Summary
In this paper we review progress made in developing United Kingdom Continental Shelf (UKCS) heavy oil fields. Reservoir productivity is compared with existing light oil developments and three categories of heavy oil reservoir are identified, which require the application of different well technologies to achieve acceptable offshore production rates. Case histories from existing developments and fields under appraisal are used to illustrate how advances in technology and effective risk management allow increasingly difficult heavy oil fields to be developed. Finally, the future direction for these heavy oil developments is discussed, looking at the scope for improved oil recovery (IOR) techniques and further technology developments to drive down costs and to increase reserves in fields currently under waterflood or to improve the economics of hitherto subeconomic fields.
Introduction
Early production from UKCS oil fields has been of light oil. However, a significant number of "heavy" (taken to refer to reservoirs with in-situ viscosities greater than 5 cp) oil fields have also been discovered. Most UKCS heavy oil is in relatively shallow reservoirs, comprising high porosity unconsolidated sands with excellent horizontal permeability (typically 3000 to 10 000 md) and very high vertical permeability (kV:kH) in the range of 0.2 to 1.0). The oil columns are usually at least partially underlain by water and some also have primary gas caps.
This combination of reservoir parameters and the demanding offshore environment of the UKCS presents a special set of reservoir engineering challenges because of the difficulties in achieving and maintaining sufficiently high production rates to justify development. In this paper we provide an overview of the development of heavy oil fields on the UKCS, past, present and future, with an emphasis on the subsurface issues. This shows how the application of new technology, principally horizontal wells, extended reach drilling (ERD) and improvements in sand control has led to successful developments. Increasing confidence in this technology has allowed the Captain field (reservoir viscosity 88 cp) to be brought onto production and encouraged appraisal activity on other fields with viscosities as high as 1000 cp.
It is conservatively estimated that there are around 10 billion STB of heavy oil in place on the UKCS. Less than a quarter of this resource is currently being developed. Assuming that recovery factors for the undeveloped stock tank oil initially in place (STOIIP) are likely to be in the range of 20 to 40% shows that there are approximately 1.5 to 3 billion barrels of additional reserves to be produced, which will make a significant contribution to the longevity of the UKCS.
Heavy Oil Resources in the UKCS
Many of the heavy oil accumulations discovered in the UKCS are in the northern North Sea, in the eastern margins of the East Shetland Platform. Other significant discoveries are in the Fladen Ground Spur, the Halibut Horst, and west of the Central Graben. Heavy oils have also been discovered in the Atlantic margin area. Fig. 1 shows the structural elements in the central and northern North Sea and the location of heavy oil fields under production or active appraisal.
The majority of the discoveries are in Lower Tertiary sands and Fig. 2 shows the conceptual lithostratigraphy of the important reservoirs. The principal heavy oil reservoirs are in the Upper Palaeocene Maureen formation, the Heimdal sands in the Lista formation (e.g., Mariner), and the Dornach and Hermod sands in the Sele formation (e.g., Bressay), the Balder and Frigg sands (e.g., Gryphon and Harding) and the mid-Eocene Nauchlan sand (Alba). The Captain field, which was discovered in 1977, is in the Lower Cretaceous Captain sand, and has the lowest API oil and highest in-situ oil viscosity of any currently producing UKCS field.