Bullheading Drilling Operations: Understanding Anomalous Gas Displacement Efficiency

Abstract Bullheading process is a well-kill approach which is employed as an alternative method during harsh drilling situations where traditional well control operations present a limited capacity. The process involves pumping liquid from the top of a drilling well, at the liquid injection rate (Qw) high enough to push down the up-rising gas influx back into the formation without circulating it out to the surface, experiencing co-current and counter-current two-phase flow in the annulus. Understanding gas displacement mechanisms and the resulting gas removal efficiency (Reff) is a key to optimize the operation. The conventional thought is based on the simple belief that gas removal efficiency increases with increasing liquid injection rate (i.e., higher Reff at higher Qw). The objective of this study is to evaluate how gas removal efficiency changes with liquid injection rate, supported by laboratory experiments and transient simulations. This study presents three major tasks: (i) identifying an existing large-scale experimental study (about 1,900 ft deep well) that allows mechanistic models and transient simulations (using OLGA) to be validated, (ii) understanding the dynamics of gas bubble displacement by downward liquid injection during the counter-current situation in a vertical well, and (iii) visualizing gas displacement mechanisms in a small lab-scale experiment (6 ft tall vertical tube with 1 in diameter). The outcome of this study shows that the gas removal efficiency does not necessarily increase with increasing liquid injection rate. In fact, the simulation and experimental evidences demonstrate that the gas removal efficiency increases with the rate (i.e., dReff/dQw > 0) when the rate is relatively low or high, but decreases with the rate (i.e., dReff/dQw < 0) when the rate is in between. This anomalous behavior in the intermediate range of liquid rate happens during the transitioning from annular/dry slug flow regime to bubbly/wet slug flow regime.