| dc.description.abstract | To be able to drill deeper, in depleted reservoirs, and in new complicated formations, new techniques must be developed and evaluated. Conventional systems for drilling are still the most common method for drilling wells, but managed pressure drilling (MPD) has gained broad acceptance and global usage during the last few years both onshore and offshore.
The Low Riser Return System (LRRS) developed by Ocean Riser Systems (ORS) is a new approach to the MPD method for use on a mobile offshore drilling unit (MODU). The LRRS concept uses a high density mud, and an adjustable mud level in the riser controlled by a subsea pump. The returning mud is pumped to the surface using this pump. This setup makes it possible to lower the pressure gradient, hence it will be possible to drill in narrow pressure windows and precisely control the bottom hole pressure (BHP) by adjusting the height of the mud level in the riser.
New well control procedures are required for the LRRS for safe operations. The procedures are based on the conventional driller’s method, and modified to fit the new drilling system. This thesis aims to compare and evaluate these well control procedures to identify the differences.
By using the LRRS, pressure sensors on the riser and the subsea pump are introduced as kick indicators. These are considered to significantly improve the kick detection, thus a kick can be detected very quickly compared to a conventional system. The quicker the kick is detected the easier it will be to handle.
The most significant differences identified are related to the shut-in of the well and the circulation of the influx out of the well. These operations can be seen as two separate operations for conventional well control. For the LRRS, the two operations seem to merge into each other due to the continued circulation during shut-in and further circulation of the influx out of the well. The shut-in of a conventional well is performed by closing the subsea blowout preventer (SSBOP) immediately after a kick is confirmed, while for the LRRS, the first step is to increase the riser level to stop the influx for then to close the annular preventer.
A kick situation was simulated to investigate the differences between the conventional drilling system and the LRRS when circulating a kick out of the well using conventional equations and software. The results indicate that the pressures during the kick circulation will be lower for the LRRS than for the conventional system as the gas moves through the well, but the choke pressures will be higher than the equivalent conventional system as of different locations of the chokes and initial pressures.
The use of conventional equations with the LRRS is considered applicable after some small modifications regarding fluid column height and friction pressures. However, the use of standard spreadsheet software for simulations is not recommended. This type of software is not suitable for simulating operations where parameters may change over time, such as riser level and choke adjustments. Important information during the circulation may therefore be difficult to identify.
A suggestion for further work would be to investigate the contingency plans of the drilling systems to identify solutions and consequences of unexpected incidents such as pump and choke failures during a well control operation. | nb_NO |
