Dogger Aquifer Research Articles

Numerical modeling of fluid–rock chemical interactions at the supercritical CO2–liquid interface during CO2 injection into a carbonate reservoir, the Dogger aquifer (Paris Basin, France)

A project of geological CO2 storage in the deep Dogger aquifer in the Paris Basin (France) is under development. Before effective containment can be assured, investigations need to be carried out on reservoir behavior when subjected to physical, chemical and mechanical perturbations induced by CO2 injections. The aim of this study is to present the numerical results of two CO2 injection scenarios, firstly with CO2-saturated water and secondly with pure supercritical CO2. The simulation results confirm the high reactivity of CO2-saturated water, which can dramatically damage the reservoir structure. On the other hand, supercritical CO2 injection appears to be weakly reactive, with a limited modification of well injectivity. Supercritical CO2 reacts differently to CO2-saturated solution: firstly, it dissolves into aqueous solution and it increases both water acidity and mineral dissolution potential, favoring augmented porosity. Following this step, numerical simulations demonstrate that hydraulic processes induced by supercritical CO2 injection are accompanied by a desiccation phenomenon of the porous medium. Irreducible water, entrapped in pores, sustains the increase in CO2 pressure. When the pressure is sufficiently high and under a continuous dry (without water vapor) CO2 flux, an evaporation process starts leading to the precipitation of salts and possibly other secondary minerals. Although there has been little focus on this desiccation process in the literature until now, it nevertheless must constitute an important risk of both a modification in porosity and well injectivity.

Impact of the landscape evolution on the hydraulic boundary conditions of the Callovo–Oxfordian formation

The Callovo–Oxfordian formation in the Eastern part of France was recognized as a potential nuclear waste repository layer. The Andra (National Agency for Nuclear Waste Management) has launched a few years ago a research program that aims to define the mechanisms of importance in the impact of the surface environment evolution on the site hydrogeology. Based on mapping and dating results, Andra has quantified the geomorphological evolution of the Meuse/Haute-Marne site in the past and has estimated the future evolution over 1 million years. The Callovo–Oxfordian boundary conditions depend on the hydraulic heads in the two surrounding aquifers, the Oxfordian limestone above, and the Dogger one, below. Both aquifer outcrops are modified over the next million years. For the present study, the geomorphologic evolution is considered independently of other processes and translated in the hydrogeological model in terms of changes in the hydraulic boundary conditions at the surface. The hydrodynamic simulations have been performed with the code Cast3M (implemented by CEA (Atomic Energy Commissariat)) using a mixed hybrid finite-element formulation. For these groundwater flow simulations, three modelled stages are presented: the Present, 500,000 years (500 Ky) and 10 6 years (1 Ma) in the future. The landscape evolution is merely considered through the use of three different topographies on which the boundary conditions are applied. According to Andra predictions, at the Meuse/Haute-Marne site, the valley incisions on the Bure plateau will locally reach the Oxfordian limestone. Thus, the Oxfordian aquifer exhibits more changes due to topographic evolution than the Dogger aquifer. Hydrodynamic simulations show a significant impact of the valley incisions on the groundwater flow by the creation of local outlets to the Oxfordian limestone aquifer in the North of the area for the 1 My topography. It induces local perturbations of the saturation level. The global erosion of the topography also pulls down the hydraulic heads on a regional basis. Changes induced by the geomorphologic evolution in the Dogger aquifer are located around 30 km East from the Underground Laboratory site, at the outcropping areas. Thus, at the Laboratory location, the piezometric surface does not show, for the future, significant modifications compared to the present state. The 20-m general lowering of the hydraulic heads in the Dogger is also globally less important than for the Oxfordian aquifer. The consequence on the Callovo–Oxfordian boundary conditions for the future is an increase of the internal upward vertical hydraulic gradient which, at present state, is well developed only immediately to the North of the Laboratory.

Geochemical evidence for efficient aquifer isolation over geological timeframes.

