a double slip correction was applied. The three methods yielded similar permeabilities with differences being less than 30%. Therefore transient methods provide comparable results to steady state gas permeability measurements when the proper corrections are applied.