This paper presents the results of study work Granherne recently performed for an offshore carbon dioxide (CO2) Enhanced Oil Recovery (EOR) Project in the North Sea. In particular process design complexities are discussed for a new topside module comprising oil/gas separation, associated gas compression, dehydration with dense phase gas streams rich in CO2.
The importance of thermodynamic Equation of State (EOS) selection is highlighted, specifically for compressors and pumps in dense phase CO2 service. The selected HYSYS EOS results were benchmarked against a highly reliable Reference Fluid Thermodynamic and Transport Properties (REFPROP) database developed by National Institute of Standards and Technology (NIST). The comparisons are presented for compressibility, heat capacity, the heat capacity ratio and the enthalpy change across each compressor stage.
For the design of the gas dehydration unit, the experimental water equilibrium concentrations are compared with those predicted by commercially available simulators for a CO2-Water system. The HYSYS EOS did not model the acid gas-water system as accurately as Aqualibrium. The degasser flows for specifying the capacity of the LP Compression Train were under predicted by HYSYS EOS. The accuracy was improved by using the proprietary NRTL-SRK-Henry’s Law model in ASPEN PLUS.
The CO2 venting and depressurisation philosophy developed to avoid the formation of solid CO2 in the distribution headers is outlined. Also, the environmental aspects of CO2 rich gas cold venting versus flaring with assist gas is discussed.
The paper further highlights the technical design complexities compared to onshore CO2 EOR.