Abstract

This paper documents current concepts of overpressured formations, and methods of prediction and detection utilizing geological and drilling engineering data.

Abnormal pressures are present in the Pertamina-Mobil "B" Block in the central part of the North Sumatra Basin. Along the Arun-Lhok Sukon High, and adjacent deeps, the overpressured interval occurs within the lower Keutapang and Baong formations of Upper and Middle Miocene age at depths ranging from 4,000 to 8,000 ft subsea. Cause of the overpressuring is believed to be related to rapid deposition/burial of sediments.

In addition to standard industry methods used to predict and detect overpressures, Mobil has found that a normalized plot of rate-of-penetration adjusted to sonic correlation can be used to determine overpressuring and define caprocks with acceptable accuracy. Intermediate casing is set at a maximum pore pressure equivalent to 12 ppg to permit successful drilling through the overpressured interval with mud weights up to 16.5 ppg.