Determination of Gas Permeability of Tight Reservoir Cores without Using Klinkenberg Correlation

Abstract

Abstract This paper reports the laboratory measurements of gas permeability for tight reservoir core samples with various upstream, downstream, and average pressures. Measurements are conducted with backpressures from atmospheric pressure up to 9.3 MPa. It is found that exerting a backpressure at the outlet of the core sample can effectively reduce the gas slip effect and improve the permeability measurements. When the backpressure reaches a certain level, defined as the minimum backpressure, the gas slip effect can be eliminated so that non-slip gas permeability can be obtained without using Klinkenberg correlation. When the backpressure exceeds the minimum backpressure the measured non-slip permeability is a constant for a given core sample. From this finding, a method of directly determining gas permeability of tight reservoir cores is provided. The experimental results also indicate that the minimum backpressure increases with decrease in permeability of tight reservoir cores. The concept and technique can be applied in the measurement of effective/relative gas permeability in multiphase systems where Klinkenberg correlation cannot be used to correct the gas slip effect. Introduction Tight gas reservoirs are defined as formations with permeability less than 0.1 millidarcy. Gas reserves in tight reservoirs constitute a significant percentage of the natural gas resources worldwide and offer tremendous potential for future reserve growth and production.1–5 Tight gas reservoirs often exhibit unusual characteristics that require more research on it. Reliable reservoir description and assessment of such low permeability tight gas reservoirs demand reliable laboratory permeability data. Gas slippage is a phenomenon when gas flows through a thin capillary tube or a fine porous medium. During this process, the velocity of gas layer in the immediate vicinity of the solid walls of the capillary or porous medium is not zero, causing an increase in gas flow rate in porous sample. Klinkenberg6 was the first to introduce the concept of gas slippage into gas permeability measurement. From the analysis, the gas velocity at the solid wall was given as6Equation 1 where ? is the mean free path of gas molecules, is the velocity gradient in the direction perpendicular to the wall, and c is a proportionality factor. Assuming gas flow in an idealized porous medium, from Poiseuille's law or Darcy's law, a relation between the apparent and "true" permeability of a porous medium was given as6Equation 2 Equation (2) is also referred as Klinkenberg correlation, where P is the mean pressure, ka is the apparent gas permeability observed at mean pressure, and k8 is the "true" permeability or Klinkenberg permeability at an infinite mean pressure. b is the gas slip factor, a coefficient depending on the mean free path of a particular gas and the average pore radius of the porous medium, as is given byEquation 3 where, r is the radius of a capillary or a pore. The idealized porous medium is that in which all the capillaries in the material are of the same diameter and are oriented at random through the solid material.6 Equation (3) suggests that the gas slip effect is significant in tight porous samples because of the extremely small pore size.