ABSTRACT: Implementation of the Ekofisk Waterflood

L. D. Hallenbeck, J. E. Sylte, D. J. Ebbs, L. K. Thomas

A full-field water injection program was begun into the Ekofisk field, located in the Norwegian sector of the North Sea. The approval of field-scale water injection was granted in 1983 and was limited to the northern two-thirds of the Tor Formation. In 1988, the waterflood was expanded to the southern part of the field and into the lower Ekofisk Formation.

The 6.7 billion barrels of oil originally in place in Ekofisk field is contained in two formations, the Ekofisk and Tor, which are separated by a thin tight zone. Each formation is characterized by low matrix permeability chalks that are naturally fractured. The natural fracturing enhances effective permeability to allow commercial production with oil expansion, solution gas, gravity drainage, and compaction drive as the primary depletion mechanisms. Spontaneous imbibition is the principal drive mechanism in the intensely fractured regions undergoing waterflood, whereas viscous forces dominate at lower fracture intensities. The Tor Formation exhibited excellent spontaneous water imbibition characteristics in laboratory experiments, which were confirmed in a pilot waterflood project. T e lower Ekofisk Formation initially gave erratic indications for spontaneous imbibition in laboratory experiments, but results from a pilot waterflood project were favorable.

The full-field waterflood included the installation of a 30-slot, fully integrated drilling and injection platform. Initial design called for 20 injection wells and a maximum total injection rate of 375000 bbl of water per day. In 1988 this concept was expanded to 31 wells using an eight-well satellite injection well platform, a converted bridge-support tripod located at the Ekofisk center 2.4 km to the south and connected by a 12-in. water-injection pipeline. Two existing production platforms were modified to allow drilling of 16 additional producers. Early drilling of injection wells commenced in 1984 from a jack-up rig and was continued from two rigs on the 2/4-K platform. Water injection was started in November 1987, and through May 1989, 115 million bbl of water has been injected with a daily injection rate approaching 375000 bbl per day.

Factors taken into account during the implementation include analyses of the two pilot waterfloods, injection-well pattern design, vertical confinement considerations, optimization of production-well sidetracks, evaluation of core fracture orientation, regional permeability variations, reservoir geology and faulting, and overall anisotropy. After the startup of injection, a comprehensive waterflood surveillance program was initiated to monitor the early results, as well as creation of three-dimensional models to predict ultimate recoveries.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990