Field and numerical determinations of pneumatic flow parameters of unsaturated fractured porous rocks on various scales

Abstract

[1] Air permeability is measured in the fractured crystalline rocks of the Roselend Natural Laboratory (France). Single-hole pneumatic injection tests as well as differential barometric pressure monitoring are conducted on scales ranging from 1 to 50 m, in both shallow and deep boreholes, as well as in an isolated 60 m 3 chamber at 55 m depth. The field experiments are interpreted using numerical simulations in equivalent homogeneous porous media with their real 3-D geometry in order to estimate pneumatic parameters. For pneumatic injection tests, steady-state data first allow to estimate air permeability. Then, pressure recovery after a pneumatic injection test allows to estimate the air-filled porosity.Comparison between the various studied cases clarifies the influence of the boundary conditions on the accuracy of the often used 1-D estimate of air permeability. It also shows that permeabilities correlate slightly with fracture density. In the chamber, a 1 order-ofmagnitude difference is found between the air permeabilities obtained from pneumatic injection tests and from differential barometric pressure monitoring. This discrepancy is interpreted as a scale effect resulting from the approximation of the heterogeneous fractured rock by a homogeneous numerical model. The difference between the rock volumes investigated by pneumatic injection tests and by differential barometric pressure monitoring may also play a role. No clear dependence of air permeability on saturation has been found so far.Citation: Guillon, S., M. T. Vu, E. Pili, and P. M. Adler (2013), Field and numerical determinations of pneumatic flow parameters of unsaturated fractured porous rocks on various scales, Water Resour.

Barometric pumping of a fractured porous medium

“…The barometric pressure parameters chosen in this paper are They represent variations in weather patterns at the Roselend site [ Guillon et al , ]. The molecular diffusion coefficient is taken as This value compares with the order of magnitude of diffusion coefficients for most gaseous tracers [ Marrero and Mason , ].…”

“…The molecular diffusion coefficient is taken as This value compares with the order of magnitude of diffusion coefficients for most gaseous tracers [ Marrero and Mason , ]. As for the transport parameters, air viscosity, compressibility coefficient, matrix permeability, and porosity are taken as following Guillon et al []. The fracture network properties were determined from the stereological analysis of fractures observed in the Roselend tunnel [ Patriarche et al , ].…”

“…Some parametric studies on the influence of fracture density and porosity are detailed. These numerical simulations are conducted with parameter values relevant for the Roselend Natural Laboratory (Alps, France), where flow parameters of the crystalline rocks were determined [ Guillon et al , ; Patriarche et al , ]. The influence of some other parameters such as the molecular diffusion coefficient, the fracture aperture, and the percolating character of the fracture network is summarized.…”

Monitoring CO2 concentration and δ13C in an underground cavity using a commercial isotope ratio infrared spectrometer

“…Third, the δ 13 C dependence on CO 2 concentration is known to be one of the largest sources of uncertainty for IRIS analyzers [3,7,9,18,19] and needs to be carefully checked. Reference gases with 17,800 as well as 2,002 ppm CO 2 concentrations (F and G, Table 1) were stepwise diluted by CO 2 -free synthetic air with 78 % N 2 , 21 % O 2 (Air Products), using two mass flow controllers (SLA5850S, Brooks, range 1 L min −1 ).…”

“…Commercial IRIS instruments are often designed for atmospheric monitoring and present good accuracy at low and not strongly variable CO 2 concentration [3,7,8]. But there is also a need for carbon isotope monitoring at higher and more variable concentrations, in volcanic [2] or underground [9] environments, as well as for soil [10] and fermentation plants monitoring [11].…”