Valhall Field - Still on Plateau after 20 Years of Production

Abstract

Abstract The estimated recoverable reserves from the Valhall field at start of production in 1982 were 250 MMSTB. Now, after twenty years on primary depletion, a total of 500 MMSTB has been produced and additional 500 MMSTB remains to be produced of the original 2.6 BSTB in place. This corresponds to a recovery factor of 40%. Possibilities to increase ultimate recovery have been defined. These will require further insights into the detailed architecture of the field and improved understanding of the reservoir energy and displacement efficiency. Approximately 50% of the drive to date has been from rock compaction. The field is a North Sea Chalk field producing from Tor and Lower Hod Formations, porosity is exceeding 50% in places, some fractured permeability is present, but matrix permeability is generally low, less than 10 mD. Application of new technology has been fundamental in bringing the field up to current performance. New completion techniques combined with dedicated draw down schedules has been established to reduce influx of the weak chalk formations and reduce well failures. Special well design for a compacting and subsiding environment has also been implemented. Drilling of the overburden has been a key challenge. This has been partly mitigated by detailed geological mapping of problem zones versus structure, lithology, overpressure and wellbore stability analysis, but has also resulted in replacing extended reach wells by the installation of new flank drilling and production facilities. The particular characteristics of the overburden and chalk also pose challenges for data acquisition. Coring technology & practices are very tightly controlled, as is selection of logging devices. A water flood scheme has been approved based on a successful water flood pilot, but issues related to sweep efficiency, presence of fault and fracture zones and large variation in thickness need to be understood to capture it's full potential. The reservoir will also experience extra water induced compaction and increased subsidence. New logging technology has recently made it possible to define and characterize the fracture system, which combined with a range of surveillance techniques will be key in this work. Seismic imaging has generally been a challenge at Valhall due to the presence of gas in the overburden. Use of 4C seismic technology has improved this. In addition, the 4D seismic response appears to be very sensitive to production-induced depletion, compaction of the reservoir and subsidence of the overburden and seafloor. Seismic techniques will be critical in future development. For a more detailed review of the first 10 years of Valhall, reference is made to the paper by Ali and Alcock [1]. Field History The Valhall field is an over-pressured, under-saturated Upper Cretaceous chalk reservoir located in the North Sea approximately 290 km offshore southern Norway in 69 m of water. The field is located in the most southwestern corner of the Norwegian continental shelf, Figure 1. The Valhall field was discovered in 1975, after exploration drilling in the period 1969 - 1974. Decision to develop the field was made in 1978 and the field started producing in October 1982. During appraisal drilling the first issues with soft chalk and solids production were experienced. Although early core tests indicated a high compaction potential, it was believed that the relatively thin chalk section would not result in significant subsidence. The platforms, however, were built tall enough to withstand some subsidence. The initial development consisted of a 3-platform complex (quarters platform, drilling platform and process/compression platform). Later, in 1996, a fourth platform was added (wellhead platform) to provide additional slots for infill drilling. In 2000, the Valhall partnership approved a water injection program, from an additional Injection platform, IP. With continued subsidence, drilling wells in the overburden became increasingly difficult. To provide efficient access to the flank areas in the North and South, installation of two new wellhead platforms was approved in 2001.