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It is widely acknowledged that CO2, a greenhouse gas, is largely responsible for climatic changes that can lead to warming or cooling in various places. This disturbs natural processes, creating instability and fragility of natural and social ecosystems. To combat climate change, without compromising technology advancements and maintaining production costs at acceptable levels, carbon capture and storage (CCS) technologies can be deployed to advance a non-disruptive energy transition. Capturing CO2 from industrial processes such as thermoelectric power stations, refineries, and cement factories and storing it in geological mediums is becoming a mature technology. Part of the Mesohellenic Basin, situated in Greek territory, is proposed as a potential area for CO2 storage in saline aquifers. This follows work previously done in the StrategyCCUS project, funded by the EU. The current investigation includes geomechanical and petrophysical methods to characterise sedimentary formations for their potential to hold CO2 underground. The work is progressing under the Pilot Strategy, funded by the EU.

We study the circles of the Apollonian band circle super-packing, which are also the images of the real projective line by all linear fractional transformations of the group PLS (2 ,Z [ i ]) . This super-packing takes place on the torus rather than on the plane. We focus first on the connection between these circles and lattices in the ring of Gaussian integers. Then we explore the group structure of the set of such circles with given curvature. The end of this paper is more geometrically-minded and describes first a test to determine whether a Gaussian circle is Apollonian or not, then a process which yields quickly all Gaussian circles with curvature smaller than a given integer. We develop algorithms for each of the computational problems we encounter. MSC Classification: 11E25 , 11G99 , 20K01 , 52C26

The water supply in drylands mainly relies on groundwater, making it a crucial resource. Springs in southern Africa are often underutilized, and are neither protected nor monitored. Thus, the aim of this study was to evaluate their quality in a sample area in northwestern Namibia and to propose solutions for the sustainable use of springs. In total, 35 springs and hot springs were evaluated in the study area located in the drier part of Namibia (Pmean = 150–400 mm/year), an area highly impacted by ongoing climate change with longer and more frequent drought seasons. The springs there are mostly uncaptured and the discharge is in the form of surface runoff, which is mainly lost to the atmosphere by evaporation. Most of the studied springs were perennial, despite a severe drought period. Local communities rely on the springs mainly for livestock and human consumption, as well as for irrigation. However, 71% of the springs do not have any protective measures. The temperature, pH, conductivity and alkalinity were tested in situ. In total, 20 samples were collected and analyzed for major ions (boron, fluoride, silica and strontium) and total dissolved solids (TDS). The physical and inorganic results mostly indicated good and excellent quality water for human consumption, while the hot springs tended to have poor water quality in terms of Namibian standards, indicating that the water was not fit for human consumption.

International audience; Applied to Enhanced Oil Recovery, microemulsions are valuable systems for extracting the crude oil trapped by capillary forces in the porous reservoir rocks. The performances of the injected formulations are often assessed by quantifying oil composition in model systems that contain relatively high amount of surfactant/co-surfactant. Recently, the question of representativity of such systems was raised because kinetics aspects and complexity of crude were neglected in model systems and are likely to impact the process efficiency. The current quantification techniques limit the characterization of representative model systems as they are destructive, time consuming and not often applicable to dark or opaque systems. In the original aim to provide a quantitative kinetic study of such microemulsions, we propose a high resolution T1-weighted imaging technique to have access to 1D-composition profiles of co-surfactant, oil and brine in Winsor I, Winsor III and Winsor II microemulsions. The analysis is carried out on model systems at equilibrium for proof of concept. Results are correlated with X-Ray Micro-CT experiments to provide better interpretations and assess the method accuracy. We provide conditions of validity of the developed NMR method and discuss its potential limitations. To a larger extent, the method could be of interest to other applications that use similar systems.

Time varying media can dramatically modify the emission of embedded sources by producing time reversed waves refocusing on the source. Here we show that such a back action can create an angular momentum by setting the source in a spontaneous spin state. We experimentally implement this coupling using self-propelled bouncing droplets sources coupled to the surface waves they emit on a parametrically excited bath. The spin state dynamics result from a self-organized interplay between the source motion and the time reversed waves. The discrete stability analysis agrees with the experimental observations. In addition, we show that these spin states provide a unique opportunity for an experimental access to parameters enabling comparison and calibration of the various existing models.

