Gravity monitoring of CO2 movement during sequestration: Model studies

Sequestration/enhanced oil recovery EOR petroleum reservoirs have relatively thin injection intervals with multiplefluidcomponentsoil,hydrocarbongas,brine,andcarbon dioxide, or CO2, whereas brine formations usually have much thicker injection intervals and only two components brine and CO2. Coal formations undergoing methane extractiontendtobethin3‐10 mbutshallowcomparedtoeitherEORorbrineformations.InjectingCO2intoanoilreservoir decreases the bulk density in the reservoir. The spatial pattern of the change in the vertical component of gravity Gz is correlated directly with the net change in reservoir density. Furthermore, time-lapse changes in the borehole Gz clearlyidentifytheverticalsectionofthereservoirwherefluidsaturationsarechanging.TheCO2-brinefront,ontheorder of1 kmwithina20-m-thickbrineformationat1900-mdepth with 30% CO2 and 70% brine saturations, respectively, produceda 10-Galsurfacegravityanomaly.Suchananomaly would be detectable in the field. The amount of CO2 in a coal-bed methane scenario did not produce a large enough surface gravity response; however, we would expect that for an industrial-size injection, the surface gravity response wouldbemeasurable.Gravityinversionsinallthreescenarios illustrate that the general position of density changes causedbyCO2canberecoveredbutnottheabsolutevalueof thechange.Analysisofthespatialresolutionanddetectability limits shows that gravity measurements could, under certaincircumstances,beusedasalower-costalternativetoseismicmeasurements.