CO2 Injection in Carbonates

Abstract

Abstract Started as an EOR technique to produce oil, injection of carbon dioxidewhich is essentially a greenhouse gas is becoming more and more important.Although there are a number of mathematical modeling studies, experimentalstudies are limited and most studies focus on injection into sandstonereservoirs as opposed to carbonate ones. This study presents the resultsof computerized tomography (CT) monitored laboratory experiments tocharacterize relevant chemical reactions associated with injection and storageof CO2 in carbonate formations. Porosity changes along the core plugs andthe corresponding permeability changes are reported for differing CO2 injectionrates and with differing salt concentrations. CT monitored experiments aredesigned to model fast near wellbore flow and slow reservoir flows. It wasobserved that permeability initially increased and decreased for slow injectioncases. As the salt concentration decreased the porosity and thus thepermeability decrease was less pronounced. The experiments were modeledusing a commercial simulator where solution and deposition of calcite wereconsidered by means of chemical reactions. The calibrated model was thenused to analyze field scale injections. It was observed that solubilitystorage of CO2 is larger compared to mineral trapping. Introduction Anthropogenic greenhouse gases, such as carbon dioxide (CO2), are considereda major contributor to global warming.[1] Sequestration of power plantgenerated CO2 by injection into groundwater aquifers and (petroleum and gas)reservoirs has been proposed as a possible alternative for the reduction ofexcessive greenhouse gases in the atmosphere. Ideally, injected CO2 willmigrate through an aquifer from injection wells to remote storage sites, andremain isolated from the atmosphere for a considerable period of time. CO2has been used for enhanced oil recovery (EOR) purposes since the 1950's (thefirst carbonated water floods were tested as early as 1951 and slugs of pureCO2 for oil displacement were tested as early as 1963.[2] Research on useof CO2 for EOR continues with an ever growing interest; however, researchconcerning terrestrial sequestration of CO2 for environmental purposes isrelatively recent.[3] Fundamental topics of interest in sequestrationresearch have concerned not just scientific and technical aspects, butpractical concerns such as the economic feasibility of storage, safety, and themaximum possible amount of CO2 storage globally and for specified regions.[1]Fundamental research questions include the following: Can typical regionalscale aquifers and depleted oil and gas reservoirs provide the residence timeneeded to achieve stated global temperature reduction goals; what geologicconditions, e.g., rock properties, depositional environments, structure, etc., provide optimal CO2 flow and transport, and optimal storage in a sedimentarybasin? CO2 can be sequestrated in geological formations like saline aquifers bythree mechanisms: solubility trapping through dissolution in the formationwater,[1] mineral trapping through geochemical reactions with the aquiferfluids,[2,4] and rocks, and hydrodynamic trapping of CO2 plume.[1] Thesemechanisms lead to storage of CO2 as free-phase gas in pore spaces, dissolvedphase CO2 in formation water and CO2 converted to rock matrix. Geological disposal of CO2 would ideally be made at supercritical conditions(31.04°C and 1070.7 psi), in order to avoid adverse effects of prior separationof CO2 into liquid and gas phases in the injection system1. The desire ofsupercritical injection limits minimum depths to approximately 800 m to sustaina supercritical regime5. Although supercritical injection and storage of CO2would be ideal, for all practical reasons, it is not a necessary condition.