LOW WATER PERMEABILITY MEASUREMENTS OF CLAY SAMPLE. CONTRIBUTION OF STEADY STATE METHOD COMPARED TO TRANSIENT METHODS

摘要

Very low permeability geomaterials (order of nanoDarcy (10-21 m~2)), such as clays rocks, are studied for many industrial applications such as production from unconventional reserves of oil and gas, CO2 geological storage and deep geological disposal of high-level long-lived nuclear wastes. For these last two applications, clay efficiency as barrier relies mainly on their very low permeability. Laboratory measurement of low permeability to water (below 100nD (10-19 m~2)) remains a technical challenge. Some authors [1] argue that steady state methods are irrelevant due to the time required to stabilize water fluxes in such low permeability media. Most of the authors (e.g. [2]) measuring low permeabilities use a transient technique called pulse decay. This study aims to compare objectively these different types of permeability tests performed on a single clay sample. For the steady state method, a high precision pump was used to impose a pressure gradient and to measure the small resulting water flow rate at steady state. We show that with a suitable set-up, the steady state method enables to measure a very low permeability of 0.8 nD (8 10-22 m~2) in a period of three days. For a comparable duration, the pulse decay test, most commonly used for such low permeability measurements, provides only an average estimate of the permeability. Permeability measurements by pulse decay require to perform simulations to interpret the pressure relaxation signals. Many uncertainties remain such as the determination of the reservoirs storage factor, micro leakage effect, or the determination of the initial pulse pressure. All these uncertainties have a very significant impact on the determination of sample permeability and specific storage.