Reprint of: Multiphase flow mechanism of sand cleanout with supercritical carbon dioxide in a deviated wellbore

Supercritical carbon dioxide (CO2) is of great interest as a sand-flushing fluid with significant potential advantages; therefore, it is of fundamental and practical importance to investigate its sand transporting mechanism. Numerical simulation was conducted to analyse the heat transfer along the whole deviated wellbore, and the results served as boundary conditions for the multiphase flow model of CO2 transporting sand. In addition, the physical properties of CO2 as a function of temperature and pressure were considered in the numerical simulation. The multiphase flow model was solved to investigate the effects of various factors on the sand transporting efficiency of CO2, including displacement, sand production rate and annulus eccentricity, in which the sand size distribution at the annulus entrance was set based on the measurement data from the experiments. The computed results show that the sand transporting efficiency decreases with inclination abruptly at first and subsequently increases, which is consistent with the experimental results. The inclination where sand is most difficult to transport varies from 48° to 72° and depends on the sand's diameter and production rate at the bottom hole. The efficiency can be improved by increasing displacement with a trend that is greater at first and later slows. A larger sand production rate at the bottom hole means it is more difficult to transport sand. The efficiency decreases with increasing eccentricity at first and subsequently begins to increase when the eccentricity reaches 0.8. Under the same conditions, the pressure drop of sand-flushing with CO2 is 52.3% that observed with water. The results lay an important foundation for practical application.