Hydrodynamic modeling of gas influx migration in slim hole annuli

Abstract

Existing well control prediction techniques have not yet found wide acceptance in slim holes. This study investigates the two-phase transient pressure behavior when a gas kick occurs in slim holes. This paper presents an improved two-phase model that will account for the change in annular pressure transients. The two-phase model is based on the drift-flux model. Newly developed drift-flux correlations with specific target to narrow annuli are applied. An explicit scheme is adopted to solve the two-phase model. Results show the high inaccuracies that occur from using conventional models instead of this new model. The velocities of each phase are seen to increase tremendously with assumption of slim holes. This in turn causes significant increase in the equivalent circulating densities (ECDs) and underestimation of bottom hole pressures. On the other hand, the hydrostatic pressures are seen to decrease as well leading to a counterbalance. Pressure signatures for slim holes are seen to shift to the right consequently affecting the void fraction distribution and other fluid properties. Case studies are performed in the simulations to examine the effect of varying annulus ratio on results for constant influx and mud rate. From the results, as annulus ratio goes beyond 0.8, the changes in bottom hole ECDs become much more substantial. The understanding of hydraulics in narrow annuli is vital for real-time pressure prediction during a gas influx. This study provides an improved simulation tool for more accurate prediction of influx behavior in slim holes.