Oseberg: Increased Recoverable Resources by Optimal Reservoir Management and Use of New Technology

Abstract

Abstract The giant Oseberg Field is located offshore Norway 140 km Northwest of Bergen at approximately 100-m water depth. The field is developed with four offshore installations and the 1998 reserves estimate is 337 MSm3. Pressure maintenance is implemented by reinjection of associated gas and import of gas from the nearby Troll Field. The field is by September 2000 producing approximately 42000 Sm3/d from 46 wells. Plateau production of 81000 Sm3/d ended in 1997. 82 percent of the expected reserves have been produced and the original oil leg of 215 m is now reduced to between 20 to 40 m in the main reservoirs. Gas treatment capacity is the bottleneck controlling the oil production rate and production is optimized on a daily basis based on individual well gas oil ratio. The Oseberg field has been an offshore laboratory to implement new technologies as they have emerged during the late eighties and the nineties. The paper will discuss both successful and less successful technologies. More than 80 percent of the production is currently coming from horizontal wells that have been drilled in the field since the early nineties. Typical horizontal well lengths are between 1200 m to 1700 m and are placed close to the water oil contact. Geo-steering by use of navigator and autotrack has been applied for optimum well placement. Three multilateral wells with both branches exceeding 1200 m have been successfully established. To increase the recovery and accelerate tail end production, intelligent completions in the horizontal wells are assumed to play an important role. Currently four wells with up to four independent zones have been completed. Reservoir management on Oseberg has also involved WAG techniques and attempts with foam to shut of gas influxes. To monitor the gas flooding of the reservoir and well inflow profiles, wireline, coiled tubing and well tractors have been applied at a large scale. Oseberg has been in the front-line to utilize geophysical technology and shot full field 3D seismic in 1982. The first 4D seismic was acquired in the central area of the field in 1989. In 1998 part of the field was covered with seabed seismic and a new conventional 3D seismic to map out remaining reserves was shot in 1999. Seismic attribute analyses have been used to map out channel sand deposits and dedicated production wells have been targeted successfully with this technique. Lately, seismic data have been visualized by "CAVE" technology for geo-steering purposes.