Analysis of Seals and Pressure Regimes in some Thamama Group Reservoirs, Abu Dhabi

Douglas F. Goff, Harry W. Mueller III, Erik J. Oswald, Hassan Al-Habshi, Salem Al-Matroushi

In this study we tested the assertion that faults segmented flow within the Thamama carbonate reservoir units, resulting in production anomalies. Several data sets were used to evaluate this problem, including structure maps, pressure and production history and test data, Mercury Injection Capillary Pressure (MICP) tests, and analysis of the oil/water contact (OWC).

Comparison of MICP derived and log derived oil saturations for the Thamama reservoir zones in several wells suggests that the OWC may not be tilted (as previously interpreted), but may actually be up to 100^prime shallower than previously mapped. Additionally, a new pattern of pressure segmentation above and below the OWC has been identified throughout the field. Within the crest of the field, pressures within the oil column show depletion due to production, whereas crestal and flank water wells show little pressure decrease.

MICP tests indicate that dense zones (carbonate argillaceous packstones and wackestones) are good capillary seals capable of supporting oil columns greater than 1000 ft, and can prevent flow across them. However, pressure communication documented between the upper Thamama three main reservoir horizons (A, B, C) indicate that there are pathways for fluid flow between these units. Based on the analysis of OWC, pressure monitoring and pressure history matching, MICP tests of the Thamama dense zones, and structural juxtaposition, it seems likely that cross leak (or possibly dip leak) through faults provides the conduits for fluid flow between reservoir units.

Results from this study suggest that fault seal and associated field segmentation is not likely to be a problem during production, However, the presence of a permeability barrier associated with the OWC may require the modification of production strategies (flank water injection vs. crestal gas injection) to deal with the poor pressure communication across this zone.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995