- © 2013 Mineralogical Society of America
Geological carbon storage will require that less than ~0.01% of the mass of CO2 stored escapes per year if significant climatic impacts are to be avoided (Hepple and Benson 2005). This requires that the geological storage sites retain much of the CO2 for more than 10,000 years. Predicting the security of CO2 in storage sites for such time periods raises questions which relate to a number of poorly understood fundamental processes concerning fluid-rock interactions in the near subsurface of the Earth. Because many of these processes are sluggish it is not possible to predict their significance from observations on active injection experiments with durations of, at most, a few tens of years. Nor do these experiments yet sample the full spectrum of potential behavior of CO2 in storage sites. For these reasons it is useful to study sites where natural CO2 has been retained in geological strata for periods which range from tens of thousands to millions of years.
Geological storage of CO2 will be mainly in depleted oil and gas reservoirs or saline aquifers at depths greater than about 800 m (DePaolo and Cole 2013, this volume). Under these conditions the CO2 will be in the denser supercritical state, but less dense than formation brines. As such it will tend to rise buoyantly and be retained by an impermeable caprock. A key concern is that the CO2, or CO2-charged brines will react with and corrode caprocks or faults and allow the CO2 to migrate upwards. CO2-rich waters are known to react with minerals but predicting the rates of fluid-mineral reactions at low temperatures is problematic (White and Brantley 2003) and the consequent changes in permeability of the caprocks or fault zones are uncertain (e.g., …