Geological sequestration of CO2 in deep saline reservoirs is one of the ways to reduce its continuous emission into the atmosphere to mitigate the greenhouse effect. The selection among prospective saline reservoirs can be expedited by developing some analytical correlations which can be used in place of reservoir simulation study for each and every saline reservoir. Such correlations can reduce the cost and time for commissioning a geological site for CO2 sequestration.
The efficiency of a CO2 sequestration operation depends on risks associated with storage, several of which can be estimated by i) the time the plume takes to reach the top seal; ii) maximum lateral extent of the plume and iii) the percentage of mobile CO2 present at any time. A database has been created from a large number of compositional reservoir simulations for different reservoir parameters including porosity, permeability, permeability anisotropy, reservoir depth, thickness, dip and perforation interval. We use a dimensionless ratio of gravity to viscous forces to formulate different correlations with the factors that contribute to sequestration efficiency. We update a previously reported correlation for time to hit the top seal and develop a new correlation for the maximum lateral extent of the plume using a newly created database for different reservoir and operating properties. A correlation for percentage of mobile CO2 during the buoyancy dominated post injection period is also developed.
We find that normalizing the maximum lateral extent by a characteristic length yields a reasonable correlation with the gravity number. This characteristic length is determined as the maximum lateral distance traveled by plume at any time assuming constant sand face velocity. The correlation confirms that low gravity number allows the plume to travel laterally due to high viscous forces while a high gravity number allows it to move faster in vertical direction due to strong gravity forces. The change in mobile CO2 after injection ends also correlates well with gravity number. We normalize the change in mobile CO2 fraction (or, equivalently, the change in trapped CO2 fraction) after the end of injection by a characteristic CO2 saturation. The characteristic saturation is obtained by considering the volume filled by vertical, buoyancy-driven movement through the area associated with the maximum plume extent.
The correlations reproduce almost all simulation results within a factor of two, and this is adequate for rapid ranking or screening of prospective storage reservoirs.