Vincent Lagneau

Module-oriented modeling of reactive transport with HYTEC

The paper introduces HYTEC, a coupled reactive transport code currently used for groundwater pollution studies, safety assessment of nuclear waste disposals, geochemical studies and interpretation of laboratory column experiments. Based on a known permeability field, HYTEC evaluates the groundwater flow paths, and simulates the migration of mobile matter (ions, organics, colloids) subject to geochemical reactions. The code forms part of a module-oriented structure which facilitates maintenance and improves coding flexibility. In particular, using the geochemical module CHESS as a common denominator for several reactive transport models significantly facilitates the development of new geochemical features which become automatically available to all models. A first example shows how the model can be used to assess migration of uranium from a sub-surface source under the effect of an oxidation front. The model also accounts for alteration of hydrodynamic parameters (local porosity, permeability) due to precipitation and dissolution of mineral phases, which potentially modifies the migration properties in general. The second example illustrates this feature.

Presentation and application of the reactive transport code HYTEC

Assessment of hazardous metal dissemination and remediation studies requires hydrogeological models able to describe the groundwater flow paths, migration along the paths and geochemical and/or biochemical reactions with mobile (colloidal) and immobile (mineral) phases of the medium. Both geochemical and hydrocdynamic facets are tightly linked together. Therefore, a strongly coupled modeling approach is required, solving the equations involved in geochemistry and hydrogeology simultaneously. HYTEC, currently used for groundwater pollution studies, safety assessment of nuclear waste disposals, geochemical studies and interpretation of laboratory column experiments, meets all these requirements. The code is based on a series of specialized modules, where among we can cite the geochemical code CHESS and differnet hydrodynamic transport codes communicating via a specific interface, currently based on MPI (Message Passing Interface). Several possibilities of the model is illustrated with the help of an operational application involving migration of toxic metals (zinc) in an aquifer. The example also demonstrates the impact of colloidal matter, capable of increasing the mobility under certain conditions.

Operator-splitting-based reactive transport models in strong feedback of porosity change: The contribution of analytical solutions for accuracy validation and estimator improvement.

Reactive transport is a highly non-linear problem requiring the most efficient algorithms to rapidly reach an accurate solution. The non-linearities are increased and the resolution is even more demanding and CPU-intensive when considering feedback of dissolution or precipitation reactions on hydrodynamic flow and transport, commonly referred to as the variable porosity case. This is particularly true near clogging, which leads to very stiff systems and therefore small time-steps. The operator-splitting approach often cited is a widely use method to solve these problems: it consists in solving sequentially the transport then the chemistry part of the problem. Operator-splitting appears to be an accurate approach, provided that the solution is iteratively improved at each time-step. The paper details analytical solutions and test-cases for this class of problems. They demonstrate that iterative improvement is then compulsory. They also helped develop an improved estimator/corrector method which allows to reach convergence faster and to reduce stiffness. The efficiency improvement is significant as illustrated by an example of carbonation of a cement paste, a well-known problem that leads to complete clogging of the interface layer.

Modeling spent nuclear fuel alteration and radionuclide migration in disposal conditions

A physico-chemical model developed for spent fuel alteration was integrated in a global reactive transport model of a spent fuel disposal system, considering both decaying and stable isotopes, corroded steel canisters, bentonite backfills and a clayey host-rock. Fuel evolution took into account radiolytic-enhanced corrosion and long-term solubility-controlled dissolution as well as instantaneous release fractions. The calculations show that spent-fuel dissolution has no significant alteration effect on the near-field components except an oxidizing plume in the vicinity of the waste packages. The dissolved uranyl species, partly precipitate as schoepite on the fuel pellets, and partly diffuse in the near-field where magnetite and pyrite reduce U(VI) to yield uraninite precipitation. Under disposal conditions, preliminary calculations indicate that steel corrosion may generate sufficient dissolved hydrogen as to react with radiolytic oxidants and inhibit fuel dissolution. The formation of a protective schoepite layer could also reduce the alteration of fuel pellets. Radionuclides migration (Am, Cs, I) in the near-field is discussed in a second stage discriminating between sorption, precipitation and radioactive decay processes. The migration of Cs is translated in terms of cumulative activity profiles useful for integrated performance assessment.

A Scenario for the Creation of H2S Heterogeneities in Acid Gas Reservoirs in Contact with an active Aquifer: a Simulation Study

Compositional heterogeneities of H2S have been noticed in many sour gas reservoirs. Its occurrence is an important factor of economic depreciation. Thus, the knowledge of its content and distribution is a critical parameter when planning field development. The paper aims at exploring the role of an active aquifer in the creation of H2S heterogeneities in high H2S-bearing gas reservoir. Indeed, under conditions of pressure and temperature of typical reservoirs, H2S is far more soluble than hydrocarbons and other gases. A preferential leaching of H2S (e.g. versus CH4) over time is thus possible. This mechanism is controlled by: (1) Differential solubility of gases, which change the relative amounts of each gas near the gas-water contact (GWC); (2) Contact with an active aquifer, which can export the dissolved gases thus enhancing dissolution on the long-term; (3) Diffusional transport in the gas phase, which transfers the compositional anomalies farther from the gas-water contact; (4) Geological parameters (type of aquifer, permeability heterogeneities) which can modify the transport scenario. To illustrate and quantify this process, we show the results of numerical simulations, performed with the two-phase transport and geochemical software Hytec. First, a very schematic reservoir with a composition considered uniformly distributed within the reservoir, has been simulated to quantify the leaching of H2S. The results highlight the potential role of the active aquifer, which can leach the gases and export them outside the reservoir. In a second phase, the effect of geological parameters on the H2S heterogeneity development was studied: additional simulations were performed on geometries closer to natural cases. The amount of leached H2S depends strongly on the geometry: the larger the GWC area, the larger the amounts leached.

Reactive transport simulation applied on uranium ISR: effect of the density-driven flow

The in situ recovery operation of a perched uranium mineralization within a thick, permeable aquifer can become a sensitive issue. Indeed, density difference between the injected high-density acidic solutions and fresh groundwater are liable to trigger density-driven migration of solution towards the bottom of the aquifer. The goals of this study are (1) to represent with a reactive transport model, the hydrogeological behaviour of such an ISL operation for quantifying the lost part of uranium and acid linked to the density driven flow, (2) to suggest some possible solutions to optimize the production.

Role of hydrodynamism in compositional heterogeneities in acid gas reservoir

Acid gases (H2S and CO2) compositional heterogeneities have been noticed in many sour gas reservoirs. Their occurrence is an important factor of economic depreciation. Thus, the knowledge of the acid gases distribution is a critical parameter for the design of field development. The mechanisms to explain compositional heterogeneities of acid gas in a reservoir are various. The paper aims at exploring the role of an active aquifer in contact with an initial high H2S content reservoir. The major mechanisms may be controlled by: * Differential solubility of gases which can change the relative amounts of each gas near the contact; * Active aquifer solubilization and transport which can export dissolved gases thus enhancing dissolution on the long-term; * Diffusional transport in the gas phase which can transfer the compositional anomalies farther from the gas-water contact. To test the influence of several parameters on the efficiency of the acid gases leaching, simulations on basic geometries have been performed with the diphasic transport and geochemical software Hytec. The simulation results show a major role of the occurrence of horizontal impermeable barriers yields to sharp heterogeneities, including a decrease in acid gas near the contact, while farther areas H2S concentration remain unaffected.