Non-Fickian dispersion in porous media : 2. Model validation from measurements at different scales

International audienceWe aim at testing and validating a mobile-immobile mass transfer model from a set of single-well injection withdrawal tracer tests in a heterogeneous porous aquifer. By varying the duration of the injection phase, different volumes of aquifer are investigated by the tracer. Hence, we focus the transport model validation not only on reproducing a single breakthrough curve (BTC) but also on the model's capacity to predict the amount of mixing as a function of the volume visited by the tracer. All the BTCs are strongly asymmetric, as expected when dispersion is controlled by diffusive mass transfers between the mobile water and the immobile water part of the porosity. However, the BTC cannot be modeled by a conventional mobile-immobile mass transfer model with a simple power law memory function. To account for that, we implement a continuous time random walk model in which the transition time distribution y (t), which is related to the excursion time probability of the tracer in the immobile domain, is a dual-slope power law distribution. The model best fits the BTC data set with a transitional regime controlled by y(t) t2 and an asymptotic regime characteristic of the conventional double-porosity model with y(t) t1.5 . This work emphasizes that high-resolution concentration measurement and multiple-scale tracer tests are required for assessing solute dispersion models in heterogeneous reservoirs and for subsequently obtaining reliable predictions

Time-resolved 3D characterisation of flow and dissolution patterns in a single rough-walled fracture

Chapter 43International audienceAn application of X-ray computed microtomography (XCMT) for 3D measurement of fracture geometry is presented. The study demonstrates the ability of XCMT to non-invasively measure the fracture walls and aperture during the course of a reactive flow experiment. The method allows estimation of both the local and global scale dissolution kinetics of a fractured limestone sample percolated by acidic water. The measured fracture geometry was then used as an input for flow modelling, in order to compare the hydraulic aperture calculated by numerical simulation with different evaluations of the aperture: hydraulic aperture measured from pressure drop during the flow experiment, mechanical aperture measured with XCMT, and chemical aperture deduced from calcium removal in the sample. The effects of reactive transport on geometry and fluid flow are discussed. Dissolution appears heterogeneous at both the small scale due to the presence of insoluble clays in the rock, and at larger scales with the formation of preferential flow pathways. These heterogeneous dissolution patterns are not predictable simply by the identification of the areas of higher fluid velocity, where transport of the chemical reaction products (i.e. rate of aperture increase) is presumed to be higher

Numerical computations of rock dissolution and geomechanical effects for CO 2 geological storage

International audienceThe paper is motivated by the long term safety analysis of the CO 2 geological storage. We present a methodology for the assessment of the ge-omechanical impact of progressive rock dissolution. The method is based on the use of X-ray tomography and the numerical dissolution technique. The influence of evolution of the microstructure on the macroscopic properties of the rock is analysed by using periodic homogenization method. The numerical computations show progressive degradation of all components of the stiffness (orthotropic) tensor. Moreover, the evolution of associated mass transfer properties (as tortuosity and conductivity tensors), by using the periodic homogenization method, is also calculated. The correlation between the mechanical parameters and the transfer properties during the dissolution pro-cess is presented. The results show that the highest increase of the hydraulic conductivity (in direction Y) is not associated with the highest decrease of Young modulus in this direction. Moreover, the highest decrease of Young modulus (in the direction X) is not associated with percolation in this direction. Finally, an incremental law to calculate settlement, in case of a rock with evolving microstructure, is proposed. The solution of the macroscopic settlement problem under constant stress and drained conditions showed that the geomechanical effects of the rock dissolution are rather limited

Non-Fickian dispersion in porous media explained by heterogeneous microscale matrix diffusion

