The Development of Fundamental Research on Supercritical Carbon Dioxide Drilling

Supercritical CO2 drilling,a new type of drilling technique uses CO2 as its sole drilling fluid and has great potential in accomplishing the resource utilization of CO2 and improving the efficiency of drilling unconventional oil/gas wells.The key theoretical and technical challenges in supercritical CO2 drilling mainly involve supercritical CO2 flow law in the borehole,the cuttings carrying capacity,rock jet-breaking and wellbore stability in multiphase flow and fluid-rock interactions.In this paper,the supercritical CO2 wellbore flow model was used to analyze the factors affecting the distribution of supercritical CO2 drilling annulus pressure;theoretical calculation and cuttings carrying experiments were conducted to analyze the cuttings carrying ability of supercritical CO2. According to theoretical analysis and rock jet breaking tests,the mechanism of supercritical CO2 jet breaking was analyzed.Wellbore stability in critical CO2 drilling was analyzed through mechanical and thermal coupling model of critical CO2 and surrounding rock in combination with the effect of supercritical CO2 on rock mechanical properties.The results show that circulation flow rate and wellhead back pressure were the main factors in the distribution of supercritical CO2 drilling annulus pressure.When the well inclination was 48°-72°,the minimum return velocity of supercritical CO2 cuttings carrying was relatively high and the cuttings carrying efficiency was closed to that of fresh water which was remarkably higher than that of air;the reduced stress from temperature change when rock broke down from supercritical CO2 jetting could effectively reduce the threshold pressure of the rock breaking down and improve the efficiency of rock breaking.When drilling through horizontal beddings of hard shale strata with supercritical CO2,a perfect wellbore stability was observed.The research established a foundation for the perfection of supercritical CO2 drilling theory and the establishment of supercritical CO2 drilling techniques.