A silica gel was applied in a porous gas reservoir, with the purpose of testing mitigation and remediation of CO 2 leakage from geological storage reservoirs. The gel has a high strength and a very low water-like viscosity, that extends its applicability to small pore diameters and low permeability media. The gel was prepared and applied on-site with oilfield equipment. Mixing was upscaled from laboratory- to field-scale, including one unsuccessful attempt. Environmental concerns and additional health and safety requirements were modest as the formulation was composed only of the non-toxic commercial silica-based product Betol K28 T, acetic acid and fresh water. A well was selected and prepared in the Becej natural CO 2 field in Serbia, the well and reservoir were prepared and the gel was placed into a 600 m deep CO 2 - and CH 4 -bearing sandstone layer. The reservoir was selectively sealed to the gas cap through a fast, CO 2 -selective gelation, while the hydraulic pathways of the liquid-filled part of the reservoir remained open. In the regions where no CO 2 was present the gelation reaction was slower and the kinetic was temperature-accelerated. A numerical model was used to simulate the impact of the pre-injection operational procedures and to quantify the impact on the temperature-dependent gelation time. The workflow aligns the needs of a research project with the interests and practical priorities of an operating company. A© 2019 Elsevier Ltd

International audience; We present a general novel technique to monitor saturation changes on small rock samples of only 15 mm in diameter and 20 mm in length for the purpose of assessing the kinetics of spontaneous imbibition processes. With a fully-3D imbibition configuration involving counter-current flows through all faces of the sample, the method is based on a NMR technique in which the sole oil phase present within the sample is monitored. The experimental method is fast for two reasons that are i) the possibility to perform accurate measurements on tiny samples, ii) the adoption of a 3D flow geometry. The kinetics of oil desaturation during spontaneous imbibition is analyzed with the help of an analytical 3D diffusion model, according to which the kinetics is proportional to the value of a "capillary" diffusion coefficient. For the purpose of demonstrating our methodology, we used this technique to compare the spontaneous imbibition of restored sandstone miniplugs from a sandstone reservoir, with and without alkali in the imbibing brine. The imbibition kinetics was quantified as capillary diffusion coefficient values. The studied case results revealed mixed impacts of alkali on the spontaneous imbibition kinetics, involving both a brine-oil interfacial tension change and a wettability alteration of the rock, the latter requiring further investigation beyond the scope of this article. List of symbols EOR Enhanced oil recovery IFT Interfacial tension [N/m] NMR Nuclear magnetic resonance D, 2r Diameter of the core (r: radius) [m] D c Capillary diffusion coefficient [m 2 /s] D e Effective molecular diffusion coefficient [m 2 /s] D m Molecular diffusion coefficient [m 2 /s] K Single-phase permeability [m²] k rw , k ro Relative permeabilities to water (w) and oil (o) L, 2l Length of the core (l: half length) [m]  w,  o Relative mobility for water (w) and oil (o) [Pa-1 .s-1 ]  w ,  o Viscosities of water (w) and oil (o) [Pa.s] Pc Capillary pressure [Pa]  Porosity of the core S w , S o Water (w) and oil (o) saturation S w *, S o * Scaled water (w) and oil (o) saturation (cf eq (3)) S wi , S oi Initial water (w) and oil (o) saturation S of Final oil saturation T 2 Transverse relaxation time [s] V Bulk volume of the sample [m 3 ] V p Porous volume [m 3 ] V o Oil volume in the core [m 3 ] V oi Initial oil volume in the core [m 3 ] ACKNOWLEDGEMENTS We thank OMV for allowing us to publish the results. Wolfgang Hujer is also acknowledged for providing laboratory data and rock description.

Investigations on geochemical processes involved in H2 generation in natural reservoirs are important for the present energy transition from fossil fuels. Most studies related to native H2 focus on the serpentinisation of ultra-mafic rocks, in which the source of H2 is linked to oxidation of Fe2+-bearing minerals and a reduction of H2O. In this study, we investigate abiotic H2 generation from a biotite-rich granite using an approach based on a study case of the Soultz-sous-Forêts geothermal site in France. Using a geochemical and reactive transport model and an existing database, we simulated the hydrothermal alteration of the granite at 130–200 °C with a redox potential from −100 mV to −300 mV. The simulations show that generation of native H2 is possible in an open system by hydrothermal alteration of biotite as a source of Fe2+, which oxidises to Fe3+ leading to precipitation of Fe3+ minerals and reduction of H+. The amount of H2 produced depends on the type of Fe2+/3+ minerals which precipitate. The optimal conditions for H2 generation are related to magnetite precipitation as in the serpentinisation process. We assessed that the upper bound for H2 production for this site is 102 kt per km3 of granite. This is in case of complete oxidation of all the ferrous iron contained in the biotite of the geothermal reservoir. In practice, only part of this potential may be attained owing to the limited current exploitation methods which are unlikely to result in a complete disequilibrium of the whole biotite. Future research needs to focus on how to reach most of the H2 potential. Simulations with increasing CO2 pressures suggest that CO2 injection can stimulate the H2 production. This study has implications for possible coupling of heat extraction and future exploitation with H2 production. Fil: Murray, Jesica María. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentina. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; Francia Fil: Clément, Alain. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; Francia Fil: Fritz, Bertrand. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; Francia Fil: Schmittbuhl, Jean. Université de Strasbourg; Francia. Centre National de la Recherche Scientifique; Francia Fil: Bordmann, Vincent. TotalEnergies SE; Francia Fil: Fleury, Jean Marc. TotalEnergies SE; Francia

