Thermal Expansion of Adsorbing Microporous Media, Application to Thermally Induced Desiccation of Clays – TEAM2ClayDesicc

The unusual thermo-mechanical behavior of clay is attributed to adsorbed water, that is to the water that penetrates into micropores less than 1 nanometer large and strongly interacts with the solid particles. However, the current understanding of the effect of water is still poor and the geo-mechanics models remain empirical and do not take into account water adsorption. In this project, we propose a multi-scale approach that brings together the adsorption at the molecular scale, the thermal expansion at the rock scale and finally the stability at the reservoir scale. At the molecular scale, the mechanics of an adsorbing micropore is investigated in detail starting from the elementary properties of atoms and molecules (e.g., electrical charges). At this scale, one finds very peculiar behaviors because of the structuration of the adsorbed water that arranges in a finite number of layers. Thus many deformations are unstable and thus ‘forbidden’, whereas others are (meta)stable and thus ‘authorized’. At the rock scale, no such behavior exists, and all deformations are possible. To understand the transition from one scale to the other, we learn from the theory describing the shape memory alloys, which face a similar situation with different possible phases at the atomic scale and forbidden deformations between phases. Starting from this theory, one needs to adapt the approach to the case of clay-rich rocks, in particular by accounting for the microstructure, the mineral composition and the geological history (consolidation state of the rock). Finally, at the geo-mechanical scale, we will perform stability analysis of typical situations with so called bifurcation methods that can predict the formation of shear bands, cracking, collapse...

We started by working on the molecular scale with simplified 2D representations of clay micropores and we currently pursue this work by considering realistic representations of montmorillonite. As expected, the mechanical behavior at this scale is oscillating because of the layered structuration of the fluid. More interesting, we could study how this behavior evolves with temperature in drained conditions, which is not studied in the literature because it requires a high precision and an excellent calibration of the parameters. At the scale of the micropore, the effect of temperature is hard to interpret: some part of the behavior exhibit thermal expansion, while other parts exhibit thermal contraction. We made a significant progress when we succeeded in bridging the gap between the behavior of the micropore and the experimental behavior of clays. To do so we chose an up-scaling approach that was not planned initially, because usual up-scaling approach are not compatible with the oscillating mechanical behavior of the molecular scale. It turns out that this oscillating behavior can be interpreted as the existence of different stable regions or phases, separated by unstable regions with forbidden deformations. This situation is analogous to the case of shape memory alloys , for which a well established mechanical theory has been developed for the last twenty years. Therefore, we learnt from this theory to propose an up-scaling approach valid for clays. The application of this approach was successful and lead to a macroscopic behavior consistent with the heating experiments on clays (expansion / contraction and reversibility irreversibility).

Although the heart of the project is mostly fundamental, ultimately, the interest is to have models and numerical tools capable of providing answers to practical geo-mechanical issues. Regarding the issues of thermal stimulation, the project should provide interesting answers in terms of the conditions favorable to underground instabilities, which can be immediately useful. Thus, for geothermal energy and hydrocarbon recovery, it should be possible to known whether drained heating is a possible alternative to hydraulic fracturing as was suggested by a national working panel in 2012. For the storage of gas or nuclear wastes, it should be possible to have an idea of the acceptable temperature increase preserving the sealing capacity.

A poster was presented at the conference of the French clay association in May 2016. A long presentation is planned at the EMI international conference in October 2016. These presentations are focused on the drained thermo-mechanical behavior at the scale of the micropore and on the up-scaling approach that captures the experimental behavior of clays. A journal article is under preparation on these subjects.

Clays are nanostructured materials that contain adsorbed water, i.e., water molecules interacting with the solid skeleton. Clay hydration and dehydration is well known to induce important deformations of the material that may end up to instabilities such as desiccation cracking of soils in dry conditions. Cracking of clay-rich rocks can be detrimental (nuclear waste or CO2 storage) or beneficial (oil and gas recovery). Clay desiccation can originate from heating since an increment of temperature induces dehydration and shrinkage. Thermal stimulation is considered as a potential alternative to hydraulic fracturing for shale oil and gas recovery from clay-rich deposits. But the technique is exploratory and its feasibility has to be demonstrated. In this project, we will investigate in detail the physics of thermal expansion of adsorbing microporous media, in particular that of clays, and ultimately assess the feasibility of thermal stimulation of shales. Adsorption in microporous solids is known to induce unusual deformations that can be understood at the molecular scale and captured by thermodynamic integration. Adsorption can induce both shrinkage and swelling depending on the molecular interactions between the fluid and the solid. Accordingly the thermal expansion of adsorbing media can be complex and we propose in this project to study it from the molecular scale to get insight into the physical mechanisms involved. We will investigate various model situations by molecular simulation and derive analytical description of the phenomena from thermodynamics. We will pay a special attention to the physical mechanisms that are relevant for clays. Clays are complex multi-scale materials in which the nanostructure is made of planar micropores where adsorption is structured in layers and induces a swelling orthogonal to the layers, with sharp transitions in function of water chemical potential and temperature. In contrast, macroscopic experiments on clays show continuous thermal deformation with both contraction and expansion, depending on the pre-consolidation state of the material and on the temperature. In this project, we will investigate the thermal expansion of clays from the molecular scale to the macroscopic scale and bridge the gap between the two scales. A fine understanding of the behavior of clay will enable to develop a thermo-hydro-mechanical constitutive modeling with a good predictive ability over a wide range of temperatures and in-situ stresses, relevant for application to thermal stimulation of shales. Finally, this constitutive modeling will serve as a basis for a stability analysis of shale reservoirs and thus to determine the conditions favorable to desiccation cracking. This project is structured as a comprehensive multi-scale approach that involves molecular simulation, thermodynamics and statistical physics, mechanical homogenization and rock mechanics. This project will provide interesting scientific results for the understanding of microporous solids in general and of clays in particular. The project will also have a relevant impact for applications in emerging geotechnical issues involving clays, especially for shale oil and gas recovery.