Gaseous or supercritical CO2 storage
the Basics of Gaseous or supercritical CO2 storageAlthough not really petroleum related this short essay does address subsurface storage of CO2 in porous reservoirs and touch on applications for enhanced recovery; Comments are welcome!
In various areas around the world CO2 is already being stored in depleted reservoirs. Depleted reservoirs are very useful for this as their structure is well known and they have the capacity to hold a gas or liquid. Problems mainly arise when the input of CO2 reacts with the minerals either leading to increased impermeability of the reservoir rock or, when seal rock or faults are chemically altered, to the escape of CO2. Thus, geologists will not only have to study reservoir characteristics but need to focus on seals and fault characteristics as well. Because the discharge of CO2 along a fault is a self-enhancing process  extra attention should be paid to fault structures.
In some reservoirs CO2 is used to push out oil, called Enhanced Oil Recovery. The goal is to provide a long term storage scenario for CO2 while oil is removed. The advantage of this method is that pore space stays filled during the extraction process in contrast to depleted reservoirs that are refilled with CO2. This increases storage space as a reduced pore pressure can lead to compaction and land subsidence, another negative effect of oil & gas extraction.
In addition to the effects of CO2 on the reservoir rock Geologists will need to understand the way CO2 flows in the reservoir rock as to predict the efficiency of pushing away the oil. In the past porosities of reservoirs were an average of the entire formation but recent findings have lead to a more detailed view. A rock may contain several pore-structures that range in size and form, causing unexpected problems when different fluids (oil & water) or different gasses pass through them. Detailed modelling of pore space and structure are key to the success of most subsurface storage projects, and for Enhanced Oil Recovery in particular. Recent technology has made it possible to make 3D detailed pore architecture models (PAM) from simple thin sections  aiding research on pore characteristics.
The main disadvantage of storing CO2 in depleted or productive reservoirs is that the are not always easily accessible and not very widespread. Other subsurface alternatives have therefore been suggested.
Deep ocean (deeper than 3 km) sediments can provide easily accessible storage as most densely populated areas are near the oceans and CO2 could be stored near it’s source. The principle relies on the storing the CO2 under supercritical conditions that is the result of the pressure & temperature regime at this depth. Supercritical CO2 is denser than water and thus will remain buried. Theories are that when calcareous sediments are selected the dissolution of carbonates will cause increased permeability, thus creating more storage space . But, more studies on the porosity, permeability and capacity of these areas will still need to be done.
Also saline aquifers are present in most parts of the world and storage in these reservoirs also relies on storing CO2 in liquid form. In deep aquifers the supercritical form of CO2 could be used. In shallower reservoirs CO2 could be dissolved under pressure in a brine extracted from the reservoir and then re-injected. This would reduce the risk of having too much gaseous CO2 in an aquifer. Gaseous CO2 is much more buoyant than CO2-free brines and could migrate up and out of the aquifer. Research has shown that increasing a brines PH will increase it’s capacity for storing CO2. Like with storage in oil reservoirs, the characteristics of the seal rock need to be known to predict the possibilities of CO2 leaking back to the surface. Also not a lot is known about the physical and chemical processes that can improve the dissolution of CO2 in these brines. Geologist could also focus on the acid recovery from a brine, to raise the PH.References
 García, J.; Pruess, K. (2002) Multiphase flow dynamics during CO2 disposal into saline aquifers Environmental geology, Volume: 42 Issue: 2-3 (June 1, 2002), pp: 282-295
 Zhang, X.; Wu, K.; Dijke, M.I.J.; Couples, G.D.; Jiang, Z.; Ma, J.; Sorbie, K.S.; Crawford, J.; Young, I. (2006) 3D Stochastic Modelling of Heterogeneous Porous Media – Applications to Reservoir Rocks Transport in porous media, Volume: 65 Issue: 3 (December 1, 2006), pp: 443-467
 House, K.Z.; Schrag, D.P.; Harvey, C.F.; Lackner, K.S. (2006) Permanent carbon dioxide storage in deep-sea sediments Proceedings of the National Academy of Sciences of the United States of America (PNAS), Volume: 103 Issue: 33 (August 7, 2006), pp: 12291-12295