الملخص الإنجليزي
CO2 injection into saline aquifers as part of the CCS process induces chemical
processes represented with CO2 dissolution into water and minerals reactions which
can disturb the brine equilibrium and changes the aquifer’s petrophysical properties.
This thesis aimed for implementing numerical tools for assessing the interactions
between carbon dioxide, the original brine, and the aquifer minerals with more focus
on exploring the parameters that could optimize the amount of solubility and mineral
trapping. The study was done on Dogger carbonate aquifer in Paris Basin.
Modelling was done with PHREEQC and CMG-GEM. PHREEQC was used for
assessing geochemical reactions of injecting a fixed partial pressure CO2 into the
aquifer with the objective of assessing the effect of pressure, temperature, the pH, the
solution’s ions, and mineralogy on geochemical trapping. It was split into two parts
aimed for studying each of the mechanisms separately. CMG-GEM was used for 2D
reactive transport simulations for studying the effect of physical and chemical trapping
mechanisms in addition to assessing this software compatibility with PHREEQC.
The main results showed that the abundance of sodium in the solution relative to
calcium and magnesium could help increase CO2 solubility and the system pH. For
minerals reactions, trials were done to simplify the system with equilibrium modelling
and applying partial equilibrium into kinetic simulations which showed that silicate
minerals reaction rates would be overestimated with semi-kinetic modelling. For the
base case, mineral trapping was mainly from albite dissolution to form dawsonite,
while other carbonate minerals contributed less. As part of sensitivity analysis, higher
pressures and temperatures were found to be helpful for increasing mineral trapping
but it is the pressure that affect the speed of dawsonite precipitation. By testing
minerals effect, albite was found to lead to more mineral trapping with higher volume
fractions, while illite led to the opposite even though both are silicate minerals. With
higher calcite volume in the system, minerals’ reaction rates could be slowed down
leading to achieve lower porosity reduction and better equilibrium pH, but lower
mineral trapping. Primary carbonate minerals were essential for achieving a positive
mineral trapping from the beginning. On the other hand, minerals were found to have
no effect on solubility trapping. Ramping up the complexity with reactive transport
simulation showed that geochemistry modelling of GEM is compatible with
PHREEQC results. Moreover, mineral trapping was found to be minimal with around
1% contribution to trapping after one thousand years. The main result of RTM
suggested that carbonate precipitation is faster with CO2-rich brine compared to
supercritical CO2.
In conclusion, this thesis showed that carbonate formations could be advantageous for
better pH buffering and less effect on the aquifer storage in addition to that existence
of certain cations-holding minerals could ramp up the mineral trapping significantly
