Ormen Lange Hydrodynamic-Aquifer Model

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 153507, ’Predicting Water in the Crest of a Giant Gas Field: Ormen Lange Hydrodynamic-Aquifer Model,’ by M. Boya Ferrero, S. Price, SPE, and J. Hognestad, Shell, prepared for the 2012 EAGE Conference & Exhibition/SPE Europec, Copenhagen, Denmark, 4-7 June. The paper has not been peer reviewed. Simulation showed that hydrodynamically tilted/stepping contacts depend on the rate of water flow across the aquifer, stratigraphic baffling and faulting, effective aquifer area, and reservoir quality. The role of sealing faults over geological time was downplayed in terms of justifying the fluid distribution. However, baffling during production is expected. At Ormen Lange, the hydrodynamic aquifer has pushed gas from the crest of the structure into the south of the field, leaving behind a northward-thickening prism of residual gas, which was imaged with a seismic direct hydrocarbon indicator (DHI). Confirmation of the hydrodynamic aquifer in this field was achieved after drilling an appraisal well in the north of the structure that corroborated fluids (water with residual gas) and pressures as predicted by the hydrodynamic-aquifer model. Introduction The Ormen Lange gas field was discovered in 1997, with first production in 2007. It is a large (8×40-km) deep-water reservoir 125 km offshore Norway at 700- to 1100-m water depth. Ormen Lange sands have excellent reservoir quality, with 26 to 30% average porosity and 500- to 1,000-md permeability. The field was developed with four subsea templates and 10 high-rate large-borehole wells. The initial field-development plan was based on a stratigraphic trap filled to spill point, with the gas accumulation being delineated by a seismic DHI, which was postulated to indicate perched water on the edges of the structure. The field-development plan was defined on the basis of information available from only four exploration and appraisal wells. The need to understand the field’s charge history became apparent when, in 2008, an appraisal well in the northern part of the field encountered only residual-gas saturations in the crest of the structure and in the middle of a high DHI. Therefore, the DHI could not be used as a proxy for the current free-water level (FWL) because seismic cannot distinguish between mobile-gas and residual-gas saturations. Subsequently, three additional appraisal wells confirmed elevated contacts, residual gas, and gas communication across the field.