Analytical Correlations for Risk Parameters Involved in CO2 Storage

Abstract

Abstract Large deep saline aquifers are present in different sedimentary basins throughout the world. The storage potential in these reservoirs is estimated at several thousands Gt CO 2 . The selection among the large number of prospective saline aquifers for CO 2 sequestration can be expedited with analytical correlations which can be used in place of a full field reservoir modeling and simulation study for each and every saline reservoir. The effectiveness of any CO 2 sequestration operation depends on pore volume and the sequestration efficiency of the saline reservoir. Sequestration efficiency is defined here as the maximum storage with minimum risk of leakage to the overlying reservoirs and to the surface. This can be categorized using three variables: time the plume takes to reach the top seal of the storage formation, maximum lateral distance the plume travels and the percentage of mobile CO 2 present at any time. A database has been created by performing a large number of compositional reservoir simulation studies for different elementary reservoir parameters. A single set of relative permeability curves is used in all simulation cases to maintain the consistency of the simulation parameters. A constant pressure far-field boundary is enforced by putting a number of producers at the boundary of the reservoir producing at constant bottom hole flowing pressure. This database is used to formulate different correlations that relate the sequestration efficiency to reservoir properties and operating conditions. The various elementary reservoir parameters are grouped into a dimensionless ratio of gravity forces and viscous forces. This ”gravity number” can be calculated for any saline reservoir without performing any simulation study, which makes it handy to use in different analytical models with various risk parameters. We improve the correlation for time to hit top seal proposed by Kumar and develop correlations for other two risk parameters by assuming radial grid system in place of Cartesian grid system. We find that normalizing all risk parameters with their respective characteristic values yields reasonable correlations with different variants of dimensionless gravity number. The characteristic values are determined with a characteristic flow speed equal to its value at the sand face. All correlations confirm the physics behind plume movement in a reservoir. For low gravity number displacement the plume travels preferentially in lateral direction due to high viscous forces and contacts more rock and brine volume, resulting in more trapping and less risk. On the other hand a high gravity number allows the plume to move faster in vertical direction due to strong gravity forces. This causes less interaction time and volume of resident brine and rock and results in less trapping of CO 2 . These analytical correlations are useful in characterizing a saline reservoir in terms of its sequestration efficiency without any reservoir simulation study, which in turn reduces the cost and time for commissioning a geological site for CO 2 sequestration.