Evaluating sealing efficiency of caprocks for CO2 storage: An overviewof the Geocarbone Integrity program and results

AbstractThe objectives of the Geocarbone-Integrity program are to develop techniques, methodologies and knowledge concerning the long term confinement of CO2 in geological storage. Linked to other French programs such as Geocarbone Injectivity or Picoref, it is an integrated approach involving geochemistry, petrophysics, geology and geomechanics. Different scales must be considered in order to describe caprocks: from the pore or grain scale in petrophysics and geochemistry, to regional scale in geology and geomechanics. The program focused on a specific site of the Paris basin but the methodologies developed are general and can be applied elsewhere.At large scales, caprock formation were described using sismic and log data. They were available on the St Martin de Bossenay depleted oil field and the Dogger caprock was mainly studied. These data were compiled and analyzed in order to obtain the variation of thickness and detect possible discontinuities. At small scale, the identification of geological facies and the origin of the low porosity and permeability in terms of sedimentology and diagenesis is necessary. For clayey caprocks, the amount and type of clay is important and for tight carbonate formation, dolomitisation is a key element. The key petrophysical parameters are the entry pressure, permeability and diffusivity. Very low permeabilities, smaller than microDarcy’s, are difficult to evaluate using standard techniques and the results can vary by up to one order of magnitude depending on the method used. For the formation considered, a low permeability transition zone, the measured entry pressures were too low to prevent the CO2 migration. Finally, diffusivity of dissolved CO2 was also evaluated: There is a decrease typically of a factor of 20 compared to bulk diffusivity, despite very low permeabilities.When injecting CO2, mechanical constraints are generated in both the storage and caprock formations, and the bottom of the caprock can be affected by geochemical reactions. The effect of the increase of pressure on the stress field has been evaluated and the possibility of fault and fracture generation or reactivation is small for the injection conditions considered. At the laboratory, elastic moduli have been measured before and after alteration of caprock samples and very small effects were observed. From the geochemical point of view, the alteration by CO2 has been studied on two types of formation, either dominated by clays or by carbonate. Some reaction paths have been identified from the experiments performed at elevated temperature and pressure conditions (80/150 °C, 150 bar) during long periods of time (3 to 6 months). Finally reactive transport simulations were performed to assess the speed and extend of CO2 migration in an idealized homogeneous caprock. Considering different situations and all key parameters (diffusion, reaction, entry pressure, two phase transport), the results show that the CO2 migrates a few meters during one thousand years, and that the porosity mostly decreases by precipitation, and increases very locally at the base of the caprock by dissolution