Gas-in-riser events can lead to rapid unloading if not timely controlled in a proper manner. When gas influx enters a wellbore with non-aqueous muds (NAMs), the ability of gas being dissolved in NAMs increases the difficulty in gas kick detection and significantly alters gas migration and unloading behavior from the predictions based on water-based muds (WBMs) assumptions.
In this study, a new mathematical model for riser gas management in NAMs is developed. In this model, the desorption of dissolved gas influx from NAMs is accounted for as an instantaneous process using a solubility-based mass transfer submodel. The effects of surface backpressures and circulation rates on the unloading behavior in both WBMs and NAMs were studied. This model was validated using data obtained from a drift-flux model (DFM) based simulator. Results show that with the same amount of free gas in the risers at the mudline level, the severity of unloading is significantly more severe in the cases of NAMs. Applied backpressure can effectively control the desorption of the gas influx from the mud, and the unloading occurs later and at shallower depth with higher backpressure. The behavior of unloading tends to be independent on the time when backpressures are applied but highly dependent on the magnitude of the backpressure and the circulation rates. The new two-phase model can accurately simulate riser gas kick events utilizing a simplified approach with improved numerical stability, making it more applicable for real-time riser gas management.