Clair appraisal: the benefits of a co-operative approach

The Clair discovery lies 75 km west of the Shetlands, and comprises an elongate ridge of Lewisian basement and an associated roll-over of variable quality Devonian–Carboniferous continental red beds. Clair was discovered in 1977, when 206/8-1A tested 25° API oil at 1500 BOPD from the red beds at the crest of the roll-over. Well 206/7-1 followed, producing 960 BOPD from fractured basement on the ridge. Ten further wells drilled between 1977 and 1985 indicated an oil in place measurable in billions of barrels, but test results were disappointing. The success of the discovery well was never repeated and commercial test production rates were never achieved. In 1990, the companies with licence interests over Clair agreed to pool their resources and expertise and collaborate in a joint appraisal programme. A series of reservoir studies quantified the importance of different fracture types to productivity and predicted that the right combination of fractures and matrix could yield much higher flow rates than had previously been achieved. A 3D seismic survey over the central area of the discovery led to a significant improvement in understanding and visualization of the structure. Two joint appraisal wells were drilled in 1991. The first, a horizontal well in fractured basement, tested at 2100 BOPD after acid wash stimulation. The second well tested the red beds on the flank of the roll-over and achieved sustained flow rates of over 3000 BOPD from two zones (one of which had been hydraulically fractured) with a significant contribution from previously unidentified open natural fractures. The orientation of these open fractures suggests they are related to a late Cretaceous episode of faulting. Two further appraisal wells and an extension to the 3D seismic coverage are planned for 1992. There are still many uncertainties associated with Clair, but the 1991 wells have finally demonstrated that significant and potentially commercial flow rates are achievable. The behavioural change associated with the joint approach has been the catalyst for the success of this new initiative on Clair.