Characteristics of Wellbore Transient Flow during Sour Gas Influx

Sour gas reservoirs are generally marine sediments and usually have narrow safety mud density window.The sour gas influx occurs frequently in drilling process which lead to a complex wellbore multiphase flow,and even the overflow and blowout.In order to achieve the accurate prediction and control of wellbore flow during sour gas invasion,a numerical model was formulated in this paper to study the wellbore transient multiple-phase flow during sour gas invasion,adopting the sour gas solubility equations.In this model,the wellbore flow was coupled with the non-Darcy seepage through formation media,and the fluid properties were also coupled with the temperature and pressure fields in the wellbore.Based on field data,the characteristics of gas-liquid two-phase transient flow in the annulus were investigated.What’s more,the sensitivity analysis was performed to evaluate the key parameters effect on the wellbore transient flow during sour gas invasion.The sour gas has large solubility in downhole conditions and it’s difficult to detect the sour gas influx in the initial invasion stage.However its solubility reduces as mixed fluid flows to the wellhead and lots of gas releases which results in wellbore pressure decreasing sharply.As a result,the wellbore pressure is hard to predict and control during sour gas invasion.This study could provide guidance for the well control parameters design for sour gas reservoirs drilling.