Zeta potential in intact natural sandstones at elevated temperatures

Supporting data are included in PDF and CSV files; any additional data may be obtained from the corresponding author (e-mail: [email protected]). TOTAL is thanked for partial support of Jackson's Chair in Geological Fluid Mechanics and for supporting the activities of the TOTAL Laboratory for Reservoir Physics at Imperial College London where these experiments were conducted. The Editor thanks Andre Revil and Paul Glover for their assistance in evaluating this paper.Peer reviewedPublisher PD

Complex conductivity of soils

The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m21, NaCl, 258C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in-phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent

Comment on ‘Streaming potential dependence on water-content in Fontainebleau sand' by V. Allègre, L. Jouniaux, F. Lehmann and P. Sailhac

Allègre et al. recently presented new experimental data regarding the dependence of the streaming potential coupling coefficient with the saturation of the water phase. Such experiments are important to model the self-potential response associated with the flow of water in the vadose zone and the electroseismic/seismoelectric conversions in unsaturated porous media. However, the approach used to interpret the data is questionable and the conclusions reached by Allègre et al. likely incorrec

Thermal conductivity of unsaturated clay-rocks

The parameters used to describe the electrical conductivity of a porous material can be used to describe also its thermal conductivity. A new relationship is developed to connect the thermal conductivity of an unsaturated porous material to the thermal conductivity of the different phases of the composite, and two electrical parameters called the first and second Archie's exponents. A good agreement is obtained between the new model and thermal conductivity measurements performed using packs of glass beads and core samples of the Callovo-Oxfordian clay-rocks at different saturations of the water phase. We showed that the three model parameters optimised to fit the new model against experimental data (namely the thermal conductivity of the solid phase and the two Archie's exponents) are consistent with independent estimates. We also observed that the anisotropy of the effective thermal conductivity of the Callovo-Oxfordian clay-rock was mainly due to the anisotropy of the thermal conductivity of the solid phase

Theory of transient streaming potentials in coupled unconfined aquifer-unsaturated zone flow to a well

A semianalytical solution is presented for transient streaming potentials associated with flow to a pumping well in an unconfined aquifer, taking into account the effect of flow in the unsaturated zone above the water table. Flow in the unsaturated zone is modeled with a linearized form of Richards\u27 equation using an exponential model for soil moisture retention and unsaturated hydraulic conductivity. Archie\u27s law is invoked for unsaturated electrical conductivity. The unsaturated electrokinetic coupling coefficient is modeled with a decaying exponential, where the maximum value is at and below the water table. The coupled flow and electrokinetic problem is solved using Laplace and Hankel transforms. The results of the model predicted behavior are presented and compared to that observed in laboratory simulations of pumping tests. The early time polarity reversal predicted the model is observable in the experiments. Other nonmonotonic streaming potential behaviors predicted by the model are also evident in experimental measurements. The model is used to estimate hydraulic parameters from SP data and these compare well to those obtained from drawdown data. For example, a hydraulic conductivity of 3.6 × 10−4 m/s is obtained from SP data compared to 3.4 × 10−4 m/s from drawdown data

A two-phase model for sheet flow regime based on dense granular flow rheology

International audienceA two-phase model having a μ(I) rheology for the intergranular stresses and a mixing length approach for the turbulent stresses is proposed to describe the sheet flow regime of sediment transport. In the model two layers are considered, a dilute suspension layer and a dense sediment bed layer. The concentration profile is obtained from the dilatancy law Φ(I) in the sediment bed layer and from a Rouse profile in the suspension layer. The comparison of velocity profile, concentration profile and macroscopic parameters (sediment flux, thickness and roughness) with experimental data shows a good agreement. These comparisons demonstrate that the dense granular rheology is relevant to describe intense bed-load transport in turbulent regime (sheet flow). The transition from the dense static bed to the dilute suspension is well described by the present model. Also, the different regimes of the dense granular rheology seems to be able to capture the transition between collision dominant and turbulent fluctuations dominant sheet flows, depending on the particles characteristic