International audience; The mechanism behind the 1H nuclear magnetic resonance (NMR) frequency dependence of T1 and the viscosity dependence of T2 for polydisperse polymers and bitumen remains elusive. We elucidate the matter through NMR relaxation measurements of polydisperse polymers over an extended range of frequencies (f0 = 0.01–400 MHz) and viscosities (η = 385–102 000 cP) using T1 and T2 in static fields, T1 field-cycling relaxometry, and T1ρ in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times T1LM ∝ f0 and T2LM ∝ (η/T)−1/2 with a phenomenological model of 1H–1H dipole–dipole relaxation, which includes a distribution in molecular correlation times and internal motions of the nonrigid polymer branches. We show that the model also accounts for the anomalous T1LM and T2LM in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the T1 ∝ f0 dispersion and T2 of similar polymers simulated over a range of viscosities (η = 1–1000 cP) are in good agreement with measurements and the model. The T1 ∝ f0 dispersion at high viscosities agrees with previously reported MD simulations of heptane confined in a polymer matrix, which suggests a common NMR relaxation mechanism between viscous polydisperse fluids and fluids under nanoconfinement, without the need to invoke paramagnetism.

The mechanism behind the

As part of the MiReCOL three-year European project ( www.mirecol-co2.eu ) on storage remediation technologies, we studied in the laboratory the capacity of foams to reduce gas flow for CO2-brine systems in rock core sample with common surfactants, as a function of interstitial velocity and gas to water fraction. Two different types of experimental set-up are used. They both allow local measurement of the water saturation for low and high pressure/temperature condition. A small MRI core-flood set-up is used to perform experiments at 35 °C and 10 bars pore pressure, in CO2 non-dense condition. In order to work under reservoir condition, at 40 °C and 130 bars, with dense super-critical CO2 we used a classical core-flood system coupled to an X-ray detection set-up. All experiments were carried out in similar Clashach sandstones with permeability between 220 and 1500 mD, and porosity in the range 10-20%. The gas and the surfactant-brine solution were co-injected at the core inlet face with a gas fraction around 0.7. We vary the interstitial velocity within two decades from about 3 ft/day up to 100 ft/day. The performance of the generated foams was evaluated from the relative foam viscosity, the ratio of the measured pressure drop in the presence of foam to the pressure drop in single phase condition for the same interstitial velocity. Whatever the pressure and permeability/porosity, the relative foam viscosity can be described as a power law vs. the shear rate evaluated from the interstitial velocity, permeability and porosity. The exponent is close to -1 describing the shear-thinning behavior.

A method for treating the surrounding of a well using a reactive suspension is studied, using silicate based solutions. They present the following key qualities: high performance, long term chemical stability, good injectivity, little environmental impact. We present an experimental investigation of the precipitation of commercial low-cost potassium silicate solutions, using a weak acid to lower the pH. Experiments were performed between 20 °C and 60 °C, by adding a non-hazardous acidic compound to the silica based solution. Bulk gelation times were estimated between a few minutes to 4 days. A coreflood injection experiment was then performed on an analog sandstone sample, representative of CO 2 formation, by using an optimum mixture with viscosity close to water. After a few days, a breakthrough experiment was performed by increasing gradually the differential pressure across the sample. Preliminary results indicate a very large strength of the order of 600 bar/m.

In the unlikely case of CO 2 leakage from a storage reservoir, it is desirable to close the leak efficiently and permanently. This could be done by injecting a reactive solution into the leak path, thereby immobilizing migrating CO 2 by consuming the gas and forming solid reactants. With regard to permanent closure, it is important to consider materials that are stable in the long-term, ensuring permanent CO 2 containment. Numerical modelling was applied to assess the feasibility of injecting calcium-rich water as a CO 2 -reactive solution to form calcite. A scenarios analyses of key parameters showed that the success of leakage remediation can be up to 95% when carefully balancing the injection rate and distance versus the permeability and leakage rate.

International audience; In this work, we studied experimentally and numerically the pressure–flow rate relationship for yield stress fluids in porous media. We developed and validated 3D numerical simulations of the velocity field via a lattice Boltzmann method based on the TRT scheme, and a specific experimental setup allowing yield stress fluids to flow in a closed-loop system to obtain stable rheological properties over a wide range of flow rates (4 decades). The porous medium studied experimentally is a sandstone. The flow properties were also simulated on a regular sphere packing and a random sphere packing as well as on a 3D geometry of a sandstone obtained by x-ray tomography. These different geometries allow highlighting the role of the heterogeneity of the pore structure on the flow properties. All results are expressed as the flow rate ˜Q versus the difference of the pressure gradient to the critical pressure (Δ˜P−Δ˜Pc); Pc defines the pressure below which there is no flow. We observed both numerically and experimentally three specific scaling regimes, already identified by Talon and Bauer [Eur. Phys. J. E 36, 139 (2013)] for a Bingham fluid in 2D porous media. We also evidenced the existence of two critical pressures: the “true” critical pressure ˜Pc defined as the pressure below which there is no flow and the “pseudo” critical pressure threshold ˜P∞c determined by fitting the data in the high-flow-rate regime. We show that the “true” critical pressure is always lower than the “pseudo” one in heterogeneous porous media and can be equal only in the case of regular porous structures. We explain these observations using an energy minimization principle.

