Long-term alteration of Rousse rock samples for CO2 storage: an experimental and numerical study

CO2 injection into the Rousse depleted gas reservoir, located in the south-west of France, started in January 2010 and 50,000 tons of this greenhouse effect gas were ultimately injected until March 2013 (Prinet et al. 2013). CO2 fate in the reservoir represents a problem of primary importance, that have been studied by several complementary research works led in collaboration by Total and IFPEN (e.g. Girard et al. 2013, Chiquet et al. 2013). One of the aspects covered by these studies concerned geochemical reactions between the injected fluid, reservoir water and rock minerals. A long-term CO2-exposure experiment was conducted in pressure vessels, and allowed the ageing of reservoir mini-plugs at in situ temperature and pressure conditions (150°C and 80 bar) during almost 24 months. Solid characterization was led every 2 to 3 months using an X-ray microprobe. Elementary maps were then acquired and converted into mineralogical maps using a statistical analysis based on pixels composition. This method allowed the identification of main minerals and of their evolution with alteration. Solution elementary composition was analyzed every week using an ICP-MS. These compositions were used for speciation calculations using the IFPEN's geochemical code Arxim. Initial reservoir mineralogy is mainly constituted of two types of dolomites. One of which forms the rock matrix, the other fills the main reservoir fractures. Quartz, pyrite and calcite traces, as well as two types of phyllosilicates (a potassic illite and a magnesian mixture of chlorite and mica) were also evidenced thanks to our analytical method. After some months of alteration, mineralogical evolutions were observed at the sample edge. Magnesian carbonates (dolomite and magnesite) seemed to precipitate, while an increase of iron, sodium and sulphur (probably due to anhydrite formation) was detected. The combination of these experimental observations to numeric speciation and minerals Saturation Index (SI) calculations allowed us to propose a reactive pathway. A first phase would consist in a slight dissolution of dolomite, potentially accompanied by ordered-dolomite precipitation. A second phase would lead to formation of magnesite and ordered-dolomite, consuming calcium released by calcite dissolution, magnesium released by dissolution of dolomite and of the chlorite/mica mixture, and CO2, thus demonstrating the mineralization potential of the Rousse reservoir. References Chiquet, P., Thibeau, S., Lescanne, M., and Prinet, C. [2013] Geochemical assessment of the injection of CO2 into Rousse depleted gas reservoir. Part II: geochemical impact of the CO2 injection. Energy Procedia, 1-12. Girard, J., Chiquet, P., Thibeau, S., Lescanne, M., and Prinet, C. [2013] Geochemical assessment of the injection of CO2 into Rousse depleted gas reservoir. Part I: Initial mineralogical and geochemical conditions in the Mano reservoir. Energy Procedia, 1-7. Prinet, C., Monne, J., and Thibeau, S. [2013] Lacq pilot injection test: The history of selecting the reservoir, injecting 50 kt of CO2 with monitoring-modeling verification and on-going long term stability demonstration, publication to the EAGE SES Conference, Pau, France, September 30th to October 4th 2013.