As a part of planning the first well utilizing underbalanced drilling technology in Norway, a comprehensive risk analysis is performed, comparing risk associated with underbalanced and conventional, overbalanced drilling. The development of a new risk analysis method is described. It is concluded that for the given case, underbalanced drilling reduces blowout risk considerably.


The Gullfaks field is located in the northern North Sea, in the Tampen area on the Norwegian Continental Shelf (NCS). The main field is developed with three fixed concrete platforms. Since 1989, development wells are drilled from 135 slots. Sidetracks make the total number of wells considerably higher. In the recent years, construction of development wells has become difficult in certain areas of the field. Abnormally high pressures have been encountered while drilling in the top layer of the cap rock formation above the reservoir. Considerable problems related to pressure control, such as lost circulation and kick have resulted in delays and even in wells being plugged and temporarily abandoned.

The high pressure is the result of extensive water injection and communication between the reservoir and the porous and permeable top layer of the cap rock. Reservoir fluids and pressure are transferred through faults or cracks or along existing wells. A limestone bed prevents pressure to propagate further upwards.

Safe drilling by means of conventional (overbalanced) drilling technology implies that the wellbore pressure has to be maintained at some level between the pore pressure and the fracturing pressure of the formation throughout the operation. The uncertainty related to the pore pressure has complicated well planning. Several experiences of narrow pressure margins close to the undrillable are made. The high rate of undesirable incidents suggests that conventional overbalanced drilling technology is inappropriate for the purpose of drilling the cap rock sections.

Underbalanced drilling (UBD) technology allows drilling with wellbore pressure lower than the pore pressure, and was suggested as an alternative to conventional drilling to enable safe construction of the cap rock sections within the problem area.

Conventional drilling is based on the principle that the pressure exerted by the well fluid column on the formation at all times is kept higher than the pore pressure, and thus prevents formation fluids from flowing into the well. The overbalanced mud column is defined as a barrier against uncontrolled flow of formation fluids via the well to the surroundings (blowout). UBD is conceptually different. The system is designed to operate with wellbore pressure below the pore pressure and will thus, if a porous and permeable formation is penetrated, produce formation fluids while drilling. The established definition of the well barrier system against blowout, which is used for conventional operations, is not applicable for UBD. Different definitions are used, which to greater extent depend on mechanical blowout preventer (BOP) elements.

Well C-5A was pointed out as the first candidate well for UBD. The planning of what would be the first application of UBD for the operator and on the NCS involved a large element of technology qualification. Large efforts were laid down to ensure that the UBD operations in the cap rock section could be carried out in accordance with national rules and regulations, industry standards and requirements given in the operator's internal governing documents.

A governing principle used by the operator implies that introduction of new technology shall not increase the risk, as compared to a reference technology. In order to study the impact on blowout risk if using UBD instead of conventional drilling technology for this particular case, the project undertook a quantitative risk analysis (QRA). Subobjectives of the analysis were to assess risk related to formation fracturing and wellbore collapse and to serve as an instrument for identifying factors representing main contributors to risk and measures for reducing risk.

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