Nuclear Magnetic Resonance (NMR) relaxation methods are key techniques for evaluating shales, from both cores and logging data. First, very small pore sizes, down to nano-meter length scales, can be detected and quantified if the NMR tool has the ability to measure relaxation times in the range [0.1–1 ms], and second the different proton populations (water, oil, gas, and kerogen) can be distinguished using 2D T1–T2 maps. We use a NMR instrument working at a higher frequency (23 MHz) that yields a much higher sensitivity than standard plug-size benchtop apparatus. In this work, we show the position of each proton population in this map: hydroxyls from the clay (T2

We consider the motion of a droplet bouncing on a vibrating bath of the same fluid in the presence of a central potential. We formulate a rotation symmetry-reduced description of this system, which allows for the straightforward application of dynamical systems theory tools. As an illustration of the utility of the symmetry reduction, we apply it to a model of the pilot-wave system with a central harmonic force. We begin our analysis by identifying local bifurcations and the onset of chaos. We then describe the emergence of chaotic regions and their merging bifurcations, which lead to the formation of a global attractor. In this final regime, the droplet's angular momentum spontaneously changes its sign as observed in the experiments of Perrard et al. (Phys. Rev. Lett., 113(10):104101, 2014)., Comment: Accepted for publication in Chaos: An Interdisciplinary Journal of Nonlinear Science

International audience; The interfacial area between two immiscible phases in porous media is an important parameter for describing and predicting 2 phase flow. Although present in several models, experimental investigations are sparse due to the lack of appropriate measurement techniques. We propose two NMR techniques for the measurement of oil-water interfacial area: (i) a time dependent NMR diffusion technique applicable in static conditions, similar to those used for the measurement of the solid specific surface of a porous media, and (ii) a fast relaxation technique applicable in dynamic conditions while flowing, based on an interfacial relaxation mechanism induced by the inclusion of paramagnetic salts in the water phase. For dodecane relaxing on doped water, we found an oil interfacial relaxivity of 1.8 μm/s, large enough to permit the measurement of specific interfacial surface as small as 1000 cm2/cm3. We demonstrate both NMR techniques in drainage followed by imbibition, in a model porous media with a narrow pore size distribution. While flowing, we observe that the interfacial area is larger in imbibition than in drainage, implying a different organization of the oil phase. In a carbonate sample with a wide pore size distribution, we evidence the gradual invasion of the smallest pores as the oil-water pressure difference is increased.

International audience; Spontaneous imbibition processes can play an important role in oil production. It can be enhanced or influenced by wettability changes generated by properly designed chemicals or by the natural surfactants resulting from reactive crude oils in the presence of alkaline solutions. The reaction of basic salts with some components of oil can, indeed, lead to the formation of natural soaps that reduces the interfacial tension between oil and brine. The latter scenario is studied herein on samples and oil from the St Ulrich oil field in the Vienna basin. To that end, spontaneous imbibition experiments were performed with two brines differing by the absence or presence of alkali. We first present a general novel technique to monitor saturation changes on small rock samples for the purpose of assessing the efficiency of a given recovery process. Samples of only 15 mm in diameter and 20 mm in length and set at irreducible saturation were fully immersed in the solution of interest, and the evolution of the samples' saturation with time was monitored thanks to a dedicated NMR technique involving the quantification of the sole oil phase present within the sample. A fully-3D imbibition configuration was adopted, involving counter-current flows through all faces of the sample. The experimental method is fast for two reasons: (i) the kinetics of capillary imbibition process is proportional to the square of sample size, i.e. very rapid if accurate measurements can be acquired on tiny samples, (ii) the present 3D situation also involves faster kinetics than the 1D configuration often used. The NMR technique was crucial to achieve such conditions that cannot be satisfied with conventional volumetric methods. The kinetics of oil desaturation during spontaneous imbibition is interpreted with the help of an analytical 3D diffusion model. For the alkaline solution, the diffusion coefficient is reduced by a factor of only two compared to the non-alkaline brine, although the interfacial tension between the oil and the imbibing solution is reduced by a factor of 10. Hence, a wettability change to a more water wet state has to be assumed when the alkaline solution replaces the non-alkaline solution in the imbibition process. However, no significant impact on the final saturation was observed.

