Gravity displacement gas kick law in fractured carbonate formation

Abstract

AbstractThe gravity displacement gas kick often occurs due to the fractures developed in fractured carbonate formation. The improper control of the gravity displacement gas kick is prone to blowout accidents. In this study, the gravity displacement gas–liquid two-phase flow law was simulated by CFD software, and the variation of fracture gravity displacement rate under two boundary conditions was obtained. Besides, the simplified gravity displacement models were established for fractures with two boundaries based on the gas–liquid two-phase flow theory. The analysis results of the influence factors related to the gravity displacement gas kick revealed that the gravity replacement rate of a fracture with an inclination angle of 90° was almost three times that with an inclination angle of 30°. When the wellhead backpressure was not applied, the gravity replacement rate was 0.35 m3/h; while, after the wellhead backpressure of 12 MPa was applied, the gravity replacement rate decreased to 0.22 m3/h. When the consistency coefficient was larger than 1.0 Pa sn, the fluidity index was larger than 0.6. In conclusion, the gravity displacement gas kick will inevitably occur under large inclination closure fractures. The larger the inclination and opening of the fracture, the smaller the wellhead backpressure, and the more serious the gravity displacement gas kick. During field operations, the gravity displacement overflow can be alleviated under low density and high viscosity drilling fluid by applying wellhead backpressure.