Gas Migration: Fast, Slow or Stopped

Abstract

Abstract We review the conflicting literature on gas migration velocities during kicks. We consider the laboratory and large scale test data that shows that for any local gas void fraction of more than 10%, the influx migrates at approximately 100 ft/min. We also review the evidence from field experience that shows that gas can migrate much more slowly (the typical rule of thumb suggests that gas bubbles move at approximately 15 ft/min) and in some cases remain stationary. We show that the yield stress of the drilling mud which holds cuttings in suspension whilst making connections, can also hold gas bubbles in suspension, and report an experimental study of these gas suspension effects. Significant volumes of gas can be held in suspension during a gas kick, this trapped gas remaining stationary until the mud is circulated out of the well. We consider the implications of this for well control operations, and present field data where gas was injected into a marine riser, it dispersed and remained stationary until circulated out. We show that a single bubble migration model, which neglects gas suspension, predicts that as the gas rises and expands it unloads the riser. By simulating the gas suspension characteristics we model the field data. We conclude that gas in moderate concentrations (more than 10%) migrates quickly, typically at 100 ft/min. This migrating influx leaves a trail of suspended gas in the mud that remains stationary. For small kicks in deep wells the entire influx can be distributed, at a low concentration, and remain in suspension until the gas-cut mud is circulated out of the well.