International audience; Dispersion of a passive tracer in a tube has been extensively studied in the case of Newtonian fluids since the pioneer work of Taylor (1953). However, the influence of more complex rheological behavior on the transport has only be scarcely investigated. Non-Newtonian fluids are increasingly used in the industry and transport in this type of fluid merits therefore thorough investigations. An example of industrial application is Enhanced Oil Recovery, that is based on the injection of non-Newtonian fluids as polymers or surfactant solutions in porous media, which are then submitted to dispersion phenomena.This work deals with transport of a passive tracer in shear thinning fluids with and without yield stress whose constitutive behaviors are representative of a large number of industrial fluids. We focus on transport in capillary tubes, essential for the understanding of dispersion in porous media. Transport is investigated at different time scales by solving the advection–diffusion equation using a Two-Relaxation-Time Lattice–Boltzmann method. We also derived an analytical expression of the Taylor dispersion coefficient for a large range of fluid rheologies. Dispersion coefficients of all fluids described by the Herschel–Bulkley model can now be determined. Analytical and numerical results are compared and very good accordance is obtained.We discuss the characteristic time scales of the transport before reaching steady state as a function of fluid rheology and Péclet number. We show that the time to reach the dispersive regime is nearly independent of the fluid rheology whereas the effective dispersion coefficient is a function of the rheological parameters.We also present the displacement distribution of the tracer molecules (propagators) as a function of time and show that they are strongly conditioned by the fluid rheology. Indeed, propagators give valuable information on the temporal evolution of the concentration profile towards the stationary Taylor regime.

Low field NMR T(2) distribution is an efficient tool for characterizing pore size distribution in porous media. In NMR relaxation experiments, we measure the magnetization decay characterized by the so-called T(2) relaxation time, resulting essentially from liquid-solid interactions of the spins carried by molecules and exploring the pore space by diffusion. For multimodal systems, a new technique called T(2)-store-T(2) allows the analysis of diffusional pore to pore exchange that is extremely useful for the characterization of connectivity. This technique uses 2D inverse Laplace techniques and produces T(2)-T(2) maps. Qualitatively, a system is coupled when off-diagonal peaks are observed. Based on an analytical solution describing diffusional coupling between two pore populations, we propose defining a coupling factor that quantify the degree of coupling between the two populations, providing an easy understanding of the complex analytical solution. This analysis allows understanding 1D T(2) experiments as well, and indicates some limitations of the T(2) characterization when interpreted as a pore size distribution. We provide examples of bimodal pore structures in which we applied our methodology: a clay gel system, a shaly sandstone and two double porosity carbonates. These systems are also described by conventional techniques (mercury injection, SEM visualization) and illustrate weak, intermediate and strong coupling. Despite the presence of distribution of pore sizes, the two pore system exchange model gives satisfactory results for the quantitative analysis of the coupling and T(2)-store-T(2) experiments. In carbonates, more complex exchanges can occur between micro-, meso- and macropores.

With the perspective of long term CO2 storage, thermodynamic data are necessary for the simulations of CO2 injection in aquifers of depleted oil reservoirs. We focus here on the determination of the resistivity and viscosi ty of aqueous NaCl solutions with dissolved CO2. It is well known that dissolved CO2 increases slightly the water density. Recently, Bando et al. (2004) measured the viscosity of CO2 saturated brines and found an increase of viscosity with increasing amoun t of dissolved CO2. For resistivity, no data is available to our knowledge. We evaluated the effect of dissolved CO2 simultaneously on the resistivity and viscosity of three NaCl solutions covering the range of salinity usually encountered in potential CO2 storage geological formations. At a constant temperature of 35 ∘C, we show that the variation of resistivity and viscosity are proportional to the amount of dissolved CO2. For viscosity, our data are in agreement with previous observations. The observed variations are small, at maximum of the order of 10%. The increase of viscosity is well correlated with the variation of conductivity because ions are moving in a more viscous solvent and th erefore conductivity decreases. For the temperature dependence, w e used the Arps model to describe the measured variation of conductivity as a function of temperature. We propose a simple model to take into account the small modifications of conductivity and viscosity as a function of the dissolved CO2 fraction and temperature. Finally, we examine two potential impact s of these findings.

Water diffusivity D measured by using NMR techniques in Na-smectite suspensions decreases with increasing smectite fraction (up to 50 wt%), but increases with increasing salinity (NaCl or CaCl2 aqueous solutions) at a fixed clay fraction. The increase, larger for CaCl2 solutions, is explained by aggregation of clay particles when high salinities are reached. Macroscopic organisation of dense mixtures of clay and aqueous solutions can be inferred by T 2 transverse NMR relaxation times which are sensitive to the volume to surface ratio. Dispersed suspensions exhibit mono-modal T 2 distributions, whereas bimodal T 2 distributions are observed for flocculated systems. The bimodal T 2 distributions are interpreted as a measurement of the spacing between clay particles within aggregates and between aggregates. Finally, the diffusion data can be gathered in an unique curve using the Debye length and the measured spacing between particles. When the thickness of the electro-diffuse layer (Debye length) is of the same order as the spacing between clay particles, the water diffusivity decreases. Otherwise it is constant at about 2.22 ± 0.25 × 10 −9 m 2 / s . This last result illustrates clearly the effect of electro-chemical properties of smectite on water diffusivity.

