In the Danish Central Graben, the Lower Cretaceous reservoir contains significant in-place oil volumes. This formation historically has posed a real challenge to develop because of the extremely low permeability. The low permeability is compounded by the fact that the formation is soft, highly heterogeneous, and can have significant clay content in some areas. Although the Lower Cretaceous chalks represent a wide spread play in the central North Sea, there are no producing analogs.
Thicker Lower Cretaceous reservoir sections in the Valdemar field have been successfully proppant fracture stimulated. Acid stimulation is preferred for the thinner sections; however, early attempts at matrix acid stimulation were very disappointing. Recently, the need to re-enter a well for maintenance allowed three zones to be selectively restimulated using improved execution techniques and a more appropriate acid formulation. Production increase following restimulation of this first well was very encouraging and the lessons learned were applied to a second well; this time a newly drilled well completed in the Tyra Lower Cretaceous reservoir.
This paper summarizes the lessons learned, starting with a review of the previous acid stimulations. Wormhole testing with Lower Cretaceous core material provided important insight toward the selection of the most appropriate acid formulation. Computer modeling was used to determine the most effective perforation patterns, based upon variable formation characteristics along the long, horizontal wellbore. Treatment execution and monitoring included diagnostic injection tests and real-time calculations of acid coverage during the limited-entry perforated completions.
The results of these two wells has been very encouraging and has opened up the possibility for continued testing of acid stimulation techniques in both the Valdemar and Tyra Lower Cretaceous reservoirs.