Geologic CO2 Storage in Oil Fields: Considerations for Successful Sites
Authors: Holtz M. H. 
Published: November 2009

Geologic storage of anthropogenic CO2 is being considered and tested in several subsurface settings. Deep brine-bearing formations hold the promise of storing large volumes of CO2 but typically have poorly defined seals and the effect of brine displacement on fresh water aquifers is only beginning to be researched. Depleted natural gas reservoirs are good storage sites with typically well known seals and pore volume. However it is likely that only special gas reservoir cases will produce situations where added economic benefit will occur form enhanced gas recovery. CO2 storage projects in oil reservoirs has the same advantages as gas reservoirs and with the possible benefit of enhanced oil recovery thus adding economic benefit plus a defined regulatory fromwork.

When analyzing an oil field as a possible geologic CO2 storage site it is critical to delineate the economic EOR benefit, storage capacity, and injection capacity. This is accomplished through integrated reservoir characterization typical to oil industry practices with the added considerations of the rate and total volume of CO2 coming from the anthropogenic source. Specifically, hydrocarbon properties, initial drive mechanism, and reservoir heterogeneity/geometry are characteristics which are salient when considering an oil field as a subsurface geologic storage site. Hydrocarbon properties determine the CO2-oil interaction such as the minimum miscibility pressure, oil solubility, and oil swelling. Initial drive mechanism along with the previously applied production practices determine the pore level saturation state in which the CO2 is being injected. Reservoir heterogeneity/geometry influences per well injectivity, pore volume storage capacity, movement of the CO2 in the reservoir, and the type of enhanced oil recovery (EOR) application applied.

Successful CO2 oil field storage sites will result from successful gas displacement recover (GDR) EOR projects. Thus understanding the applications of GDR is the key considerations for combining GDR and EOR. There are basicly five GDR applications including miscible, immiscible, pressure maintance, gas assisted gravity drainage (GAGD). and mixed gas GDR Applications. Pattern flood water after gas (WAG) miscible displacement is the most common GDR application currently in use. This is an application for reservoirs with high vertical heterogeneity and low reservoir dip. It results in a variying saturation states mixed with both mobile and residual, oil, water, and CO2. Continuos injection of CO2 can also be a misclble and could result in higher CO2 storage saturations than a WAG. The immiscible GDR application would likely be applied in a similar manner to miscible but storage would occur with lower CO2 saturation and higher oil saturation. Pressure maintance GDR is typically applied to gas condensate reservoirs or oil reservoir gas caps. In this application CO2 storage saturation could be high because CO2 is displacing hydrocarbon gas, The GAGD application by design results in the lowest weater and oil saturations and the highest OC2 storage saturations. GAGD is applied for more homogenous reservoirs with reservoir dip angles typically greater than 7 degrees. GAGD can result in high EOR recovery efficiencies as well as high pore level CO2 storage efficiency. Todate most mixed gas applications you been implemented as GAGD with mixtures of CO2, N2, and light hydrocarbons.