In certain situations, such as in viscous liquids or nano-porous media, the transversal relaxation time T 2 may become so short that the pulse duration or the detector dead time is no longer negligible. These cases raise fundamental questions about relaxation during the radio-frequency pulses used in classical CPMG or Inversion-Recovery sequences. For an ideal system having a single relaxation time, we examine the effect of the finite pulse duration on the magnetization decay after the pulses occurring in FID, CPMG and Inversion-Recovery sequences. We solved analytically the Bloch equations during and after the pulses and compared the theoretical predictions with experimental data. Finally, we propose approximate simple expressions to correct the sequences for the magnetization attenuation during the pulses. IR curves are affected by transversal relaxation during the pulses, yielding asymmetric curves even if T 1 is very large. The magnetization decay obtained during a CPMG sequence is not affected by relaxation during the pulses. This is valid provided the time origin is chosen in the middle of the first pulse.

We present a preliminary study of the detection of flocculation by NMR relaxometry. For this purpose, various asphaltene solutions in the presence of solvents inducing aggregation and flocculation were prepared. The asphaltenes were extracted from one crude oil and various fractions of toluene and n -heptane were prepared to vary the solvent quality. We measured the transverse magnetization decays at 60 MHz using standard CPMG sequences. We show first that asphaltene aggregates have a well-defined signature typically at relaxation times in the order of 1 ms. In general, the signal can be split into two parts characterizing the asphaltene aggregates and the solvents. The asphaltene signature becomes bimodal near the flocculation threshold. The solvent relaxation rate varies linearly with the asphaltene concentration and this effect can be associated with viscosity variations. To cite this article: A. Prunelet et al., C. R. Chimie 7 (2004).

International audience; We propose a versatile model specifically designed for the quantitative interpretation of NMR velocimetry data. We use the concept of mobile or immobile tracer particles applied in dispersion theory in its Lagrangian form, adding two mechanisms: (i) independent random arrests of finite average representing intermittent periods of very low velocity zones in the mean flow direction and (ii) the possibility of unexpectedly long (but rare) displacements simulating the occurrence of very high velocities in the porous medium. Based on mathematical properties related to subordinated Lévy processes, we give analytical expressions of the signals recorded in pulsed-field-gradient NMR experiments. We illustrate how to use the model for quantifying dispersion from NMR data recorded for water flowing through a homogeneous grain pack column in single- and two-phase flow conditions.

CO2 capture, transport and storage (CCS) has the potential to significantly reduce the carbon emission that follows from the use of fossil fuels in power production and industry. Integrated demo-scale projects are currently being developed to demonstrate the feasibility of CCS and the first such projects are expected to start operating in Europe under the Storage Directive in the period 2015 – 2020. As part of the license application, these projects must develop a corrective measures plan, which describes the measures that can be taken when the CO2 in the subsurface behaves in an unexpected way. The MiReCOL project supports the development of corrective measures plans and helps building confidence in the safety of deep subsurface CO2 storage by providing a toolbox of techniques to mitigate and/or remediate undesired migration of CO2. MiReCOL aims to support the dialogue between CCS project operators and regulators by providing a clear description of the scope and feasibility of corrective measures.

International audience; New geophysical data collected at the Aden‐Owen‐Carlsberg (AOC) triple junction between the Arabia, India, and Somalia plates are combined with all available magnetic data across the Gulf of Aden to determine the detailed Arabia‐Somalia plate kinematics over the past 20 Myr. We reconstruct the history of opening of the Gulf of Aden, including the penetration of the Sheba Ridge into the African continent and the evolution of the triple junction since its formation. Magnetic data evidence three stages of ridge propagation from east to west. Seafloor spreading initiated ∼20 Myr ago along a 200 kmlong ridge portion located immediately west of the Owen fracture zone. A second 500 kmlong ridge portion developed westward up to the Alula‐Fartak transform fault before Chron 5D (17.5 Ma). Before Chron 5C (16.0 Ma), a third 700 km‐long ridge portion was emplaced between the Alula‐Fartak transform fault and the western end of the Gulf of Aden (45°E). Between 20 and 16 Ma, the Sheba Ridge propagated over a distance of 1400 km at an extremely fast average rate of 35 cm yr−1. The ridge propagation resulted from the Arabia‐Somalia rigid plate rotation about a stationary pole. Since Chron 5C (16.0 Ma), the spreading rate of the Sheba Ridge decreased first rapidly until 10 Ma and then more slowly. The evolution of the AOC triple junction is marked by a change of configuration around 10 Ma, with the formation of a new Arabia‐India plate boundary. Part of the Arabian plate was then transferred to the Indian plate.

