|dc.description.abstract||Today the world is experiencing an ever increasing energy demand. Since petroleum resources are an important contributor to the energy demands of the modern world, exploration efforts are being pushed to new frontiers. Deep-water offshore exploration involves a number of challenges that has pushed for new technology development. Dual-gradient drilling systems like Subsea Mudlift Drilling (SMD) - and Low Riser Return System (LRRS) are leading contactors in this new area of drilling technology.
LRRS has a subsea mud pump that enables having the riser only partially filled with mud, as the pump will transport mud from the riser to rig surface. Above the mud is air or an inert gas mixture. This makes it possible to drill with a higher mud-weight compared to conventional offshore operations, and also regulate BHP quickly by changing the riser fluid level.
It is very important to evaluate the safety of this new system, especially with regards to well control. In determining the pressure buildup during an unscheduled influx of formation fluids (kick), and compare it with a conventional offshore drilling system, it is possible to determine how much influx LRRS can take compared to the conventional setup. Doing so will determine the operational safety and limitations of LRRS compared to conventional drilling operation.
The major benefit of LRRS is the ability to have a much lower pre-kick casing pressure than a conventional system. Since the mud weight of LRRS will be higher in deep water operations, the pressure gradient is equally higher. When both systems have the same BHP, LRRS will have a much larger difference between casing shoe pressure and fracture pressure at the casing shoe. As a result LRRS can take much larger influxes (82,9% in 650 m water depth) compared to the conventional system when a kick is encountered at the same depth and with the same kick intensity.
When a gas bubble is pressure relieved as it migrates up hole, a certain amount of mud must be bled form the well to induce a given pressure reduction within the gas. Since LRRS has a heavier mud-weight, the loss of hydrostatic will be higher. This leads to a steeper pressure buildup curve, which will reduce the benefits of LRRS in slim holes and when large influxes are taken. In the case of 650 m water depth LRRS has a benefit in slim-hole of 30,6%.
The benefits of LRRS are magnified in deep waters, where the mud-weight difference is large. In a deep water case, LRRS has a kick-margin benefit of 170,7% over conventional. However, when the influx volume is increased from 10 m3 to 15 m3 LRRS’s kick margin benefit over conventional is 105,3%.
Compared to the leading dual-gradient drilling system (SMD) LRRS has a 25,6% kick margin benefit, in 650m water depth. This is because LRRS can operate with even higher mud-weight than the SMD system. In deeper waters the relative benefit of LRRS compared to SMD with regard to the conventional system, will be lower.||nb_NO