Quantifying Oil-Recovery Mechanisms During Natural-Gas Huff n PuffExperiments on Ultratight Core Plugs

摘要

Despite promising natural gas huff'n'puff(HnP)field-pilot results,the dominant oil-recovery mechanismsduring this process are poorly understood.We conduct systematic natural-gas(C1 and a mixture of C1/C2with the molar ratio of 70/30)HnP experiments on an ultratight core plug collected from the Montneytight-oil Formation,under reservoir conditions(P = 137.9 bar and T = 50°C).We used a custom-designedvisualization cell to experimentally evaluate mechanisms controlling(i)gas transport into the plug duringinjection and soaking phases,and(ii)oil recovery during the whole process.The tests also allow us toinvestigate effects of gas composition and initial differential pressure between injected gas and the plug(ΔPi= Pg-Po)on the gas-transport and oil-recovery mechanisms.Moreover,we performed a Peclet number(N_(Pe))analysis to quantify the contribution of each transport mechanism during the soaking period.We found that advective-dominated transport is the mechanism responsible for the transport of gas intothe plug at early times of the soaking period(N_(Pe)= 1.58 to 3.03).When the soaking progresses,N_(Pe)rangesfrom 0.26 to 0.62,indicating the dominance of molecular diffusion.The advective flow caused by ΔPiduring gas injection and soaking leads to improved gas transport into the plug.Total system compressibility,oil swelling,and vaporization of oil components into the gas phase are the recovery mechanisms observedduring gas injection and soaking,while gas expansion is the main mechanism during depressurization phase.Overall,gas expansion is the dominant mechanism,followed by total system compressibility,oil swelling,and vaporization.During the'puff period,the expansion and flow of diffused gas drag the oil along itsflowpaths,resulting in a significant flow of oil and gas observed on the surface of the plug.The enrichment ofinjected gas by 30 mol% C2 enhances the transport of gas into the plug and increases oil recovery comparedto pure C1 cases.