Horizontal and Vertical Permeability Restrictions in Upper Beryl Sandstone Reservoir of Beryl Field--Challenge for Reservoir Management

Craig A. Knutson, Ragnhild Erga, Fiona M. Macaulay

The Beryl A field is the seventh largest oil field in the UK sector of the North Sea. The upper Beryl Sandstone reservoir (250-500 ft thick) contains about 75% of the total field reserves. After 12 years of production, it is exhibiting some unexpected reservoir behavior: (1) water moving over oil within a massive sandstone and (2) adjacent areas in close pressure communication having gas-oil contacts differing by over 300 ft. Subtle vertical and horizontal permeability restrictions within the upper Beryl Sandstone are recognized as controlling these unusual conditions and can be used to optimize oil recovery.

Faults with sand-on-sand offsets act as horizontal permeability restrictions and divide the upper Beryl Sandstone into four major reservoir fault blocks. Examination of pressure decline curves reveals the magnitude and direction of pressure differentials between the fault blocks and explains how faults can allow good pressure communication between adjacent fault blocks yet maintain different fluid or gas contacts and how fluids can move between fault blocks that do not appear to be in good pressure communication.

Vertical permeability restrictions resulting from thin, interbedded siltstones, shales, or coals further complicate the generally massive Upper Beryl reservoir. The degree to which these thin layers act as reservoir seals has been determined semiquantitatively by analyses of pressure transient tests, repeat formation tests (RFT), production performance, and cased-hole logs. The vertical zonation of the upper Beryl Sandstone causes differential depletion and, eventually, gas and water cusping. A detailed understanding of the zonation allows identification of zones of by-passed reserves and provides a means for effective reservoir management.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.