Simulation of Enhanced Oil Recovery in Pre-Salt Reservoirs: The Effect ofHigh CO2 Content on Low Salinity Water Alternating Gas Injection

摘要

The high CO2 content of Brazil's pre-salt fields,which may reach values from 20% to 44% molar,presentsboth a challenge as well as an opportunity.CO2 stripped from the produced gas cannot be released intothe atmosphere due to environmental restrictions.Therefore,the whole amount of CO2 produced shouldbe continuously reinjected into the reservoir.This work investigates the effect of CO2 content on thelow salinity water alternating CO2 injection technique (CO2LSWAG) using a commercial compositionalreservoir simulator.In these field-scale simulations,CO2 is stripped from the produced gas and reinjectedinto the reservoir.Primary oil recovery methods such as CO2 flooding and LSW flooding are also simulated.Chemical reactions between CO2 and the minerals present in the reservoir are modeled.Wettability changeis assumed to be the main mechanism for improved oil recovery due to low salinity water injection.Compositional simulations of CO2 injection usually assume a constant injected gas rate.In this case,CO2is supposed to come from an external source.In many petroleum reservoirs this assumption is true.Threefactors are assessed in the present work.The first one is the natural reservoir pressure,which is the maindriving force in primary production.The second factor is the amount of CO2 available for injection.Thethird one is the wettability change promoted by the reaction involving CO2.It is shown that in primaryproduction,higher CO2 content leads to quicker depletion of the natural energy of the reservoir,leading tolower oil recovery.Nevertheless,higher CO2 content also means that more gas is available for reinjection,potentially leading to increased oil production.Finally,as CO2 reacts with minerals it promotes a change inwettability from an oil-wet to a water-wet state.It is shown that the CO2 content is an important variableto be assessed in a high CO2 content reservoir.Optimal injection practices must take these three aspectsinto consideration.