A b s t r a ct Worldwidelong-termtrendsshowthatconventionaloilfielddiscoveriesare declining while demandforoilisincreasing. EnhancedOilRecovery (EOR)isatermappliedtomethodsusedforrecoveringoilfroma petroleumreservoirbeyondthatrecoverablebyprimary and/orsecondarymethods.Whenthe gasinjectionprojectsarerevieweditcanbeseenthatnon-hydrocarbongasesespeciallyCO 2 is themostcommoninjectant.Inanycomprehensivereservoirstudy,avery importantstep,whichisnecessarytobedone before estimating the fluid in place, initializing reservoir simulators, and planning the reservoirdevelopment,isaccurateassessmentof thespatialdistributionof thefluid componentsinhorizontalandverticaldirections.Thefluidcompositionvarieswithdepthin many reservoirs and this phenomenon is reffered to as “compositional grading” whichcan significantlyaffectdifferentaspectsofreservoirdevelopment.Althoughthereare numerousstudiesaboutcompositionalgrading andallauthorsemphasized thatvariationofcompositionmakesvariationofmiscibility condition,but allofthemtry to formulatethisphenomenonbasedonthethermodynamic approachto reachtoa proper model in order to predict composition and other reservoir fluid roperties along the reservoir column, and thereis a lack of attention to its effects on gas injection.Inthisstudy, an Iraniansouthwestoilreservoir withlowshrinkageundersaturated oilof API gravity of30wasselected. Becauseofnearbypowerplants,thisreservoirisa good candidate forCO 2 injection.Afull compositionalsimulationmodelwasfirst preparedwith complete petrophysical, geological, and PVT data. composition at different depths wit h respect to datum were calculated, using the isothermal compositional grading model and imported into the model to simulate the case of compositional grading.The optimize gas rate for the oil rate of 1500 STB/d is 8 MMSCF/d and the injection should done on all the layers.Although the optimize gas rate for the oil rate of 2500 STB/d is 10 MMSCF/d and the injection should be done on the top layer. Key Words : Enhanced oil recovery, CO 2 injection, Compositional grading, Reservoir simulation.
