A field-wide permanent seismic array was installed at the Valhall field during the summer of 2003. The seismic array covers an area of 45sq. km. More than 120 km of permanent 4-component seismic cables were trenched into the seabed and will be used for seismic monitoring through the future life of the Valhall reservoir.
The aim is to provide "on demand" images of the 4D seismic responses due to production, to support placement of new wells, to guide interventions in existing wells for improved production rates, and to manage the water injection program currently being implemented over the field.
The first survey was acquired late last year, and a second survey is scheduled for February 2004.
The Valhall field is an over-pressured Upper Cretaceous chalk reservoir located 290 km southwest of Stavanger, Norway in 70 m water depth, figure 1. The field was discovered in May 1975 and first production from the field was delivered in October 1982. After more than 20 years, the field is still producing at plateau and is expected to produce economically for 25 more years,. To date, around 500 million barrels of oil, out of the estimated 2,7 billion barrels in place, have been produced. Current plans aim at draining around 40 % of the total oil in place by 2027. These plans include a water injection scheme, which is now being implemented.
The weak chalk reservoir, 6 km wide and 13 km long, with high porosity but low permeability, poses a number of technical challenges. The reservoir is relatively thin and structurally complex,. It is compacting during depletion, resulting in subsidence of the overburden, and occasionally causing chalk influx into the wells. The dynamic overburden is causing difficulties in drilling the overburden, with well bores frequently failing, months or years after drilling.
Producing from a low- permeability reservoir like Valhall requires a relatively large number of wells. Most of the current producers and new wells to be drilled are long-offset near-horizontal wells, parallel to bedding in the reservoir.
More than 5 billion NOK was invested in new infrastructure at Valhall during 2002 and 2003. Two new wellhead platforms were installed to accelerate development of the southern flank and the northern flank of the field. A third platform was installed at the Valhall center to support the water injection program. In the next few years more than 20 production and injection wells are to be drilled.
The "Life of Field Seismic", LoFS program is designed to assure that these investments will deliver the promised production and to improve recovery rates by optimizing existing well programs and identifying new upsides.
It is expected that "LoFS" will enable the Valhall partnership to:
Identify and realize upside reserves from improved reservoir description and management decisions such as optimization and expansion of the water flood project
Reduce cost by improved drilling trajectories, prognosis and extending the life time of the well
Improve recovery by high-grading low productivity areas