MECHANISM OF WELL CONTROL DURING DRILLING IN SOUR GAS WELLS

The content of CO2 and/or H2S is so high even to the supercritical state in some sour gas wells in the northeastern Sichuan basin.The volume of CO2 and H2S only expand abruptly when returning very close to the wellhead in the annulus,which often induces blowouts.In this paper the corresponding mathematical model has been established and a software program developed based on the analysis of the special phase behavior of supercritical CO2 and H2S.And the annular temperature field,pressure field,compressibility factor and volumetric expansion in critical sour gas wells have been simulated.The results indicate that there is enough time to warn overflows in conventional deep gas wells,while there is hardly sufficient time to warn gas overflows in sour gas wells due to the abrupt expansion of sour gas in the upper annulus.So the monitoring of instantaneous flow rate of drilling fluid in annulus should be stressed during drilling of sour gas wells in order to detect the overflow as early as possible to ensure safe drilling.