Depleted reservoir drilling : a study of the Ula field in the North Sea

Abstract

Ula is an oil field operated by BP in the southern part of the Norwegian North Sea. As the field matures depleted intervals are drilled to access remaining reserves. Drilling in highly mature fields is challenging, both with respect to loss of circulation and differential sticking. On the Ula field there is a pressure barrier/baffle isolating parts of the Ula reservoir units. This pressure barrier creates a pressure differential between the units. The units above the barrier (1A1-1A2) have been produced since 1998 without any pressure support, resulting in a pressure decrease in this zone. The units beneath the barrier (Units 2-3) have been produced since 1987 and have been supported by both water- and WAG-injection, giving a much higher formation pressure in these units compared to the units above the barrier. The pressure differential between Units 1 and 2 have produced challenges drilling this interval in terms of losses during drilling and cementing operations. This thesis describes the challenges in drilling depleted formations with respect to lost returns and differential sticking. The Ula field in the North Sea will be the primary focus throughout the thesis. The study involves reviewing BP’s drilling practices on the Ula field and proposing potential recommendations and improvements. Maximum overbalance on the Ula field and different options for drilling depleted reservoirs will be evaluated. Wellbore strengthening techniques and the potential benefits of using such techniques will also be presented.