International audienceMobile-immobile mass transfer is widely used to model non-Fickian dispersion in porous media. Nevertheless, the memory function, implemented in the sink/source term of the transport equation to characterize diffusion in the matrix (i.e., the immobile domain), is rarely measured directly. Therefore, the question can be posed as to whether the memory function is just a practical way of increasing the degrees of freedom for fitting tracer test breakthrough curves or whether it actually models the physics of tracer transport. In this paper we first present a technique to measure the memory function of aquifer samples and then compare the results with the memory function fitted from a set of field-scale tracer tests performed in the same aquifer. The memory function is computed by solving the matrix diffusion equation using a random walk approach. The properties that control diffusion (i.e., mobile-immobile interface and immobile domain cluster shapes, porosity, and tortuosity) are investigated by X-ray microtomography. Once the geometry of the matrix clusters is measured, the shape of the memory function is controlled by the value of the porosity at the percolation threshold and of the tortuosity of the diffusion path. These parameters can be evaluated from microtomographic images. The computed memory function compares well with the memory function deduced from the field-scale tracer tests. We conclude that for the reservoir rock studied here, the atypical non-Fickian dispersion measured from the tracer test is well explained by microscale diffusion processes in the immobile domain. A diffusion-controlled mobileimmobile mass transfer model therefore appears to be valid for this specific case

Non-Fickian dispersion in porous media : 1. Multiscale measurements using single-well injection withdrawal tracer tests

International audienceWe present a set of single-well injection withdrawal tracer tests in a paleoreef porous reservoir displaying important small-scale heterogeneity. An improved dual-packer probe was designed to perform dirac-like tracer injection and accurate downhole automatic measurements of the tracer concentration during the recovery phase. By flushing the tracer, at constant flow rate, for increasing time duration, we can probe distinctly different reservoir volumes and test the multiscale predictability of the (non-Fickian) dispersion models. First we describe the characteristics, from microscale to meter scale, of the reservoir rock. Second, the specificity of the tracer test setup and the results obtained using two different tracers and measurement methods (salinity-conductivity and fluorescent dye­optical measurement, respectively) are presented. All the tracer tests display strongly tailed breakthrough curves (BTC) consistent with diffusion in immobile regions. Conductivity results, measured over 3 orders of magnitude only, could have been easily interpreted by the conventional mobile-immobile (MIM) diffusive mass transfer model of asymptotic log-log slope of 2. However, the fluorescent dye sensor, which allows exploring much lower concentration values, shows that a change in the log-log slope occurs at larger time with an asymptotic value of 1.5, corresponding to the double-porosity model. These results suggest that the conventional, one-slope MIM transfer rate model is too simplistic to account for the real multiscale heterogeneity of the diffusion-dominant fraction of the reservoir

Nutrient dynamics and primary production in the eutrophic Berre Lagoon (Mediterranean, France)

1 - The brackish Berre Lagoon is one of the largest Mediterranean coastal lagoons (155 km2). Since 1966, it has been influenced by a large freshwater discharge from a hydroelectric power plant, which has led to strong changes in the ecosystem structure and functioning. 2 - During 2005 and 2006, we monitored physico-chemical variables, primary production, nitrogen assimilation and regeneration in the water column using the dual-isotopic 15N/13C technique. 3 - Significant seasonal variations were observed for salinity, however summer values were similar to those observed in the lagoon before the power plant opened (30). Nitrate concentrations (NO3-) varied, yielding very low values in the summer to values of 220 µg N-NO3-.1-1 in winter. Ammonium (NH4-) and phosphate (PO43-) were observed in significant concentrations throughout the year (>7 µg N-NH4+.l-1 and 3 µg P-PO43-.1-1 respectively). Total chlorophyll increased slightly from February 2005 to June 2006, never exceeding 17 µg.1-1 until summer 2006, which was marked by a large accumulation of biomass (> 30 µg.1-1) maintained until the end of the year. 4 - Mean primary production rates ranged from 50 to 1 600 µgC.1-1.d-1, giving a total annual production of 507 gC.m-2.yr-1 in 2005 and 742 gC.m-2.yr-1 in 2006. This production is based on the assimilation of 15 000 and 24 000 tons of dissolved inorganic nitrogen (DIN) in 2005 and 2006 respectively (uptake rates ranging from 3 to 77 µg N-NH4-+.1-1.d-1 and from 1 to 63 µg N-NO3-.1-1.d-1). Regeneration processes were very active: 15 to 85 µg N-NH4+.1-1.d-1 were regenerated in the water column and sustained at least 60% of ammonium uptake. 5 - 95% of the measured annual primary production was regenerated production. These budgets need to 5 - be analysed with caution as biological processes are characterized by high interannual and seasonal variabilities. Regeneration processes in the water column certainly fed by other processes (acting in the sediments) are the basis of eutrophication maintenance in the Berre Lagoon