Effects of Pore Network Geometry on Permanent Storage of Sequestered CO2

Mark Holtz and Steven Bryant
The University of Texas at Austin, Austin, TX

Sequestration of CO2 in saline aquifers holds promise in reducing emissions. Favorable aspects include both the location of saline aquifers relative to anthropogenic CO2 sources and the volume of CO2 that could be stored in them. The outstanding question is whether projects can achieve verifiable storage over geologic time. A very promising approach to achieving permanent sequestration takes advantage of the mechanism of capillary trapping. Because CO2 is the nonwetting phase, brine can trap CO2 as residual phase saturation within the pores of the aquifer. The magnitude of residual saturation within rock, and hence the fraction of injected CO2 that can be stored in this form, is a function of the rock's pore network geometry.

Through pore network simulation, rock modeling, petrophysical measurement, and petrography the residual nonwetting phase saturation has been investigated for a range of rock types. Poorly consolidated rocks have been analyzed through pore network simulation. Rocks dominated by intergranular pore geometry have been analyzed through grain modeling, petrophysical measurements, and petrography. Rocks dominated by secondary pore geometry were also analyzed through petrophysical measurements and petrography description. The magnitudes described in these analyses are verified from field examples.

Intergranular pore network geometries display an inverse relationship between residual gas saturation and porosity. We hypothesize that this relationship is a result of non wetting phase snapoff within the pores. Secondary porosity that results in the dissolution of grains increases the overall aspect ratio of the pore network and thus increases the residual saturation. Certain authigenic clays have the opposite effect, decreasing the aspect ratio and residual saturation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005