In the period May 1991 to April 1992 Ranger Oil (U.K.) Ltd., drilled five separate wells close to salt diapir structures in the Central Graben area of the North Sea. Two different salt diapirs were appraised, one in block 23/27 and the other straddling blocks 29/2a and 22/27a.
Without exception all of the wells exceeded their planned dry hole drilling time estimates. In addition to two planned geological sidetracks, there were three further unplanned mechanical sidetracks, all caused by unstable hole conditions. A breakdown of non-productive time associated with the wells is outlined.
This paper emphasises the prerequisite of good seismic and geological data in planning salt diapir wells. The use of high angle and horizontal drilling techniques are outlined for appraisal and development, as a means to satisfy both drilling and reservoir engineering requiements in these often marginal prospects.
The history of the salt diapir drilling in other parts of the world is also reviewed. It is concluded that drilling close to salt diapirs is usually complex and that wells must be designed and managed with a high degree of flexibility.