The use of the geologic underground, either to extract natural resources (water, heat, gases, useful mineral substances, etc.) or to inject / store resources (water, heat, hydrogen) or undesirable com-pounds (industrial waters and brines, CO2, etc.) requires the knowledge and control of reactive transport processes in porous and fractured geological media in various scales of time and space.

Being able to investigate at lab scale the combined effects of thermodynamics, hydrodynamics, geo- and biochemical processes occurring in such deep geological environment is still challenging to address these topics. However, over the last fifteen years, new types of transparent high-pressure micro- and millifluidics reactors have been developed based on the idea of combining the advantages of micro-fluidics (size reduction, rapid screening, in situ analysis, reproducibility, control of hydrodynamics, low consumption of reagents during optimization phases, etc. ) with fluid systems under realiqstic reservoirs conditions (istudies of geofluid flows in model porous media, biology under extreme conditions, etc.). These tools allow to study more precisely the phenomena taking place at small scales and are complementary to the classical approaches using either macroscopic batch reactors or diamond anvil cells.

In this presentation, we will first detail the technologies available for the fabrication of high-pressure micro and millifluidics reactors, and then we will discuss their use in several applications related to the use of deep geological environments such as: (i) the geological storage of CO2 (fluid flows, carbonation processes, solubility measurements, etc.), (ii) the effect of deep underground microorganisms on gas storage (CO2, H2) along with possible biochemical reactions (in particular methanogenesis), (iii) the production of hydrogen from iron and iron-bearing minerals in subsurface conditions in link with CO2 storage and (iv) the use of microfluidic reactors to investigate clogging mechanisms in the case of geothermal energy recovery.