Abstract
VapourLiquid Equilibria (VLE) calculations, particularly involving the phase behaviour of CO_{2} and H_{2}S, are used in scale prediction modelling. In this work, the impact of VLE calculations for CO_{2} and H_{2}Srich gas phases and for acid and sour gas mixtures on scale prediction calculations is evaluated. Three Equations of State (EOS), viz. SoaveRedlichKwong (SRK), PengRobinson (PR) and ValderramaPatelTeja (VPT), are implemented in the HeriotWatt model and used in VLE calculations. The solubility of singlecomponent CO_{2} and H_{2}S in water and the solubility of a gas mixture in water were compared with experimental data in terms of the Absolute Relative Deviation (ARD). The solubility data were then used in PHREEQC to calculate the impact of using different EOS on carbonate and sulphide scales, particularly on CaCO_{3} and FeS. Average ARDs of 6.04%, 4.10% and 3.77% between experimental and calculated values for CO_{2} solubility in water were obtained for SRK, PR and VPT EOS, respectively. Similarly for H_{2}S solubility in water, average ARDs of 6.49%, 6.66% and 6.48% were obtained. For the solubility of sour and acid gas mixtures in water, average ARDs of 13.92%, 13.25% and 10.78% were obtained. It has thus been concluded that VPT EOS performs better than SRK and PR in VLE calculations for the analysed data. The errors introduced in VLE calculations have been found to impact the calculation of the amount of CaCO_{3} precipitated with consequences for scale inhibitor selection. Higher deviations were found in the amount of CaCO_{3} precipitation for gas mixtures when compared to single component CO_{2} rich phase. Furthermore, the large errors occurring in VLE calculations for H_{2}S solubility have not been found to impact the calculation of the amount of FeS precipitated when H_{2}S is in excess with respect to Fe^{2+}. Scale prediction calculations carried out using PHREEQC demonstrate that VLE calculations can have a high impact on mineral precipitation. Thus, it is recommended that the best VLE model available should always be used for scale prediction modelling, particularly when mixtures of gases are present.
Original language  English 

Title of host publication  SPE International Oilfield Scale Conference and Exhibition 
Publisher  Society of Petroleum Engineers 
ISBN (Electronic)  9781613994559 
DOIs  
Publication status  Published  2016 
Event  SPE International Oilfield Scale Conference and Exhibition 2016  Aberdeen, United Kingdom Duration: 11 May 2016 → 12 May 2016 
Conference
Conference  SPE International Oilfield Scale Conference and Exhibition 2016 

Country/Territory  United Kingdom 
City  Aberdeen 
Period  11/05/16 → 12/05/16 
ASJC Scopus subject areas
 Energy Engineering and Power Technology
 Geochemistry and Petrology
 Geotechnical Engineering and Engineering Geology
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Eric James Mackay
 School of Energy, Geoscience, Infrastructure and Society, Institute for GeoEnergy Engineering  Professor
 School of Energy, Geoscience, Infrastructure and Society  Professor
 Research Centres and Themes, Energy Academy  Professor
Person: Academic (Research & Teaching)