Perforation Optimisation of a Layered Reservoir
Authors: Costaschuk B. 
Published: December 2007

B. Costaschuk SPE, Agip (U.K.) Limited

Abstract
A new coiled tubing perforating deployment system which allows long gun lengths to be run and retrieved in a live well was used to optimise perforating operations and performance in an oil well which encountered two reservoir layers which were differentially pressure depleted. This perforating technology was used to perforate intervals of 583 ft and 138 ft separately in underbalanced conditions and successfully resulted in optimum perforating performance and maximum well productivity. This perforating technology has application where long intervals are required to perforated underbalanced and guns cannot be released because of limited rat hole or retrieved conventionally because of limited lubricator length.

Introduction
The success of well perforating depends on several criteria, including: the programme design, the operational performance and achieving maximum productivity. In the case of the Tiffany platform oil well 16/17-A5z, the perforation programme was designed based on the need to satisfy reservoir objectives, completion considerations and drilling concerns. The Tiffany reservoir part of the Upper Jurassic Brae formation in Block 16/17 in the UK sector of the South Viking Graben and can be characterised by two main reservoir layers which are subject to differential pressure depletion. The drilling of the 6" hole in well A5z, encountered the top reservoir unit with a pressure 1400 psi less than the bottom less depleted reservoir unit. This reservoir pressure profile presented significant concerns for drilling risks and for reservoir objectives in achieving optimum perforations and maximum productivity. Field data indicated that formation skin damage was sensitive to the degree of mud overbalance pressure and that underbalanced perforating and immediate clean up of a zone were critical factors for minimising this damage and maximising well productivity.
Perforating options for well A5z included drilling extra rat hole for perforating underbalanced and releasing the TCP guns, however, this option presented risks for differential sticking and formation skin damage as a result of required mud overbalance pressures. Running a dedicated TCP gun string to perforate underbalanced was an option, however there was potential of productivity damage from killing the well. Perforating conventionally using through tubing wireline guns was an option which did not require additional drilling, however, because the intervals within each layer were long and of uniform pressure, there were concerns that significant and effective underbalance conditions could not be achieved once part of each interval was perforated.
A new coiled tubing deployment perforating system which allows guns to be run and retrieved in a live well regardless of lubricator length, was another option which did not require additional drilling. This option provided several advantages for the A5z well since each entire unit could be perforated with sufficient underbalance in separate stages and effectively cleaned up. This system successfully resulted in optimum perforating performance and maximum well productivity compared to other drilling/perforating options.

Discussion
The Tiffany field is the northernmost of the four oil fields - Tiffany, Toni, Thelma and S.E. Thelma discovered in Block 16/17. The T Block fields are in the UK sector of the South Viking Graben and are operated by Agip (U.K.) Limited. The Tiffany field was discovered in 1979 and six appraisal wells were drilled between 1979 and 1989. Development drilling to date has included five producers and two water injection wells.
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