Bullheading Drilling Operations: Understanding Anomalous Gas Displacement Efficiency

Summary Bullheading process is a well-kill approach that is used 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 uprising gas influx back into the formation without circulating it out to the surface, experiencing cocurrent and countercurrent 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 the Reff increases with increasing liquid injection rate (i.e., higher Reff at higher Qw). By performing (i) simulation and model fits to large-scale experimental data (about 1,900 ft deep well) from LSU PERTT laboratory and (ii) small lab-scale experiments (6-ft-tall vertical tube with 1 in. diameter) with visualization capacity, this study proves for the first time that the Reff increases with Qw (i.e., dReff/dQw 0) when Qw is relatively low or high, but decreases with Qw (i.e., dReff/dQw 0) when Qw is in between. This anomalous behavior, reminiscent of multivalued folding solution surface in catastrophe theory with unstable region surrounded by two adjacent stable regions, is shown to coincide with flow regime change from mist/annular to slug and, finally, to bubbly flow regime. Although this new finding has a significant implication in bullheading field operations, further investigations with dimensionless numbers and in annulus geometry require future studies.