Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Effect of Natural CO2 Occurrence on CO2 Storage in Geological Formations
Yitian Xiao1; Godron Macleod1; David Advocate1; Chris Reaves1; 
Robert
Pottorf1
(1) ExxonMobil Upstream Research Company, Houston, TX.
Natural CO2 occurrence can have significant impact on CO2 injection performance and capacity estimation. The primary control on naturally occurring CO2 in the Earth’s crust results from its origin. The generated CO2 is further controlled by the migration processes, trapping mechanisms, and eventually, mineral-gas-fluid buffering interactions. We investigated the controls on natural occurring CO2, including its concentration as a function of temperature, mineralogy, and fluid chemistry. We coupled CO2 thermodynamic modeling with field data to describe the geological controls on CO2 occurrence, and refined a theoretical correlation for estimating CO2 concentrations in geological formations. The primary observation used for estimating CO2 volumetrics is that the partial pressure of CO2 (pCO2) covaries with temperature, while the reservoir mineralogy plays a significant role in determining the pCO2 - T correlation. This trend can be related to pH buffering associated with minerals and pore water interactions based on fundamental thermodynamic principles. The theoretical correlation matches the field data reasonably well, and should apply to a wide range of CO2 concentrations for reservoirs with constrained access to CO2 rich fluids.
We applied CO2 injection reactive transport modeling (RTM) to investigate how natural occurring CO2 affect CO2 storage. Our RTM simulates large-scale CO2 injection into subsurface reservoirs, with the ability to capture the complex interplay of multiphase flow, capillary trapping, diffusion, convection, and chemical reactions. The simulation results suggest that reservoirs with initial CO2 concentration below the buffering capacity tend to favor CO2 storage with higher solubility trapping, resulting in smaller CO2 plumes that can be better contained by structural trapping. The under saturation conditions also help to avoid potential mineral precipitation and formation damage near the injection well and therefore, is a better choice for CO2 storage over reservoirs with initial CO2 exceeding the buffering capacity. We also investigated CO2 injections in different types of siliciclastic and carbonate reservoirs. This study expanded our fundamental understanding of CO2 occurrence, buffering and sequestration processes at multiple spatial and temporal scales in nature. The results can help us make better judgments of CO2 storage capacity and site selection from appraisal to development to monitoring.