BLOCH, GERARD, Nederlandse Aardolie Maatschappij, Assen, Netherlands, MARK DOYLE, Petroleum Development Oman, Muscat, Oman, and PETER KUKLA, KSEPL/Shell Research, Rijswijk, Netherlands
ABSTRACT: Subsurface Pressures and Leak-off Tests in the Netherlands and their Implications for Prospectivity, Safety, and Well Design
More than 4500 formation pressures and leak-off tests (LOT) of Dutch wells have been compiled and validated. When plotted vs. depth, these data exhibit a remarkable separation, the formation pressures being consistently lower than the LOT measurements. The band that separates these two sets of data has been called the "lower bound" with respect to the LOTs. This lower bound corresponds to the maximum expected formation pressures, which seem to be related to the minimum formation strength of the rock.
In the Dutch northern offshore, numerous valid dry structures have formation pressures close to or at the lower bound. At a depth of 2000 m, this corresponds to overpressure of only 75 bars (1100 psi).
Where a regional aquifer can be established, the protected trap concept is applicable. These types of traps are located downdip of potentially leaky structures and have a greater chance of having retained their charge, and maximum hydrocarbon column lengths can also be established. Furthermore, overpressures can lead to porosity preservation and increase the volume of gas in place. Prospects should be ranked accordingly.
The identification of overpressured zones and the prediction of the most likely pressures to be expected have a significant effect on exploration programs and planning. Adequate rig selection and well design will reduce costs and increase drilling safety.
AAPG Search and Discovery Article
#90990©1993 AAPG International Conference and Exhibition, The Hague,
Netherlands, October 17-20, 1993.