Non-aqueous drilling fluids used in deepwater drilling operations are conducive to the dissolution of formation gas during wellbore influx events, increasing the risk of gas kicks going unnoticed. This can lead to hazardous riser gas unloading events if kicks are allowed to pass the subsea BOPs and come out of solution at the bubble point at shallow riser depth. One possible solution to handle and even prevent these events is to generate enough backpressure using Managed Pressure Drilling (MPD) with a Rotating Control Device (RCD) to keep the dissolved gas in solution. However, for large kicks, the required backpressure may exceed riser pressure limits.
A novel kick handling procedure using a dilution control strategy (DCS) is proposed here to handle gas influxes and subsequent gas unloading events. The idea behind this DCS is to inject mud into the riser through the boost line while simultaneously slowing down the circulation rate through the drillstring as the dissolved kick passes the open subsea BOPs. The kick will then get diluted and will be distributed across a larger annular space leading to a significantly decreased gas concentration that can be more easily handled by the MPD system with lower backpressure.
The feasibility of the DCS is investigated in this paper using a multi-phase flow model which is validated against experimental data for a gas kick in oil-based mud. Simulation results for a demonstration case show that a kick entering the well at 18,500 ft with 2,250 kg gas can be thoroughly eliminated with a 3:1 dilution ratio (which is the ratio of riser boost rate to drillstring circulation rate) with approximately 620 psi backpressure when using an MPD system. To improve the applicability of the proposed DCS procedure in field practice, a data-driven approach is implemented using simulated data points to provide a fast estimation of the optimum dilution ratio (DR) to control the kick in real-time.