Coupled hydro-chemo-mechanical instabilities in geomaterials: laboratory scale experimental analysis of phenomena occurring in CO2 geological sequestration

Significant contributions in the literature have been focused on the study of the response of the reservoir rock to CO2 injection, however less results are available concerning the direct interaction between the acidified solution stored in the aquifer and the caprock. The following scenarios could be considered as the most representative of the hydro-chemo-mechanical interactions between the acidic brine solution and the caprock. S1) The pressure of the scCO2 at the top of the reservoir is lower than the gas entry pressure of the caprock. As a consequence, the CO2 cannot flow through the caprock but cations just diffuse through it. Geo-chemical alteration of minerals prone  to acid attack, can occur because of the chemical disequilibrium between the brine, saturating the clayey rock, and  the acidified solution. S2) Pre-existing fracture network/faults, having gas entry pressure lower than the scCO2 pressure and intrinsic permeability higher than that of the surrounding clay-rich rock (typically two order of magnitude), act as a flow conduit for the acidified solution. In this case, the scCO2 does not directly enter into the rock matrix but penetrates the caprock through the fractures. S3) The pressure of the scCO2 at the reservoir top exceeds the gas entry pressure of the caprock matrix. In this case, the CO2 penetrates the caprock and a drainage process takes place (a non-wetting fluid displacing a wetting one).
The experimental campaign will be firstly based on oedometer tests, which will be carried out during a secondment period at University of Strathclyde (UK), limited to 1D stress-strain paths allowing for injection of non-wetting fluid to test the effect of calcite dissolution on breakthrough. Samples for the oedometric test will be prepared with different pore-fluid chemistry in terms of pH (to simulate acidification due to scCO2) and electrolyte concentration (to  simulate brine at various salt concentration). Ranging the pre-consolidation stress of the oedometer between 1 and 10 MPa will allow simulating open and close micro-structure and therefore establishing an equivalence between conditions representative of laboratory tests which are going to be carried out in the second part of the experimental campaign. The second part of the experimental campaign will be based on biaxial tests, carried out using BIAX a
suitable experimental setup already available at GeM Laboratory in ECN, which is going to be adapted to let acidic solutions be used either to saturate or to diffuse/to inject the specimen. BIAX is a unique biaxial loading apparatus, which guarantees air-tightness of the specimen keeping it directly in contact with two sapphire windows, and whose control system allows to separately drive the upstream and the downstream fluid pressure. It recently allowed to identify fingering formation through granular media during drainage process.
The objectives of the research activity will be to reproduce at the laboratory scale the above-mentioned
scenarios S1 and S3 representative of the interactions between the acidic solution stored in the reservoir
rock and the sealing caprock. To this purpose analog materials will be designed which will allow to carry
out tests within the loading capacity of BIAX (about 1.5 MPa) being representative of the response of
real shale-like geomaterials under in-situ loading conditions (hundred of MPa).
The scientific program should respect the following itemized list of tasks
– developing the experimental campaign based on oedometer tests, during the secondment period at Strathclyde University (UK);
– calibrating the upgraded experimental apparatus;
– exploiting the experimental protocol relative to the campaign to be conducted together with a Post-Doc researcher (to be recruited) in the aim of investigating the above-mentioned scenarios at laboratory scale; a parametric analysis with respect to loading conditions should be taken into account
 

The PhD contract will start in September 2026. It will take place at the Laboratoire GeM, Ecole Centrale de Nantes (France) with a 6 months secondment period at Strathclyde University. The recruited researcher will be part of the UTR Environmental Geomechanics (https://gem.ec-nantes.fr/utr-geomec/) which is leading cutting edge investigations on multi-physics and multi-scale mechanics of geo-materials.

Profile - Background in civil engineering/geosciences, recommended in mechanics of geomaterials. A solid background in chemistry of materials will also be appreciated. Advanced knowledge in scientific programming (e.g. Python, Matlab) is also required.