A comparative study on methods for determining the hydraulic properties of a clay shale

Abstract

Summary A comprehensive characterization of clay shale behaviour requires quantifying both geomechanical and hydromechanical characteristics. This paper presents a comparative laboratory study of different methods to determine the water permeability of saturated Opalinus Clay: i) pore pressure oscillation, ii) pressure pulse decay, and iii) pore pressure equilibration. Based on a comprehensive dataset obtained on one sample under well-defined temperature and isostatic effective stress conditions, we discuss the sensitivity of permeability and storativity on the experimental boundary conditions (oscillation frequency, pore pressure amplitudes, effective stress). The results show that permeability coefficients obtained by all three methods differ less than 15 per cent at a constant effective stress of 24 MPa (kmean = 6.6E-21 to 7.5E-21 m2). The pore pressure transmission technique tends towards lower permeability coefficients, whereas the pulse decay and pressure oscillation techniques result in slightly higher values. The discrepancies are considered minor and experimental times of the techniques are similar in the range of 1–2 days for this sample. We found that permeability coefficients determined by the pore pressure oscillation technique increase with higher frequencies, i.e. oscillation periods shorter than 2 hours. No dependence is found for the applied pressure amplitudes (5 per cent, 10 per cent, and 25 per cent of the mean pore pressure). By means of experimental handling and data density, the pore pressure oscillation technique appears to be the most efficient. Data can be recorded continuously over a user-defined period of time and yield information on both, permeability and storativity. Furthermore, effective stress conditions can be held constant during the test and pressure equilibration prior to testing is not necessary. Electron microscopic imaging of ion-beam polished surfaces before and after testing suggests that testing at effective stresses higher than in-situ did not lead to pore significant collapse or other irreversible damage in the samples. The study also shows that unloading during the experiment did not result in a permeability increase, which is associated to the persistent closure of micro-cracks at effective stresses between 24 and 6 MPa.

A Model and Measurement Technique for Liquid Permeability of Tight Porous Media Based on the Steady-State Method

“…On the other hand, when a constant flowrate mode is applied for permeability measurement, it takes too long to arrive at equilibrium pressure. [18][19][20][21] reported that 6 to 8 weeks is required until a reasonable flow equilibrium is achieved when a light oil is used for measurement of permeability in the range of nanodarcy. Jones and Meredith, 23 Morrow and Lockner, 24 and El-Dieb and Hooton 25 measured permeability of tight rocks in the range of nanodarcy in 15 to 160 h by applying only one pressure gradient across the sample.…”

“…Since the measurement error of flowmeters is much greater than the magnitude of fluctuations of low flowrates associated with tight rocks, practically, it is not feasible to use the constant pressure mode of injection and measure the effluent flowrate unless a very accurate liquid flowmeter is available. On the other hand, when a constant flowrate mode is applied for permeability measurement, it takes too long to arrive at equilibrium pressure. − Lasswell reported that 6 to 8 weeks is required until a reasonable flow equilibrium is achieved when a light oil is used for measurement of permeability in the range of nanodarcy. Jones and Meredith, Morrow and Lockner, and El-Dieb and Hooton measured permeability of tight rocks in the range of nanodarcy in 15 to 160 h by applying only one pressure gradient across the sample.…”

A modified pulse‐decay approach to simultaneously measure permeability and porosity of tight rocks

“…For example, the gas compressibility is related to pressure through the equation of state, and the permeability coefficient k is affected by pressure due to the poroelastic and slippage effects. [31][32][33] However, as previously noted, 15,[34][35][36] when the initial pressure difference between the two ends of the sample in a single measurement is small, these variables can be regarded as constants and their values are taken under the mean pore pressure.…”

An Early‐Time Solution of Pulse‐Decay Method for Permeability Measurement of Tight Rocks

“…However, due to the tightness of these rocks, permeability measurements using the steady state method are very time consuming (Heller & Zoback, 2013; Metwally & Sondergeld, 2011). One of the most widely used transient methods is the pulse‐decay method (Brace et al., 1968; Gaus et al., 2019; Winhausen et al., 2021). In the typical pulse‐decay tests, a cylindrical rock sample is installed in a flow cell connected to two reservoirs.…”