Aquitards-layers of rock having low permeability-have been suggested as potential long-term reservoirs for toxic materials such as nuclear or chemical waste. But information about the isolation properties of aquitard layers is essential to evaluate whether they can indeed be used safely as reservoirs. Here we investigate the long-term mobility of groundwaters between two aquifers surrounding an aquitard layer in the eastern recharge area of the Paris basin, France, using helium isotopes as a geochemical tracer. The deeper Trias sandstone aquifer, which lies above the crystalline basement, accumulates radiogenic (4)He and primordial (3)He from large regions of the crust and mantle at rates comparable to the degassing of the whole crust and of mid-ocean ridges. We show that the overlying carbonate Dogger aquifer, which is separated from the Trias aquifer by an aquitard layer consisting of a approximately 600 m succession of shales and clays, is stagnant and has been extremely well isolated from the Trias over the past several million years. This finding, together with previous studies at the centre of the Paris basin, shows that diffusive mass transfer across aquitards is negligible and that cross-formational flow in basins takes place preferentially in faulted areas.

Calcul de l'evolution de la permeabilite des reservoirs sedimentaries contenant des argiles; application a la zone de la faille de Bray (bassin de Paris)

Abstract We propose a model for simulating the changes in porosity and permeability caused by hydrothermal diagenesis in sedimentary aquifer where salinity, temperature and fluid flow vary in space and time. Such modifications of the hydrodynamic properties of the medium are bounded to geochemical reactions and groundwater flow. Fluid velocity is particularly low in deep reservoirs (typically less than 1 m/year). Then, the local equilibrium simplification, which is justified by a set of world-wide data of the chemical composition of groundwater, can be implemented toward straightforward transient calculations. In the model presented here, the coupled processes of fluid flow, temperature and chemical species transport are solved using well established methods. The originality of the model is the development carried on to predict the permeability evolution controlled by the mineral dissolution and precipitation. Usually to simulate permeability changes modelers use the classical porosity-permeability model based on statistical analyses of in situ or laboratory measurements. However, hydraulic conductivity changes are not controlled solely by porosity changes, but also depend on pore-scale structure transformations. Depending on the mineral type, the precipitation or dissolution of the same quantity of volumetric quantity will induce very different changes in the hydraulic conductivity. Principally clay minerals depict a wide range of atypical organisations of different microstructural characteristics of the porous media. The spatial distribution of these characteristics cannot be modelled at basin scale. Away from both too complicated and too unrealistically simplified approach, the model presented here is based on the calculation of the permeability evolution from the change in the mineral fraction due to mineral precipitation and dissolution. To simplify, the minerals are divided into two groups: clay minerals and non-clay minerals. The specific contribution of clay minerals is controlled by a single weighting coefficient. This coefficient is associated to the proportion of poorly connected porosity that characterize clay structure, albeit it is presently impossible to propose any quantitative relationship between the value of this parameter and the microstructural characteristics of the diagenetic clays. The model is tested here to simulate the evolution of the porosity and the permeability in a peculiar zone of the Paris Basin. The study area of several hundred meters large is inside the Dogger aquifer, close to the Bray fault zone where invasion of saline water from Triassic formation takes place. This zone is characterised by high thermal and salinity gradient as well as by the superposition of sub-horizontal regional flow and ascendant fault-controlled flow: it is an ideal case study for examining the importance of taking into account the specific contribution by clay minerals when computing permeability evolution. This study is proposed as a parameter sensibility analysis: - to compare the relative influence of the clay weighting coefficient, the temperature, the salinity, and the cementation exponent on the computed evolution of the permeability, - to discuss the consequences of the introduction of the clay weighting coefficient in comparison to the classical porosity - permeability evolution model, - to simulate various evolution scenarios of past and future thermal and geochemical constraints and their consequences on the evolution of the permeability changes in the Bray fault zone taking into account uncertainties on the value of the clay weighting coefficient and on the cementation exponent. Forty-one simulations of one million years were necessary to cover a large spectrum of the expected variations of each parameter. The results show that: - the local variation of the permeability depends on the time evolution of temperature and of salinity, and on the values of the cementation exponent of the porosity-permeability law and of the clay weighting coefficient. Within reasonable ranges of these four parameters, their influence on the permeability changes is of the same order of magnitude, - the influence of the clay weighting coefficient on the porosity evolution is negligible. Feedback effects of permeability evolution on the porosity evolution, through the change in the flow regime, is minor, - by the use of a classical model without a clay weighting coefficient, permeability and porosity present the same pattern of evolution: they both increase or decrease. By the use of the clay weighting coefficient, in some places the permeability and porosity can show opposite evolution. One increases when the other decreases even for low values of the coefficient, - in the vicinity of the fault, the model predict an increase of permeability independently of potential temperature and salinity modifications and whatever the clay mineral weighting coefficient is: Bray fault sealing is unlikely as long as head gradient is maintained in the fracture zone.

