f Managing a Field with 24yrs of Production History and More Than 40 Years Still to Go

The Valhall chalk Field, located in the Southern North Sea, is unique in it’s production history, reserves growth linked to technology and expected long future ahead. It was discovered in 1979 and put on production in 1982. Starting with estimated reserves of 250 mm bo, the field today has 2 billion barrels as its ambition and potential. Being a soft, high porosity and essentially uncemented and high pressure chalk, the reservoir has been on primary depletion producing from a strong compaction drive for more than 20 yrs. The crest, hidden under a gas cloud, was the primary target area. Due to drilling problems through the compacting and subsiding, soft overburden and unsuccessful extended-reach drilling two flank platforms were put in place in 2002 and 2003. This has significantly contributed to reserves growth and the reservoir limits are pushed deeper and outwards as we drill on the flanks. In the crest, a water injection program was sanctioned, to build up pressure and increase production through matrix sweep and pressure support. The IP platform was in place in 2003 and water injection started through two converted wells first in the northwest area of the field in 2004. Today a crestal monitoring well (measuring pressure and water induced rock compaction underneath crest installations) and a first crestal water injector are in place as the first parts of a planned water injection programme with a line drive patterns consisting of injectors and producers. A key element in the water injection programme is surveillance. An advanced surveillance program allowing for steering and detecting the pattern of water flow is planned and about to be implemented. Equally important perhaps even more, is the Life of Field Seismic (LoFS) installed at Valhall. Kilometres of cables are covering the field allowing for regular acquisition of seismic data across the field. Primarily installed for following the 4D changes in the field associated with the water injection programme, LoFS has shown to have impact on essentially the entire reservoir management: Well planning, Wells management and for 4D effect in the reservoir. There are challenges to overcome in that details are still hidden underneath the gas cloud and details are still to be unlocked I the images resulting from the successive surveys. However, we are unlocking more and more of the story of the chalk reservoir, changing essentially every day as oil is produced and water is injected.