Improving our knowledge on the hydro-chemo-mechanical behaviour of fault zones in the context of CO2 geological storage

Fault systems can play a significant role regarding several risk issues related to CO2-injection-induced mechanical perturbations: fluid flow enhancement or compartmentalization within the reservoir, loss of integrity of the reservoir-caprock systems, potential triggering of seismicity and generation of new leakage pathways or permeability barriers. Several methods exist to model hydro-mechanical behaviour of fault zones. A first "conventional" approach aims at evaluating the fault response by directly post-processing the results of the large-scale coupled hydro-mechanical simulations. This consists of: estimating the changes of the effective stress field in the reservoir-caprock system during CO2 injection; computing changes of shear and normal stresses acting on the fault plane; comparing them to a fault reactivation criterion. However, this approach does not account for the effects of the presence of the fault on the stress and pressure field in the surrounding rock matrix. More sophisticated (and physically more realistic) modeling strategies have been proposed, which explicitly integrate the fault zone as a distinct element in the large-scale simulation, i.e. by representing it as a linear discontinuity with various hydraulic and mechanical properties. From a CO2 storage perspective, such a model still remains limited regarding two issues: 1. Faults are complex and heterogeneous geological systems, which do not correspond to discrete surfaces as already postulated by many authors. A fault zone is composed of an inner core made of fine material, often impermeable, and where slip is concentrated. It is surrounded by an outer damage zone that often acts as a hydraulic pathway, because of the presence of a fracture network, whose characteristics (fractures' orientation, connectivity, lengths, density, number of fractures' families, etc.) depend on the distance to the core; 2. Chemical interactions (dissolution and precipitation processes, chemically-induced weakening, etc.) between CO2-enriched brine and the minerals constituting the fault zone. can affect the mechanical and transport properties of the faulted/fractured system. In particular, chemo-mechanical processes can either stabilize the system if the compaction rate is increased or destabilize it if new micro-fractures are created. The FISIC project intends to overcome those limitations by accurately modelling the hydro-chemo-mechanical (HCM) complexity of a fault zone. The main goal is to provide appropriate theoretical and numerical models for accurate evaluation of fault stability in the context of CO2 geological storage, i.e. improving the stability analysis of a fault both undertaking pressure increase and alteration due to the presence of an acidic fluid. Four research questions are addressed and the progress regarding each of them is presented: 1. How to represent a fault zone in a tectonic setting, which has a priori been selected far from major potentially seismically-active faults, i.e. a moderate-to-low-deformed setting; 2. What are the fractures' organization within the damage zones of faults, i.e. their spatial distribution? 3. What are the dominant chemo-mechanical processes resulting from aqueous CO2 in fractured/faulted systems? 4. How to integrate (numerically) the hydro-chemo-mechanical (HCM) behaviour of fault zones in large-scale (reservoir or basin-scale) simulations? Regarding the first and second question, a geostatistical approach is adopted based on observations of fault structures at the Cirque de Navacelles. This site, located in the late Jurassic platform carbonates of Languedoc (southern France), can be considered a good analogue for CO2 storage reservoir. The third question is addressed at laboratory scale focusing on the CO2(aqueous)-induced effect on: i. the slow growth of cracks (subcritical fracturing) and their healing; ii. the degradation of mechanical properties such as fracture toughness or shear strength. Finally, the fourth question is numerically addressed by relying on: i. advanced meshing tools developed for converting complex geometries to finite element models; ii. analysis of fracture propagation condition within multiphase fractured porous media; iii. the rheological behavior derived from the experimental study