Solubility of silica polymorphs in electrolyte solutions, II. Activity of aqueous silica and solid silica polymorphs in deep solutions from the sedimentary Paris Basin

Activity coefficient for aqueous silica in saline waters and brines from the Paris Basin was calculated using Pitzer's specific interaction model. Quartz and chalcedony are the only reported authigenic silica minerals in the Dogger aquifer of the Paris Basin (France). However, the measured silica concentrations fall between those of these two phases. The silica concentrations measured in Dogger fluids seem to be controlled by a microcrystalline quartz phase with a grain size computed to be about 20 nm. Studies have shown that pressure can preserve small grain size for a long time at the geological scale. The effective mechanism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain-contact inducing a quartz supersaturation and prohibits the increase in size of reprecipitated microcrystalline quartz grains. This hypothesis is supported by other studies reported in the literature. The proposed model, which incorporates silica mineralogy and a precise calculation of aqueous silica activity, allows us to explain measured silica concentrations in the deep sedimentary solutions of the Dogger aquifers. In the Keuper brines, silica solubility can in most cases be explained by an equilibrium with either chalcedony or quartz. Another application of the present work is shown by an example, where we examined the importance of precisely evaluating the activity coefficient in basin characterisation, as the goal of reservoir characterisation is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturations.

Noble gases in crude oils from the Paris Basin, France: Implications for the origin of fluids and constraints on oil-water-gas interactions

In order to investigate the potential of noble gases to trace the dynamics of oil reservoirs, we have analysed the abundance and isotopic composition of all noble gases (He, Ne, Ar, Kr, and Xe) in crude oils from the Paris Basin, France, using a new extraction and purification procedure. The main oil reservoirs are presently located in the Jurassic (Dogger) limestone and in the Triassic (Keuper) sandstone, but hydrocarbons originated from a common source rock formation located in the interbedded Liassic sequence. Despite this common origin, the abundance and isotopic ratios of the noble gases differ between the Dogger and the Keuper. The isotopic compositions of Kr and Xe are indistinguishable from that of air. 3He/ 4He ratios, higher than those predicted from radiogenic production in the sediments or in the crust, are attributed to the occurrence of mantle-derived 3He in the basin. Each sedimentary sequence is characterised by well defined and homogeneous 21Ne/ 22Ne and 40Ar/ 36Ar ratios, which average 0.0306 ± 0.0008 and 312 ± 10 for the Dogger and 0.0367 ± 0.0012 and 664 ± 30 for the Keuper, respectively. The main source of radiogenic noble gases appears to be the continental crust underlying the basin, with possible regional contributions of noble gas isotopes produced in the sediments. The helium and argon isotopic ratios of the Dogger oils are very similar to those observed in geothermal waters flowing in the Dogger aquifer throughout the basin, demonstrating that noble gases in oils derive from associated groundwaters. Oil reservoirs in the Paris Basin therefore accumulate noble gases from wide regions of the continental crust through cross-formational flow of groundwaters and subsequent partitioning into oil. This observation implies that noble gases cannot be directly used to date oils, but can provide time constraints if (1) water/oil interactions are quantified and (2) the residence time as well as the noble gas characteristics of associated groundwaters are known. Oil-water-gas partitioning processes are well recorded in the fractionation of noble gas elemental abundance. Two distinct processes have been identified: (1) accumulation of atmosphere-derived (ANG) and radiogenic noble gases both initially dissolved in groundwaters, resulting in a positive correlation between absolute amount of ANG and the extent of heavy noble gas fractionation and (2) subsequent fractional degassing, resulting in a negative correlation between ANG abundance and heavy noble gas fractionation. Degassing is particularly evident for the Keuper oils and might have occurred in the reservoirs following hydrodynamic gas stripping. The ANG abundance in the Dogger reservoirs requires that about one order of magnitude more water than presently observed has interacted with the oil. Given current estimates of the residence time for groundwaters in the Dogger aquifer, the duration of oil-water interaction is in qualitative agreement with a Palaeocene-Oligocene age for the major episode of secondary oil migration in the Paris Basin. High xenon contents in Keuper reservoirs suggest that they have experienced hydrodynamic interactions between flowing waters and oils for long time, and that the Trias might have reached hydrostatic condition only recently.