Diffusion of dissolved CO2 is a possible transport mechanism in caprocks which should be evaluated precisely for long term storage of CO2. It is also a mechanism controlling the caprock geochemical reactivity in the vicinity of the top of the storage where CO2 accumulates driven by gravity. As a first step, we studied the diffusion properties of water in two low porosity caprock samples, giving an upper limit of the diffusion of CO2 dissolved in water. Two methods were used and compared on the same sample: standard through diffusion experiments with tritiated water, and low field NMR experiments with deuterated water. In through diffusion experiments, the sample is placed between two reservoirs; tritiated water is injected in one reservoir and its concentration measured in the other. In the NMR experiments, the sample is immersed in deuterated water and the water concentration inside the sample measured as a function of time. The NMR experiments also provide a measurement of porosity and pore size distribution, gradually invaded by D2O; in the present experiments, we observed that a small porosity fraction could not be exchanged with D2O. Both techniques give a comparable value of pore diffusion, namely 2.4 10−10 m2/s and 1.5 10−10 m2/s respectively for NMR and through diffusion methods. The porosity of the sample was 6%. Finally, we give some arguments for modeling diffusivity, useful for numerical simulators. The effective diffusivity is primarily a function of porosity and we suggest a power law dependence. Due to the similarity between diffusion and electrical properties in porous media, the exponent of the power law can be inferred from existing literature.

The objectives of the Geocarbone-Integrity program are to develop techniques, methodologies and knowledge concerning the long term confinement of CO2 in geological storage. Linked to other French programs such as Geocarbone Injectivity or Picoref, it is an integrated approach involving geochemistry, petrophysics, geology and geomechanics. Different scales must be considered in order to describe caprocks: from the pore or grain scale in petrophysics and geochemistry, to regional scale in geology and geomechanics. The program focused on a specific site of the Paris basin but the methodologies developed are general and can be applied elsewhere.At large scales, caprock formation were described using sismic and log data. They were available on the St Martin de Bossenay depleted oil field and the Dogger caprock was mainly studied. These data were compiled and analyzed in order to obtain the variation of thickness and detect possible discontinuities. At small scale, the identification of geological facies and the origin of the low porosity and permeability in terms of sedimentology and diagenesis is necessary. For clayey caprocks, the amount and type of clay is important and for tight carbonate formation, dolomitisation is a key element. The key petrophysical parameters are the entry pressure, permeability and diffusivity. Very low permeabilities, smaller than microDarcy’s, are difficult to evaluate using standard techniques and the results can vary by up to one order of magnitude depending on the method used. For the formation considered, a low permeability transition zone, the measured entry pressures were too low to prevent the CO2 migration. Finally, diffusivity of dissolved CO2 was also evaluated: There is a decrease typically of a factor of 20 compared to bulk diffusivity, despite very low permeabilities.When injecting CO2, mechanical constraints are generated in both the storage and caprock formations, and the bottom of the caprock can be affected by geochemical reactions. The effect of the increase of pressure on the stress field has been evaluated and the possibility of fault and fracture generation or reactivation is small for the injection conditions considered. At the laboratory, elastic moduli have been measured before and after alteration of caprock samples and very small effects were observed. From the geochemical point of view, the alteration by CO2 has been studied on two types of formation, either dominated by clays or by carbonate. Some reaction paths have been identified from the experiments performed at elevated temperature and pressure conditions (80/150 °C, 150 bar) during long periods of time (3 to 6 months). Finally reactive transport simulations were performed to assess the speed and extend of CO2 migration in an idealized homogeneous caprock. Considering different situations and all key parameters (diffusion, reaction, entry pressure, two phase transport), the results show that the CO2 migrates a few meters during one thousand years, and that the porosity mostly decreases by precipitation, and increases very locally at the base of the caprock by dissolution.

We investigated dispersion in homogeneous porous media (grain packs) by nuclear magnetic resonance (NMR) measurements and random walk simulations in pore networks. We measured water molecules displacements during a time interval tΔ by NMR measurements, which allows us to obtain propagators and charateristic cumulants of displacements such as the mean square displacement σ. The evolution of the cumulant σ as a function of time tΔ (σ2 ∝ taΔ) is a very sensitive test of Gaussian behaviour compared to the analysis of the shape of propagators. In a homogeneous 30μm grain pack and low Peclet numbers (15 < Pe < 45), we observed weak super dispersion in saturated conditions (a = 1.17) and gradually stronger super-dispersionas the water saturation decreases (up to a = 1.5 for 42 %) during steady-state oil-water two phase flow. Insaturated conditions, propagators and breakthrough curves are Gaussian or nearly Gaussian, whereas in two phase conditions, propagators are non symmetric and breakthrough curves show thick tails at long time. Weshow that the anomalous dispersion observed is better explained by Lévy stable laws (asymetric for longitudina ldispersion, and symetric for transverse dispersion) than by Gaussian laws. Random walk simulations were performed in a pore network constructed using high resolution images of the grain pack. They allow us to obtain the same informations than the NMR, with walkers submitted to diffusive and advective effects. The simulations show the existence of an anomalous stagnation not observed in experiments, highlighting the oversimplification of the pore network that prevent reproducing some aspects of the velocity field detected by NMR. However, the simulations indicate similarly a super-dispersion at long time in saturated conditions, La dispersion dans des milieux poreux homogènes (empilements de grains) a été étudiée par des mesures par résonance magnétique nucléaire (RMN) et des simulations de marches aléatoires dans un réseau de pores. La RMN permet de mesurer l’ensemble des déplacements des molécules d’eau durant un temps tΔ, et d’obtenir propagateurs et moments caractéristiques. L’évolution temporelle du second moment σ (σ2 ∝ taΔ) permet de caractériser de manière précise le régime de dispersion des molécules (Gaussien ou anormal). Des mesures pour des écoulements de 15 < Pe < 45 dans un empilement de grains de 30μm ont permis d’observer une dispersion anormale faiblement super-dispersive (a = 1.17) en écoulement saturé et une augmentation progressive du caractère super-dispersif avec la diminution de la saturation en eau (jusqu’à a = 1.5 pour 42 %)lors d’une co-injection stationnaire eau-huile. En écoulement saturé, les propagateurs et courbes de percée sont quasi-gaussiennes, tandis qu’en écoulement insaturé, les propagateurs sont asymétriques et les courbes de percée présentent des trainées aux grands temps. Dans ces conditions, on montre que la dispersion anormale observée est mieux décrite par des lois stables de Lévy que par des lois gaussiennes. Des simulations de marche aléatoire ont été réalisées dans un réseau de pores extrait d’un milieu poreux réel par imagerie microscanner.Elles permettent d’obtenir les mêmes informations que la RMN, les marcheurs se déplaçant par advection et diffusion. Ces simulations montrent l’existence d’une stagnation non observée dans les expériences, montrant que la simplification du réseau poreux est trop importante et empêche de reproduire certains aspects du champ de vitesses détecté par la RMN. Toutefois, l’évolution temporelle du second moment a également un caractère super-dispersif à temps long à 100 % de saturation

