Gas influxes are common during drilling operations. If not handled properly, in open loop systems gas can expand and unload the riser, however in closed loop systems, unless gas is depressurized, the top section of the riser will become over pressured. Riser unloading has not been accurately modeled due to uncertainties in gas expansion. This paper presents studies on gas expansion and unloading in a lab scale model at Texas A&M University.
We have performed experiments in a scaled riser at the Dual Gradient Drilling laboratory. We ran experiments using water and air as the gas phase. We recorded the change in volume as gas expands and liquid height changes. We mitigated the disproportion of the atmospheric pressure and the scale of the model by using vacuum pumps. We also measured overflow by allowing liquid level to rise to determine the final gas volume.
The experiments provide information on gas expansion and riser unloading. During the expansion process the top of the bubble travels rapidly building momentum, which carries a large volume of mud to the top of the riser model. This occurs through the rapid change in gas volume and the energy released as the pressure decreases. By allowing gas to expand in a controlled environment we measured the impact of gas expansion.
This paper provides information on controlled and uncontrolled gas expansion, impact on riser unloading, and benefits of a closed system.