Comment on “Helium Isotope Fluxes and Groundwater Ages in the Dogger Aquifer, Paris Basin” by B. Marty, T. Torgersen, V. Meynier, R. K. O'Nions, and G. de Marsily

Comment on “Helium Isotope Fluxes and Groundwater Ages in the Dogger Aquifer, Paris Basin” by B. Marty, T. Torgersen, V. Meynier, R. K. O'Nions, and G. de Marsily

Reply [to “Comment on ‘Helium Isotope Fluxes and Groundwater Ages in the Dogger Aquifer, Paris Basin’ by B. Marty, T. Torgersen, V. Meynier, R. K. O'Nions, and G. de Marsily”

Reply [to “Comment on ‘Helium Isotope Fluxes and Groundwater Ages in the Dogger Aquifer, Paris Basin’ by B. Marty, T. Torgersen, V. Meynier, R. K. O'Nions, and G. de Marsily”

Stable isotope conservation and origin of saline waters from the Middle Jurassic aquifer of the Paris Basin, France

Seventy-nine samples of formation water from geothermal boreholes, oil-field and ground-water wells, tapping the Dogger (Middle Jurassic) limestone reservoir of the Paris Basin, have been analyzed for their 2H, 18O, TDS, bromide and chloride contents. Geostatistical mapping of 2H and of 18O contents and their variations with chloride content indicate that both isotopes have a common origin and a similar behaviour within the aquifer. The σ 2H and σ 18O values increase progressively from the outcrops toward a zone where temperature, Cl − and TDS values are high. The σ 2H and σ 18O values are very uniform in this zone and do not show any relation with formation temperature, in contrast to what is generally observed in other sedimentary basins. A discussion of isotopes exchange with the carbonate matrix for oxygen, and with H 2S for deuterium, shows that such processes are not controlling the heavy isotope contents of groundwaters. This is attributed to the relatively short residence time of groundwaters in the aquifer. Both 2H and 18O contents of groundwater may thus be considered as conservative tracers in the Dogger aquifer of the Paris Basin and their initial contents are modified only by fluid mixings. Complex mixing processes account for stable isotope composition, TDS, Cl − and Br − values of the Dogger fluids. All mixtures include a brine component. Meteoric water percolated and dissolved halite in the Triassic aquifer (secondary brine). This saline solution mixed with some residual primary brine, then migrated via vertical faults into the Dogger aquifer. Such migration is found to be in agreement with data from fluid inclusions in late carbonate cements from the Liassic and Dogger strata. The requirement that these units were exposed implies that meteoric invasion could not have occurred before the Tertiary. Successive mixings of the brine with several types of meteoric waters depleted in heavy isotopes account for the present isotope and chemical compositions of the Dogger fluids.

Geochemistry and origin of formation brines from the Paris Basin, France: 2. Saline solutions associated with oil fields

