Consistent experimental investigation of the applicability of Biot-Gassmann’s equation in carbonates

International audienceCarbonate formations are characterized by multiscale heterogeneities which control theirflow and acoustic responses. At the laboratory scale, carbonate rocks already do not show astrong correlation between P- and S-wave velocities and porosity. The velocity disparitiesbetween carbonates of similar mineralogy and porosity result from different microstructuresderived from their sedimentary facies and subsequent diagenetic transformations. The stilldiscussed applicability of Biot-Gassmann’s equation for fluid substitution in carbonate rocksremains another key issue. We propose an integrated experimental workflow that allows aconsistent checking of the applicability of Biot-Gassmann’s equation and provide key geologicaland microstructural information to understand the petroacoustic signature of carbonate rocks.This approach is implemented on samples representative of two different carbonate formations.The obtained results demonstrate the applicability of Biot-Gassmann's equation for the twostudied carbonate families and show the interrelation of mineralogy and porosity distribution intheir acoustic response

Effective medium modeling of diagenesis impact on the petroacoustic properties of carbonate rocks

International audienceCarbonate formations are highly heterogeneous, and the velocity-porosity relationships are controlled by various microstructural parameters, such as the types of pores and their distribution. Because diagenesis is responsible for important changes in the microstructure of carbonate rocks, we have extended the standard effective medium approach to model the impact of diagenesis on the carbonate elastic properties through a step-by-step effective medium modeling. Two different carbonate rocks deposited, respectively, in lacustrine and marine environments are considered in this study. The first key step is the characterization of the diagenesis, which affected the two studied carbonate sample sets. Effective medium models integrating all of the geologic information accessible from petrographic analysis are then built. The evolution of the microstructural parameters during diagenesis is thoroughly constrained based on an extensive experimental data set, including X-ray diffraction analysis, different porosimetry methods, and ultrasonic velocity measurements. A new theoretical approach including two sources of compliance is developed to model the specific behavior of carbonates. A compliant interface is introduced around the main carbonate grains to represent grain contacts and the different pore scales are taken into account through multiscale modeling. Finally, direct calculations with the model provide elastic wave velocities representative of the different diagenetic stages. An extrapolation to permeability evolution is also introduced. This approach allows the identification of the acoustic signature of specific diagenetic events, such as dolomitization, dissolution, or cementation, and the assessment of their impact on the elastic properties of carbonates

Towards a better comprehension of reactive transport coupling experimental and numerical approaches

In this work we focus on further understanding reactive transport in carbonate rocks, in particular limestones characterized by a bimodal pore size distribution. To this end, we performed injection experiments with CO2-saturated water on a sample of Euville limestone and monitored the experiments with a medical CT scanner. Microscanner imaging was performed before and after alteration. Experiments showed that permeability increased by nearly two decades due to the alteration process. This increase could be attributed to the formation of a preferential dissolution path visualized on the CT images. Microscanner images show that preferential dissolution areas are characterized by the presence of numerous enlarged macropores. The preferential dissolution path created therefore retains a porous structure and does not correspond to a wormhole-type channel. To provide further knowledge of the small-scale physics of reactive transport, we performed Lattice-Boltzmann simulations of flow in a numerically generated model 2D porous medium having geometrical and topological features designed to approach Euville limestone. We showed that the fluid velocity increased in nearly percolating paths of macropores. Considering the experiments, this means that the CO2-saturated water starts to enter high-velocity zones earlier than low-velocity zones, inducing an earlier onset of the alteration process and a more pronounced local dissolution. However, numerical results showed that the alteration of non-connected macropores leads to an increase of permeability much smaller than the experimentally observed one. To explain this fact we used effective medium modelling that permits predicting the variation in permeability as a function of the fraction of macropores and consequently as a function of alteration. It proved that as long as there is no alteration-induced percolating path consisting of macropores, the increase in permeability is relatively low as shown by the Lattice-Boltzmann simulations. An increase in permeability of several orders of magnitude is only observed when the macroporosity is close to the percolation threshold. This fact is in accordance with the experimentally observed results

French national epidemiology of bacterial superinfections in ventilator-associated pneumonia in patients infected with COVID-19: the COVAP study

International audienceAbstract Background Description and comparison of bacterial characteristics of ventilator-associated pneumonia (VAP) between critically ill intensive care unit (ICU) patients with COVID-19-positive, COVID + ; and non-COVID-19, COVID-. Methods Retrospective, observational, multicenter study that focused on French patients during the first wave of the pandemic (March–April 2020). Results 935 patients with identification of at least one bacteriologically proven VAP were included (including 802 COVID +). Among Gram-positive bacteria, S. aureus accounted for more than two-thirds of the bacteria involved, followed by Streptococcaceae and enterococci without difference between clinical groups regarding antibiotic resistance. Among Gram-negative bacteria, Klebsiella spp. was the most frequently observed bacterial genus in both groups, with K. oxytoca overrepresented in the COVID- group (14.3% vs . 5.3%; p < 0.05). Cotrimoxazole-resistant bacteria were over-observed in the COVID + group (18.5% vs . 6.1%; p <0.05), and after stratification for K. pneumoniae (39.6% vs . 0%; p <0.05). In contrast, overrepresentation of aminoglycoside-resistant strains was observed in the COVID- group (20% vs . 13.9%; p < 0.01). Pseudomonas sp. was more frequently isolated from COVID + VAPs (23.9% vs . 16.7%; p <0.01) but in COVID- showed more carbapenem resistance (11.1% vs . 0.8%; p <0.05) and greater resistance to at least two aminoglycosides (11.8% vs . 1.4%; p < 0.05) and to quinolones (53.6% vs . 7.0%; p <0.05). These patients were more frequently infected with multidrug-resistant bacteria than COVID + (40.1% vs . 13.8%; p < 0.01). Conclusions The present study demonstrated that the bacterial epidemiology and antibiotic resistance of VAP in COVID + is different from that of COVID- patients. These features call for further study to tailor antibiotic therapies in VAP patients