Phase relations in the Longyearbyen CO2 Lab reservoir – forecasts for CO2 injection and migration

Abstract

Understanding of fluid-mixture properties relevant to the Longyearbyen CO2 Lab pilot project (LYBCO2) is of great importance for the assessment of the injection performance. Phase equilibria and density of the binary, ternary and quaternary systems containing CO2, CH4, H2O and NaCl were investigated using a Statistical Associating Fluid Theory (SAFT)-based equation of state (EoS) at ambient temperature and pressure, and salt concentrations up to 5 mol kgw-1, all relevant to LYBCO2. Binary interaction parameters of the subsystems (CO2–CH4, CH4–H2O, and CH4–NaCl) were tuned against available experimental data, using previously adjusted parameters for pure components and CO2–H2O subsystems. Solubility of CH4 and CO2 and subsequent mixture densities were predicted at 298 K and pressure up to 100 bar. It is found that by increasing the hydrocarbon in the injection stream (even in small amounts) and also the salt concentration and solubility of the CO2 in the aqueous phase, then consequently the density of the mixture will reduce. Moreover, hydrocarbon impurities like CH4 would result in a favourable density difference and faster plume migration; however, the probability of a three-phase state (two liquid and one vapour phase) near the bubble line is very high too. The results of this work are applicable to estimation of storage capacity as well as simulation of plume migration and fate in all projects involving a CO2, CH4, H2O and NaCl-bearing fluid system.

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