Contribution of the Steady State Method to Water Permeability Measurement in Very Low Permeability Porous Media

Abstract

Very low permeability geomaterials (order of nanoDarcy (10-21 m2 )), such as clay rocks, are of interest for many industrial applications including production from unconventional reserves of oil and gas, CO2 geological storage and deep geological disposal of high-level long-lived radioactive waste. In these last two applications, the efficiency of clay, as a barrier, relies on their very low permeability. Yet, laboratory measurement of low permeability to water (below 100 nD (10-19 m2 )) remains a technical challenge. Some authors (Hsieh et al. , 1981, Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 18 , 245-252) argue that steady state methods are irrelevant due to the time required to stabilize water fluxes in such low permeability media and prefer a transient technique called pulse decay. This study aims to perform and compare transient and steady state techniques on three samples. Regarding the steady state method, a high precision pump was used to measure water flow rate through the sample. We show that with a suitable set-up, the steady state method enables us to measure a very low permeability of 0.8 nD (8 × 10-22 m2 ) over a period of three days and 2.6 nD (2.6 × 10-21 m2 ) over a period of one day. While the pulse decay test provides only an average estimate of the permeability for a comparable duration. Many issues are raised in pulse decay tests: determination of the reservoirs storage factor, micro leakage effects, determination of the initial pulse pressure, 2D mechanical effect. Contrary to the widespread belief that transient techniques are required to measure very low permeability, we show that direct steady state measurement of water permeability, with suitable equipments, can be much faster and more accurate than measurement by pulse decay. In fact, low water and rock compressibilities result in fast propagation of pressure wave and it cannot be argued that steady state conditions are not reachable in a reasonable amount of time. Still, pulse decay remains an interesting alternative to steady state methods when permeability is higher than 50 nD (5 × 10-20 m2 ).