We investigated dispersion in homogeneous porous media (grain packs) by nuclear magnetic resonance (NMR) measurements and random walk simulations in pore networks. We measured water molecules displacements during a time interval tΔ by NMR measurements, which allows us to obtain propagators and charateristic cumulants of displacements such as the mean square displacement σ. The evolution of the cumulant σ as a function of time tΔ (σ2 ∝ taΔ) is a very sensitive test of Gaussian behaviour compared to the analysis of the shape of propagators. In a homogeneous 30μm grain pack and low Peclet numbers (15 < Pe < 45), we observed weak super dispersion in saturated conditions (a = 1.17) and gradually stronger super-dispersionas the water saturation decreases (up to a = 1.5 for 42 %) during steady-state oil-water two phase flow. Insaturated conditions, propagators and breakthrough curves are Gaussian or nearly Gaussian, whereas in two phase conditions, propagators are non symmetric and breakthrough curves show thick tails at long time. Weshow that the anomalous dispersion observed is better explained by Lévy stable laws (asymetric for longitudina ldispersion, and symetric for transverse dispersion) than by Gaussian laws. Random walk simulations were performed in a pore network constructed using high resolution images of the grain pack. They allow us to obtain the same informations than the NMR, with walkers submitted to diffusive and advective effects. The simulations show the existence of an anomalous stagnation not observed in experiments, highlighting the oversimplification of the pore network that prevent reproducing some aspects of the velocity field detected by NMR. However, the simulations indicate similarly a super-dispersion at long time in saturated conditions., La dispersion dans des milieux poreux homogènes (empilements de grains) a été étudiée par des mesures par résonance magnétique nucléaire (RMN) et des simulations de marches aléatoires dans un réseau de pores. La RMN permet de mesurer l’ensemble des déplacements des molécules d’eau durant un temps tΔ, et d’obtenir propagateurs et moments caractéristiques. L’évolution temporelle du second moment σ (σ2 ∝ taΔ) permet de caractériser de manière précise le régime de dispersion des molécules (Gaussien ou anormal). Des mesures pour des écoulements de 15 < Pe < 45 dans un empilement de grains de 30μm ont permis d’observer une dispersion anormale faiblement super-dispersive (a = 1.17) en écoulement saturé et une augmentation progressive du caractère super-dispersif avec la diminution de la saturation en eau (jusqu’à a = 1.5 pour 42 %)lors d’une co-injection stationnaire eau-huile. En écoulement saturé, les propagateurs et courbes de percée sont quasi-gaussiennes, tandis qu’en écoulement insaturé, les propagateurs sont asymétriques et les courbes de percée présentent des trainées aux grands temps. Dans ces conditions, on montre que la dispersion anormale observée est mieux décrite par des lois stables de Lévy que par des lois gaussiennes. Des simulations de marche aléatoire ont été réalisées dans un réseau de pores extrait d’un milieu poreux réel par imagerie microscanner.Elles permettent d’obtenir les mêmes informations que la RMN, les marcheurs se déplaçant par advection et diffusion. Ces simulations montrent l’existence d’une stagnation non observée dans les expériences, montrant que la simplification du réseau poreux est trop importante et empêche de reproduire certains aspects du champ de vitesses détecté par la RMN. Toutefois, l’évolution temporelle du second moment a également un caractère super-dispersif à temps long à 100 % de saturation.