Laboratory-scale hydraulic pulse testing: influence of air fraction in cavity on estimation of permeability

Abstract

When performing transient hydraulic pulse tests on porous materials in the laboratory it is implicitly assumed that the accessible pore space of a porous medium and the pressurised fluid-filled cavity are completely saturated with the permeating fluid. In certain instances the pressurised fluid cavity in a hydraulic pulse test can contain an air fraction introduced through either the experimental procedure or released from the unsaturated regions of the porous medium. The purpose of this paper is to examine the influence of an air fraction within the pressurised region, on the estimation of permeability derived from transient hydraulic pulse tests. A series of hydraulic pulse tests conducted on Stanstead Granite was used to estimate the permeability of the rock. It is shown that if the effect of the cavity air fraction is omitted from the analysis of the hydraulic pulse test, the permeability can be underestimated. The paper also presents a discussion of the potential role the air fraction can contribute to discrepancies in the permeabilities estimated from steady-state tests and hydraulic pulse tests.

Isothermal Permeability of the Argillaceous Cobourg Limestone

“…The permeability values for the two samples tested ranged between 3:2 Â 10 À20 m 2 to 4:2 Â 10 À20 m 2 and compared favourably with the steady state test results performed on the same samples. Both the "Patch Permeability Test" and the current "Partial Cavity Test" (Selvadurai and Najari, 2015) are innovative approaches for conducting steady state and transient pulse tests for estimating the fluid transport characteristics of low permeability geomaterials.…”

“…Although the central cavity and the attached connections were filled with water and precautions were taken to minimize the presence of air in the cavity, it is almost impossible to completely eliminate the air fraction in the pressurization volume. A practical technique was proposed by Selvadurai and Najari (2015), which takes into account the effect of air fraction on the analysis of the hydraulic pulse test results. The technique uses the pressure build-up curves to estimate the volume of air fraction and then adjusts the compressibility of the cavity fluid accordingly.…”

“…Selvadurai and Ichikawa (2013) examined the role of partial saturation of the pore fluid on the one-dimensional cavity pressure decay response and showed that the role of unsaturation becomes more important when the air fraction is large and it is particularly important when dissolved air and air solubility effects are taken into consideration (Selvadurai, 2009). The study by Selvadurai and Najari (2015) examined the role of an air fraction in the pressurized cavity on the pulse decay response; the volume of the air fraction can be estimated by recording the cavity pressurization response and the pulse decay response can be corrected to account for the effective compressibility of the pressurized fluid. These studies show that when the hydraulic pulse test results are corrected for the air fraction, they correspond closely to the results from steady state tests, which can provide the most accurate permeability estimates.…”

The thermo‐hydro‐mechanical behavior of the argillaceous Cobourg Limestone

“…Here the fluid‐filled sealed cavity was modeled as a porous medium with a porosity of unity and a relatively high permeability (i.e., K f = 10 −12 m 2 ) in order to minimize any spurious pressure gradients and convection effects within the cavity. Also, in order to capture the effect of cavity volume change on the cavity pressure change, the elasticity parameters were assumed to be ν f = 0.49 and E = 3(1 − 2 ν f )/ C eq , where C eq is the compressibility of the cavity fluid that may also contain trapped air [ Selvadurai and Najari , ]. The system of nonlinear partial differential equations governing the fluid‐filled cavity are as follows: …”

“…A simplified relationship for the compressibility of an air‐water mixture can be obtained from the form of the Voigt bound where φ is the air fraction in the fluid‐air mixture (volume of air/volume of fluid‐filled cavity), C w is the compressibility of the pure fluid without air inclusions, i.e., 4.54 × 10 −10 Pa −1 [ White , ], and C a is the compressibility of air at the temperature and pressure associated with the experiment. Alternatives to equation that take into account air solubility have also been investigated [ Schuurman , ; Fredlund , ; Teunissen , ; Selvadurai and Najari , ]. For nonisothermal phenomena the behavior of the air bubbles follows the ideal gas law: where V a is the volume of the air, P a is the absolute pressure of the air bubble, T is the temperature in degrees Kelvin, m is the number of moles, and R is the universal gas constant.…”