Leonardo do N Guimarães

THMC coupling in partially saturated geomaterials

ABSTRACT A theoretical formulation for the performance of coupled thermo-hydro-mechanical and chemical analysis (THMC) of problems involving partially saturated geomaterials is presented. Both homogeneous and heterogeneous chemical reactions are considered. The incorporation of chemical equilibrium is achieved by the direct minimization of Gibbs free energy. Although the formulation is general, particular consideration has been given to the phenomena that are likely to occur in compacted swelling clays used in isolation barriers. An example of application is described involving the coupled THMC analysis of a large scale in situ heating test of an engineered barrier.

COUPLED ANALYSIS OF DOUBLE POROSITY SWELLING CLAYS

The understanding of the coupled behaviour of highly active swelling clays is better achieved considering two structural levels and their interactions in the context of a double-porosity model. Hydraulic equilibrium between the two porosity levels is not assumed. Applications to hydration swelling tests of bentonite powder-pellets mixtures and to the chemomechanical behaviour of bentonites are presented.

Formulation for the THMC analysis of clayey materials: application to radioactive waste disposal

A fully coupled formulation combining reactive transport and an existing thermo-hydro-mechanical (THM) code is briefly described. Special attention has been given to phenomena likely to be encountered in clay barriers used as part of containment systems of nuclear waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically controlled and equilibrium-controlled reactions have been incorporated. The formulation has been implemented in the numerical code CODE_BRIGHT. An application is presented concerning the performance of a large scale in situ heating test simulating high-level nuclear waste repository conditions.

A Concurrent Efficient Global Optimization Algorithm Applied to Engineering Problems

One of the major difficulties in applying optimization to real engineering problems is that each function evaluation requires a complete model simulation which is in general computationally expensive. Moreover, some problems are known to be multimodal with several local minima. A common approach to tackle these problems is to construct cheap global approximation models of the responses often called metamodels or surrogates. These are based on simulation results obtained for a limited number of designs using global data fitting such as DACE. The optimization algorithm repetitive analysis needs are all performed using the cheap metamodels. In this study a two-stage approach is employed based on the Efficient Global Optimization algorithm, EGO, due to Jones. First an initial sample of designs is obtained using some Design of Experiments (DOE) technique such as Latin Hypercube. Parallel simulation runs for the initial sample are used to construct a kriging metamodel. In the second stage the metamodel is used to guide the search for promising designs which are added to the sample in order to update the model until a suitable termination criterion is fulfilled. The selection of designs which are adaptively added to the sample should balance the need for improving the value of the objective function with that of improving the quality of the prediction so that one does not get trapped in a local minimum. In the EGO algorithm this balance is achieved through the use of the Expected Improvement merit function. In this study the original EGO algorithm is modified to exploit parallelism. The algorithm is also modified to include general nonlinear constraints. The modified algorithm is applied to two example problems. The first is a small multimodal, two-variable structural design problem that can be graphically represented. The second is an eight-variable polymer injection problem in oil reservoir engineering.

Predicting Calcite Scaling Risk Due to Dissolution and Re-Precipitation in Carbonate Reservoirs During CO 2 Injection

Carbonate reservoirs are more geochemically reactive than sandstones and can experience big changes in porosity and permeability because of mineral reactions. In this work, we analysed the calcite dissolution and precipitation in chalk reservoirs during injection of seawater and CO2 bearing fluids. We performed reactive transport simulations with injection of seawater, carbonated water, CO2 gas, CO2 SWAG and CO2 WAG. We evaluated the mineral reactions that occur in the injector and producer blocks. Moreover, the calcite dissolution rate was calculated and its relationship with flow rate was investigated. Simulation results showed that during injection of CO2 gas alone, calcite dissolution was fast but limited, and occurred everywhere. On the other hand, for the other injected fluids the dissolution around the injector was continuous and, with the exception of the seawater scenario, precipitation was observed downstream. In addition, the calcite dissolution per injected water pore volumes for both CO2 SWAG and CO2 WAG was higher because of higher dissolution of gaseous CO2 in injected and formation waters. Moreover, the dissolution rate was found to be proportional to the water flow rate which confirms the assumption that calcite kinetics are fast compared to reservoir flow. This knowledge is valuable when planning CO2 WAG projects in carbonate reservoirs. As dissolution rates increase with flow rates, high permeability zones will show faster porosity changes, which may compromise the injector wellbore integrity and may lead to a more severe and growing calcite scaling risk around the producer wellbore.