Top hole drilling in deep water

Abstract

Summary

When drilling the top hole section in deep water regions, alternative methods to the conventional riserless drilling system may be employed to avoid the "Pump and Dump" strategy. Conventional drilling with riser is not beneficial to use in top hole drilling with no blowout preventer (BOP) installed. It was used until 1985 but today it is avoided due to security reasons. Drilling the top hole section and recirculating the drilling fluid back to the rig can be applied by utilizing the low riser return system (LRRS) or the riserless mud recovery (RMR) system.

In the LRRS an adjustable pump is located a certain distance below the sea level, mainly to reduce the hydrostatic column of mud in the well. In this situation the mud pressure at seabed may be adjusted to equal the sea water pressure. In case of a disconnection the well pressure will then be maintained even without a BOP installed. In the RMR system the mud column starts at seabed and the seawater column above the mudline will not influence the maximum allowable mud weight to be used in contrast to a conventional riser system. Therefore, the smaller pressure window in deep water due to the relatively larger column of seawater will be neutralized when applying the LRRS and the RMR system.

To control a shallow gas zone when utilizing a conventional riser system a certain static mud pressure is needed.In addition the calculated dynamic mud pressure must not fracture the formation.To avoid fracturing the conductor has to be set at the depth where the dynamic mud pressure equals the equivalent fracture gradient. When utilizing the LRRS and the RMR system, the equivalent fracture gradient at conductor setting depth will not decide the maximum allowable dynamic mud weight to be used, since the mud column is not starting from rotary kelly bushing (RKB). Then a higher mud weight may be used which results in easier controlling of the shallow gas zone, even with a lower conductor setting depth. In addition the top hole length will be extended.

The main advantage using the LRRS compared to the RMR system is that the bottom hole pressure quickly can be changed by adjusting the fluid level in the riser, hence a shallow gas influx can be controlled rapidly.However, the change in bottom hole pressure is limited by the narrow gap in pore and fracture pressure at the conductor setting depth when drilling the top hole. Still it may be determining in controlling an unexpected shallow gas zone with high overpressure. The main advantage of the RMR system compared to the LRRS is that the conventional marine drilling riser is avoided which is a benefit regarding the rig capacity. Then a smaller mud volume is needed and the necessary supported weight from the rig is reduced. These differences will be larger as the water depth increases. In addition, the amount of gas entering the well is difficult to stop in a LRRS. When gas rises using the LRRS it must be diverted through the diverter line. When gas rises in a RMR system it will most likely be diverted away by the water currents.

Further work may be applied, listed in the following stages:

. Compose a drilling program to calculate the increasing benefits of using a RMR system when the water depth enlarges.

. Compose a drilling program to calculate how quickly a shallow gas zone may be controlled with a LRRS compared to a RMR system.