Geothermal heat advected by the recharge of underground conduit. Case study of the karstic spring of Lez (Hérault, France)

The karstic flow system of the Lez spring is capturing a large proportion of the deep geothermal heat flow as observed by temperature measurements both above and below the underground karst network. For interpreting these data, an analytical model of heat/fluid interaction based on the conservation of mass and energy in a dual conduit/porous medium system is developed. In the model of energy transport, the energy of the fluid includes its enthalpy plus its gravity potential; the hydraulic head is shown to be the correct potential accounting for both pressure and gravity. The thermal features of the model are expressed as a function of a few parameters: from comparison with the actual data, the depth of the conduit appears to lie around 400 m and the number Pe characterizing the recharge is about 6. When estimated with available thermal data, the amount of geothermal energy captured by the flow system in steady state conditions is significantly lower than the actual energy output of the spring. The possible origin of this offset is discussed: effect of gravity potential, 3D convergence of geothermal heat flux lines, transient effect. Moreover, the mapping of the vertical temperature gradient at low depth indicates the general pattern of the recharge zone energy

Flow regime associated with vertical secondary migration

International audienceSecondary migration is defined as the movement of hydrocarbons through relatively permeable rocks from source to trap: a two-phase flow within a porous medium. Depending on the geometry and capillary pressure distributions of carrier beds, secondary migration has both vertical and lateral components. The present paper focuses on that part of the migration where the movement is mainly vertical. Its objective is to propose a description of the dynamics governing the vertical part of secondary migration based on the main physical aspects of two-phase flow in a homogeneous porous medium. The study is illustrated by laboratory observations performed in a vertical, 2-D Hele-Shaw cell filled with a transparent porous medium where the flow of dyed oil invading a wetting fluid is visually observed. These observations help us to understand the effect of buoyant, capillary and viscous forces on the resulting flow, the relative importance of which is characterized by non-dimensional numbers. Extrapolating these observations to natural media, it is proposed that vertical secondary migration can be described as a percolation of disconnected and vertically-elongated stringers. These stringers do not move continuously but as a succession of snap-off and re-feeding events which result in a jerky upward movement. Using parameters characterizing the physical properties of the fluid and of the porous medium, the geometry and the dynamic behavior of the stringers are estimated. The width of stringers occurring during secondary migration in geological media is centimetric and their vertical size ranges from several centimeters to a few meters. An upper limit of the mean upward velocity of stringers is proposed, as well as an estimate of their spatial density and of the minimum, average horizontal distance (decametric) between two stringers. The stringers are sparsely distributed, resulting in a low average oil loss and a high efficiency of the vertical migration process

Testing oil saturation distribution in migration paths using MRI

International audienceMagnetic Resonance Imaging (MRI) method allows to observe the distribution of different fluids in situ in porous media, and to measure oil and water saturation. Although this technique has great advantages compared to others, there remains large space for assessing the method and improving the accuracy of measurement. Using MRI, the oil secondary migration paths are scanned to measure the saturation distribution during the laboratory experiments. The resulting map can be calibrated using a device with the same pore structure as the probed sample and fully saturated with oil. This device is scanned with the probed sample at the same time in order to calibrate the saturation. The Spin-echo multi-slices sequence (SEMS) is adopted for MRI to ensure that the oil saturation in migration paths is accurately measured. The relevant spatial resolution of the mapping is defined according to the concept of REV (representative elementary volume). The oil saturation resulting from data obtained using different image formats are compared and the resulting saturation evaluation is compared to direct bulk saturation measurements. This comparison demonstrates that the calculated MRI oil saturation using DICOM image format is quite accurate, with a relative error less than 2%

Identification of Biofilm-Associated Cluster (bac) in Pseudomonas aeruginosa Involved in Biofilm Formation and Virulence

Biofilms are prevalent in diseases caused by Pseudomonas aeruginosa, an opportunistic and nosocomial pathogen. By a proteomic approach, we previously identified a hypothetical protein of P. aeruginosa (coded by the gene pA3731) that was accumulated by biofilm cells. We report here that a ΔpA3731 mutant is highly biofilm-defective as compared with the wild-type strain. Using a mouse model of lung infection, we show that the mutation also induces a defect in bacterial growth during the acute phase of infection and an attenuation of the virulence. The pA3731 gene is found to control positively the ability to swarm and to produce extracellular rhamnolipids, and belongs to a cluster of 4 genes (pA3729–pA3732) not previously described in P. aeruginosa. Though the protein PA3731 has a predicted secondary structure similar to that of the Phage Shock Protein, some obvious differences are observed compared to already described psp systems, e.g., this unknown cluster is monocistronic and no homology is found between the other proteins constituting this locus and psp proteins. As E. coli PspA, the amount of the protein PA3731 is enlarged by an osmotic shock, however, not affected by a heat shock. We consequently named this locus bac for biofilm-associated cluster

Time to reach steady state in large aquifers

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