The Dunbar Field, lying in the East Shetland Basin in Block 3/14a (U.K.C.S.), contains a significant proportion of its volatile oil in thick (90 m), low permeability sands (1-2 mD) of the Brent Group/Upper Massive Sands. To achieve secondary recovery from these ‘tight’ formations, high productivity horizontal or multi-lateral drains had to be designed as withdrawal points. The drilling of such wells in a highly compartmentalized and complex reservoir was not without significant risk, particularly in relation to the known levels of lateral and vertical heterogeneity.

An early development well, D05, originally planned as a water injector, provided an opportunity to evaluate the benefits of such horizontal drains within the ‘tight’ zones. This vertical well also contained thin, high permeability (100 mD) layers below the thick low permeability ‘Upper Massive Sands’. A lateral drain (D05Z) was planned within the Upper Massive Sands whilst the original vertical hole would be retained. The two sections would be completed in order to produce from both the permeable and ‘tight’ zones.

This paper presents the full process of the evaluation of the pre-drill vertical well and the reservoir studies that followed. These studies led to the recommendation to drill a lateral drain of very complex geometry while retaining the production from the existing vertical section. The 960 m lateral drain that was subsequently drilled complied with very strict recommendations and guidelines and constituted a major drilling achievement. The production performance of the lateral drain was also a success and increased Total and Elf's confidence in the use of such complex wells (horizontal, dual drains, multi-lateral drains). In fact, they are now the chosen option for the development of the Dunbar tight zones.

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