Riser gas equilibrium for oil based muds with MPD considerations

The offshore drilling industry has been seeking to more realistically simulate gas-in-riser events, to optimize mitigation methods and justify the adoption of new pressure control equipment. This paper presents an improved model to describe gas in riser events with direct application to oil-based muds (OBM), with which most of the gas-in-riser events take place. A mathematical model is developed and used to describe the behavior of gas influx migrating or being circulated out in a marine riser in an OBM. The release of dissolved gas in OBM is thus factored in and assumed instantaneous. Bubbly flow is assumed for the contaminated mud and simulations are performed for different choke openings as in Managed Pressure Drilling (MPD) applications and with different booster pump circulation rates. The contaminated mud section is discretized both in time and space to ensure that pressure and pressure dependent parameters are accurately quantified. Results clearly show that the severity of the rapid unloading process can be overestimated when a water-based mud scenario is assumed. The severity of rapid unloading and the depth of the riser equilibrium point during the circulation of the influx out of the riser are seen to be of little dependence on the time at which the backpressure is initiated but with very high dependence on the magnitude of the backpressure and the circulation rates. Severity of rapid unloading increases with the circulation rate. If the gas is allowed to migrate without circulation, backpressure application would have a much greater effect on the unloading results. The concept of riser equilibrium has been thus far developed in the context of water-based muds. This paper considers only OBMs where most of the events take place.