A new approach to risk control of gas kick in high-pressure sour gas wells

Abstract

During drilling in high-pressure sour gas well, the occurrence of gas kick may cause serious consequences. Generally, in order to prevent gas kick, higher density of drilling fluid is adopted to keep adequate positive pressure difference in hole bottom. The requirement of positive pressure difference in China is 5 MPa. However, due to the narrow mud weight window in most high-pressure sour gas well, this method could not keep both equivalent static density and equivalent circulation density within the safe window. The lower density of drilling fluid could not ensure hydrostatic pressure higher than a pore pressure of 5 MPa; while higher density may cause leakage and therefore trigger blowout. In this paper, an improved approach is proposed to solve this problem, which involves bottom-hole pressure control methods and necessary equipment. A case study shows that the conventional method cannot develop a reasonable mud density while the improved method can ensure a bottom-hole pressure within the narrow pressure window. The results show that the improved method not only solves the problem of circulation loss that occurs in conventional drilling, but also meets the Chinese standards of well control in high-pressure sour gas wells, providing a new technique for the risk control of gas kick.