Oil fields from the Paris Basin are located in three geological levels: Dogger (Middle Jurassic), Rhaetian and Keuper (Upper Triassic). The origin of water and dissolved salts in saline solutions associated with oil is investigated, using the hypothesis that Cl − and Br − are conservative in solution. In a diagram of Cl − Br − ratios vs. Cl −, the points of the Dogger and the Keuper aquifers lie in the zone of primary brines whereas those of the Rhaetian are located in the zone of secondary brines. The Na + Cl − ratio is very high in all of the saline solutions, suggesting either a relatively poorly evolved brine (at the beginning of halite deposition) or some halite dissolution. All of the saline solutions, especially those from the Dogger aquifer, are clearly diluted. Dilution by meteoric waters would not alter the Cl − Br − ratio. High concentrations of lithium, rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures of secondary brines formed by dissolution of halite and small amounts of sylvite and Ca-sulphate. Mass-balance calculations of the excess of Cl − with respect to Na + + K + allows the fraction x of water derived from the primary brine to be calculated. Assumptions are that all Na + and K + is derived from the secondary brine and that the Cl − content of the primary brine is constrained by bischofite saturation at ∼ 11,000 mmol kg −1. The typical proportion of water from the primary brine waters is ∼ 0.7% in the Dogger formation and ∼ 2.4% in the Keuper. Values of the concentration factor CF, expressed as the ratio of the molal concentration of a conservative ion in the evolved brine to its concentration in seawater have been calculated for lithium. Solutions from the Dogger and Keuper aquifers have CF Li + -values of 1.8 · 10 3 and of 7.7 · 10 3, respectively. In the Rhaetian aquifer, the linear relationship between heavy-isotope contents of the brine ( 2H and 18O) and the Cl − content indicates a mixture of two water sources. The first one is meteoric water as defined by the intersection of the formation water line with the global meteoric water line ( δ 2 H = −65‰ , δ 18 O = −9.3‰ ). The other end-member is enriched in heavy isotopes and in Cl −, and is attributed to seawater that has dissolved evaporites (mainly halite). However, the presence of small amounts of an extremely evolved brine is required to account for the bromide, lithium, rubidium and boron contents of the formation brine from the Rhaetian aquifer. As this contribution is very limited in amount, it cannot affect the heavy-isotope content of the formation water. Mass-balance equations are solved for ternary mixtures of meteoric water, seawater and brine. The same constraints as above (no Na + and K + supplied by the primary brine and Cl Brine ≈ 11,000 mmol kg −1) are applied. It is also assumed that the seawater component has the same ionic contents as modern seawater and that the salt content of the meteoric component is negligible. The fraction of primary brines is x ≈ 1.4% according to the Cl − balance. This estimate agrees with that obtained from the lithium balance assuming that the primary brine component was the same as that of the Keuper formation water. Mass-balance calculations also indicate that the SO 4 2− content of the Rhaetian formation water is exclusively due to the secondary brine component. In the three saline solutions, the very high Ca 2+ contents and high Ca 2+ Mg 2+ ratios, may be attributed to reaction of the very evolved brine (free of SO 2− 4), with gypsum or anhydrite. The result is an exchange of Mg 2+ for Ca 2+, which causes a release of Ca 2+ to the solution and a precipitation of secondary Mg-sulphate. This process can also account for the high strontium contents of the analysed samples. Values of the ratio 87 Sr 86 Sr are very high in the Keuper formation water due to close contact of the solutions with the granitic basement or with granite-derived detrital deposits. Values in the Rhaetian and Dogger aquifers equal those of Upper Triassic marine deposits. The stable isotope ( 34S and 18O) content of the dissolved aqueous SO 2− 4 indicates a Triassic origin for the bulk of sulphate involved in the evolution of formation waters from the Rhaetian and from the Dogger. All geochemical indicators thus show a vertical migration of Triassic brines. However, aqueous SO 2− 4 from the Keuper has a Permian isotopic signature suggesting a reworking in solution of salts from this period.

Quantitative study of geochemical processes in the Dogger aquifer, Paris Basin, France

The quantification of geochemical reactions in hydrothermal aquifers requires an exhaustive approach because of their interdependence. All chemical elements likely to have a quantitative influence on dissolution or precipitation reactions have to be taken into account. Geochemical constraints the number of which equals the number of the chemical elements, were determined with the help of chemical analyses of the solution. In the case study of the Dogger aquifer (Paris Basin, France), 12 elements are taken into account (Al, C, Cl, Ca, F, H, K, Mg, Na, O, S, Si). Three constraints apply to water-activity, neutrality and conservation of chloride in aqueous solutions. The determination of the remaining constraints was based on saturation indices, stability diagrams of minerals and partial pressure of carbon dioxide ( pCO 2). The present-day solutions are at equilibrium with respect to nine minerals (albite, anhydrite, chalcedony, calcite, dolomite, fluorite, kaolinite, K-feldspar and illite or chlorite). Thus, it was demonstrated that the composition of these solutions (including computed pCO 2) is only a function of temperature and chloride content. Moreover, it was possible to test the validity of the geochemical system by computing its speciation and comparing these theoretical results with actual chemical analyses (pH and concentrations). Finally, geochemical simulations were used in predicting what quantities would be dissolved or precipitated, as either the temperature or the chlorinity varied. Although the rock is predominantly calcareous, these quantities could not be determined if the influence of the aluminosilicates were neglected. This chemical component approach with which one can pose and solve rigorously the chemical equilibrium problem constitutes a prerequisite for the quantitative study of geochemical processes related to fluid flow.

