den Ouden, Ron¹, Claudia Guargena², Kåre Langaas², Sigbjørn Kalvenes², Joe Cartwright³, Ramon Loosveld², Nicola Moller¹, Irfan Ahmed¹, Eirik Berg¹, Liv Stuevold¹
(1) Norsk Hydro ASA, 0240 Oslo, Norway
(2) A/S Norske Shell, 4098 Tanager, Norway
(3) University of Cardiff,

ABSTRACT: Implications of the Uncertainty of the Dynamic Fault Seal Behavior, Ormen Lange, Norway

The many faults within the reservoir of the giant Ormen Lange gas field form a polygonally connected network, with throws commonly decreasing towards branch lines. They developed close to seabed, ruling out cataclasis and cataclasis-enhanced cementation. The high N/G reservoir and small throws low SGR ratios. Injection films and cements are unlikely to be continuous over the entire sand-sand windows. The scarcity of fault-linkage suggests gas communication, although flow may be tortuous. The common gas gradient, and the absence of measurable depletion during well tests also suggest that the faults do not seal on the geological time-scale.
However, “limit effects” observed during one of the well, the apparent flatspot-stepping, and the slightly shallower position of the Free Water Level in one of the wells (perched water?) indicate that faults may to some degree compartmentalise the reservoir, particularly on the production time-scale. Recognition of potential perched water pockets is not only important for saturation-height modelling, but also for well locations and completions. We have risked various configurations of geometric lows (4-way dip synforms versus faulted traps, thief sands in the seat seals, reactivation history of the faults). Fault-related perched-water pockets imply a lack of downward water escape, but not necessarily a compartmentalisation of the gas-bearing reservoir.
A more cautious approach, based on the learning-while-drilling concept, is therefore necessary. Production versus pressure drop relations will be the earliest indications of the size of the connected gas accumulation. Later time-lapse seismic surveys may detect possibly bypassed compartments.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.