According to The Intergovernmental Panel on Climate Change (IPCC) AR6 synthesis report: Climate Change 2023 in global modeled pathways, that limit global warming to 2°C or below, almost all electricity is supplied from zero or low-carbon sources in 2050, such as renewables or fossil fuels with CO2 capture and storage (CCS) in deep geological formations. CCS has consequently emerged as an important option to reduce greenhouse gas emissions and CCS facilities are continuing to grow in Europe. However, global rates of CCS deployment are far below those envisaged to limit global warming.
It is the aim of LOCCO project (“hydro-chemo-mechanical LOCalization phenomena in CO2 geological sequestration” financed by ANR – Agènce Nationale de la Recherche – in France) to increase knowledge of the interaction between the CO2 injected in geological storage reservoirs and the surrounding rocks, which should act as sealing barrier, to the CO2 migration and leakage, and guarantee long-term storage security.
In geological sequestration, CO2 is injected in liquid form, but it transforms into a supercritical fluid (scCO2 ). Having density lower than the aqueous brine, initially saturating the reservoir rock, scCO2 tends to buoy through it, in continuous contact with the brine, and therefore to accumulate below the
caprock. Different zones within the aquifer host rock at different distances from the injection well can be identified, which are differently affected by the scCO2 concentration; in particular a zone I, fully saturated by scCO2 , in the close vicinity of the injection well, a zone II, characterized by the
presence of a two-phase mixture of scCO2 and brine with possibly buffered pH, a zone III fully saturated with an aqueous solution acidified by CO2 and a zone IV unaffected by CO2 injection. In the worst-case scenarios, the solution stored in zones I, II and III could be significantly acidified with respect to the almost neutral characteristics of natural brine.

Significant efforts in the literature have been devoted to understanding the response of reservoir rock to CO2 injection; however, considerably fewer results address the direct interaction between the acidified solution stored in the aquifer and the caprock. While chemo-hydrodynamic pattern formation has been extensively studied in porous media, for instance in polymers, its implications on the deformation and structural integrity of solid porous structure remain poorly understood.
For what concerns CCS operations, the following scenarios could be considered as the most representative of 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 diffuse through it. Geo-chemical alteration of minerals prone to acid attack, can occur because of the chemical dis-
equilibrium between the brine, saturating the clayey rock, and the acidified solution. 

S2) Pre-existing fracture network/faults, having gas-entry pressure lower than the scCO 2 pressure and intrinsic permeability higher than that of the surrounding clay-rich rock (typically two order of magnitude), act as flow conduits for the acidified solution. In this case, the scCO2 bypasses to a large extent the rock matrix by flowing through the fracture paths. 

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 through the caprock via a drainage process (a non-wetting fluid displacing a wetting one).
 

Which scenario prevails depends on the characteristic time scales associated with the above processes. These can be characterized and quantified using a suitable poro-hydro-chemo-mechanical model parameterized by an appropriate set of dimensionless numbers (including Péclet number, Damköhler number, capillary number and viscosity ratio). The project focuses on formulating this coupled model and implementing it numerically, thus paving way towards practical estimates of safe storage capacities of CO2 in geological reservoirs. Moreover, while scenario S2 might intrinsically mean a non-homogeneous transport of scCO2 across the caprock via fractured pathways, S1 & S3 might present flow regimes with both homogeneous transport and transition to non-homogeneous pathways (for instance worm-holing in S1 and viscous fingering in S3). Associating such pattern-forming unstable regimes to the magnitude of the various dimensionless numbers would also be of special interest.

The objectives of the research activity will be the rigorous formulation, at a continuum-scale, of a suitable poro hydro-chemo-mechanical model to describe the above-mentioned phenomena and the numerical implementation of the same, within the LAGAMINE Finite Element code (mainly developedat UEE). In particular the scientific program consists of the following tasks:
– Modeling dissolution and precipitation of carbonate minerals within the caprock driven by diffusive and/or advective transport of cations, which can dampen or enhance visco-capillary fingering and alter the micro-structure of the solid skeleton;
– Modeling two-phase visco-capillary flow, as an extension to the phase field approach to partial saturation in porous media;
– Modeling alteration of stiffness and strength properties of the caprock by dissolution/precipitation, which could trigger deformation bands and localized strain depending on the loading condition and the material properties;
– Implementing the model within LAGAMINE FE code by updating existing subroutine on chemo-mechanical physics, and taking advantage of a subroutine implementing the phase field approach to partial saturation, which is currently under development.
– Simulating FE test-cases to reproduce laboratory-scale conditions. These results will inform on the relevant scaling laws to be used within a parallel experimental research activity carried out within the LOCCO project. This provides an unique opportunity to have a dialogue between
modeling philosophies and experimental inferences.