Helium isotope fluxes and groundwater ages in the Dogger Aquifer, Paris Basin

Helium isotopic and rare gas measurements in the Dogger aquifer of the Paris Basin have shown the helium concentration and helium isotopic ratio to be first‐order homogeneous with only slight differences in relation to specific chemical water types. Helium in the Dogger aquifer contains quantities of 3He 3–4 times in excess of radiogenic production/accumulation from natural decay of U, Th series elements. Calculations of cosmogenic production/accumulation of 3He cannot account for the observed excesses, and 3He excess is attributed to the addition of a mantle‐derived He magmatic component. Calculations of the isotopic fluxes of helium out of the Dogger (4He flux, 2×10−7 mol 4He m−2 yr−1; 3He flux, 1.8×10−14 mol 3He m−2 yr−1 (±50%)) suggest that the helium residence time and the groundwater residence time in the Dogger aquifer are 4 Myr (±50%). This age estimate is at least 10 times as great as estimates derived from hydrologic models but is more consistent with the geochemistry of the water.

Regional modelling of groundwater flow and salt and environmental tracer transport in deep aquifers in the Paris Basin

A hydrodynamic model which takes into account the aquitard storage effect was developed for the deep multilayered system including the Dogger aquifer and its surroundings in the Paris Basin. It provides a good explanation for a series of observations in the Dogger concerning, for instance, the hydraulic head, the salinity and the transmissivity. The calibration of the model also makes it possible to estimate some unmeasured parameters such as the aquifer and aquitard storage coefficients. Finally, the results of simulations of the transport of 4He and 14C strengthen the representativeness of the model. The Darcy average horizontal velocity in the Dogger, obtained with the model, is of the order of 0.33m year −1.

Detailed study of sulfide scaling at la courneuue nord, a geothermal exploitation of the Paris Basin, France

The fluid of the Dogger aquifer is always used through a closed loop formed by the production well, the heating plant and the injection well. After two or three years of exploitation of the geothermal doublets in the northern part of the Paris basin, scaling and plugging problems have appeared in some cases. The results of the detailed study carried out at La Courneuve Nord, a typical site of this area, are presented. The drawdrown of production rate, scaling in the heat exchanger and the increase of injection pressure required a rapid decision for workover operations on the wells. These cleaning operations and joint research studies allowed us to identify the cause of the plugging as well as to locate these deposits and to estimate their importance. Iron sulfide was found from the base of the production well casing all the way to the injection well casing. The thickness of this scale induces hydraulic pressure loss along the casings. Moreover, part of this sulfide is carried away by the fluid and fills up the bottom of the open hole injection well. After the cleaning operations, the hydraulic properties of the reservoir seem to be recovered. Chemical and mineralogical analyses of theses deposits identified the presence of a large variety of iron sulfide (mackinawite, pyrite, pyrrhotite) and a typical corrosion product (Fe 2(OH) 3Cl). Biochemical and bacteriological studies show a very high content of micro-organisms. At the end of the workover operations, several geophysical logs gave information on the corrosion of the casing as well as the residual scale. A balance of the iron content is calculated, between the volume of deposits and the iron removed by the corrosion of the casing. A chemical model, TPDEGAZ, is used to calculate the evolution of the saturation indexes of the fluid with respect to iron sulfide phases. The effects of parameters such as pressure, temperature, degassing and addition of iron by corrosion are simulated. The results of the observation and modeling approaches are compared.