Flow behavior analysis during supercritical CO2 drilling:consideration of varying thermo-physical properties of CO2 in wellbore

Key words:  drilling  supercritical CO₂  thermo-physical properties  cuttings carrying capability  temperature and pressure distribution

Abstract:

In order to calculate the thermo-physical parameters of CO₂, in wellbore under varying pressure and temperature conditions, the governing flow equations of fluids in wellbore were established based on specific enthalpies, in which the variations of the thermo-physical properties of supercritical (SC) CO₂ and their influence on fluid flow can be studied. The results of case studies show that the average errors using the Span-Wagner and V-W equations for calculating CO₂ 's properties are within 0.3％ and 1.5％, respectively, which are better than other methods in the temperature-pressure range considered during SC-CO₂ drilling. The density, viscosity and thermal conductivity of CO₂ in wellbore decrease with the depth increasing, while its heat capacity increases firstly, and then declines. Because the variation of thermo-physical properties CO₂ along the wellbore, the velocity, kinetic energy of the fluid, and its cuttings carrying capability increase from wellhead to bottom hole, and the borehole pressure varies nonlinearly along the wellbore. The accuracy of the flow behavior analysis for SC-CO₂ can be greatly improved via considering its variable thermo-physical properties in wellbore, otherwise, errors over 10.7％, 7.9％ and 1.1％ can be raised when calculating the cuttings carrying capability, bottom hole pressure and wellbore temperature distribution, respectively.