Research On Wellbore Flow And Cuttings Carrying Of Supercritical Carbon Dioxide

Supercritical carbon dioxide is a new type of drilling fluid which has been highly regarded in petroleum industry due to its characteristics of high density,low viscosity and easy diffusion.The previous studies showed that the supercritical carbon dioxide jet has a far lower rock breaking threshold pressure than that of the water jet,which means the supercritical carbon dioxide jet can effectively improve the drilling speed;meanwhile,supercritical carbon dioxide does no harm to the reservoir and can effectively protect it,so it is very desirable for the development of unconventional oil & gas resources.But now,the research of supercritical carbon dioxide in drilling is still in its initial stage,so it is of great importance for the development of supercritical carbon dioxide drilling technology to do the study of supercritical carbon dioxide flowing and lifting mechanism in the wellbore,and the influence of supercritical carbon dioxide on the permeability of near wellbore area.The density,specific heat,viscosity and thermal conductivity of carbon dioxide are studied.By analyzing the supercritical carbon dioxide flowing process and the heat transfer in wellbore which combines the heat transfer between the wellbore and the formation,the temperature model and the pressure model for supercritical carbon dioxide in wellbore were established.Numerical method was used to analyze the distribution of the temperature and the pressure of supercritical carbon dioxide in wellbore.The results indicate that the temperature of the CO₂ within the drill pipe and the annular space increases as the well depth increases,so does the density,but with a very small increment.Based on the stress analysis of particles in CO₂,a settling velocity model and cuttings start-up model was established for the particles in CO₂.The results show that the settleing velocity of particles increases with the increase of temperature and decreses with the increase of pressure of carbon dioxide.When the inclination angle is less than 15°,the particles startup in a rising pattern;when the inclination angle is more than 15°,the particles startup in a rolling pattern;when the inclination angle is between 50° and 70°,it's the most difficult for the particles to startup.Combining the drilling practice and the property analysis of CO₂,two experimental devices of different sizes were designed by similitude principle for the study of supercritical carbon dioxide flowing and lifting mechanism.The experimental results is well consistent with the model,but experimental results show that the terminal velocity decreases when the CO₂ is in the gas-liquid transition zone and the terminal velocity increases when the CO₂ is in the supercritical zone.The experimental results show that when the CO₂ and water exist together,the cuttings will be more dispersed and smaller in size.By using experimental and numerical methods,and taking the size distribution of cuttings into consideration,a study on the supercritical carbon dioxide lifting mechanism was done.The results show that the numerical simulation is well consistent with the experiments.The influence of factors,such as temperature,pressure and inclination angle,on the supercritical carbon dioxide lifting capacity was analyzed by using numerical simulation method.Based on the research above,the influences of supercritical carbon dioxide on the permeability of near wellbore area,the mud pollution removal were analyzed by using experimental methods.The results show that supercritical carbon dioxide can effectively improve the permeability of formations.The SEM analysis indicates that can disperse the clay in the formations and thus form corrosion holes.Meanwhile,CO₂ can effectively remove the mud pollution to the formations.In this paper SC-CO₂ flow model and cuttings carrying model in wellbore is established and also the experimental equipment to test the SC-CO₂ flow and cuttings carrying is developed.These work laid the theoretical foundation for the development of supercritical carbon dioxide in drilling and completion technology.