Effect of CO_2 Impurities on Gas-Injection EOR Processes

摘要

Carbon dioxide flooding is a promising enhanced oil recoverymethod both on technical and, if operating costs are properlycontrolled, economic grounds. Injecting this greenhouse gasalso has environmental merits. Flue gas from power plants is aready source of CO_2; however extracting CO_2 for enhanced oilrecovery from such a source will increase project costs.Furthermore, to reduce both the net CO_2 utilization and thecost of purchasing gas, it is usually necessary to recycle theproduced CO_2 with as little purification as possible. Therefore,understanding the roles of impurities in fluid phase behaviourand miscibility characteristics is necessary for designing acost-effective CO_2 enhanced oil recovery process.Laboratory studies of the effect of CO_2 impurities onphase equilibrium and minimum miscibility pressure (MMP)were conducted on two Saskatchewan light oils covering arange of densities from 29.5°API to 38°API. The resultsindicate that the MMP for these light oils could increaseunfavourably as the N2 and/or CH4 concentration increased inthe CO_2 stream. The MMP changes as the type andconcentration of impurities in the injected CO_2 stream change.However, coreflood tests showed that the near-miscible CO_2displacement might employ the same mechanisms as miscibleCO_2 flooding to mobilize and displace oil; thus, good oilrecovery can be achieved in the vicinity of the MMP. Whilelaboratory measurements are essential in the evaluation of agas injection process, an equation of state (EOS) simulationwas demonstrated to be a useful tool in analyzing the phasebehaviour of various injection gases, reservoir fluids, and thegas-oil interactions.