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Abstract

The BP Operated Rhum Field is a North Sea High Pressure High Temperature lean gas field. A key design requirement for successful development well planning and execution was to evaluate Overbalance and Dynamic Underbalance perforating techniques for their potential to optimise productivity while reducing the risk of producing hydrocarbons to surface during downhole operations.

In this paper we present a case history focussing on the evaluation of the available perforation techniques with the aim of matching or exceeding the flow performance achieved on the Appraisal Well DST. The selection of drill pipe conveyed dynamic underbalance perforation, using the Schlumberger PURE technique provided the best compromise in terms of minimising HSE exposure and maximising productivity, with the main potential downside being the potential for large cost over-runs from utilising Caesium Formate brine. The re-completion of a Rhum appraisal well provided a rare opportunity to obtain a direct comparison of well productivity from standard Underbalanced and Dynamic Underbalanced perforating operations in the same reservoir interval.

Operational issues forced the Rhum team to utilise a third perforation technique, on-balance perforation via electric line, and the paper will present the productivity achieved using this technique also.

The paper progresses sequentially through the following items;

• An overview of the Rhum Field and Reservoir;
  
• Design and Planning of the perforation techniques utilised on Rhum Development wells to overcome challenges such as Hard Rock Perforating and Sand Control requirements;
  
• The detailed design and core flood testing of two different types of Caesium Formate Kill Pills, aimed at identifying the least-damaging formulation.

Operational data is presented to demonstrate the success of dynamic underbalance in maintaining a full fluid column, effective gas suppression in clear brine and tight fluid loss control. Field production and pressure data showing the filter cake clean-up over time is examined to evaluate the actual performance of the chosen kill pill after extended exposure at downhole temperature.

The paper concludes with a comparison of the flow performance achieved on a single well in an identical reservoir interval when shot with ; a) standard underbalance during the original appraisal DST operation and b) dynamic underbalance for the development completion operation (original perforations cemented off). These results are compared with the flow performance of a second well shot on-balance using electric line.