Abstract: Quantifying Permeability-Controlled Low-Dip, Low-Rate, Oil (Secondary) Migration

Oyvind Sylta

Water-saturated cores of Bentheimer and Berea sandstones have been subjected to low rate migration of dodecan. The change in water saturation was measured indirectly by means of gamma-ray absorption. The experimentally determined dodecan saturations were compared to numerically computed saturation distributions using a standard black-oil reservoir simulator and standard capillary and relative permeability relationships. The results showed that the secondary migration process is sufficiently slow to occur at hydrostatic equilibrium. Therefore, the oil saturation distribution, which is controlled by the capillary curve, can be computed when the oil column height is known.

The oil column height, and thus the saturation distribution within flow paths, can be compute by iterative procedures when volumetric oil flow rates are known. Oil migration losses can then be computed from oil flow rates, carrier bed dips, permeabilities, capillary pressure curves etc. The resulting set of equations can be used in assessing secondary migration efficiencies from standard types of reservoir technology measurements.

The magnitudes of the oil losses depend strongly upon the subsurface permeability distribution. A simulation case study from the Viking Graben in the North Sea shows an example of significant lateral variations in migration losses. Most of the secondary migration oil losses in this basin are model led early in the migration history, i.e. prior to the Tertiary, when the oil paths are first saturated.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994