With increasing focus on identifying cost effective solutions to well design with minimal impact on productivity, this paper will focus on an alternative to cesium formate as perforation fluid in the HPHT Gudrun field operated by Statoil. Cesium formate has been used with success for drilling and perforating many HPHT wells. However, given the significant cost of this fluid coupled with low oil prices, Statoil wanted to perform testing to assess the performance of an alternative low ECD oil based mud as a perforation fluid. The paper will describe the extensive qualification testing that has been performed. This includes coreflooding using representative plugs from Gudrun under downwhole temperature and pressure conditions. In addition, eight Section IV perforation tests have been performed to compare the performance of Cs formate and the low ECD oil based mud. These tests were undertaken using gas and oil saturated cores to reflect different production scenarios. The main aspects of the perforation operation that were reflected in the test design were as follows.
Perforating at reservoir pressure and laboratory testing temperature of approximately 100°C
Simulating an extended shut in period after perforation
Undertaking a clean up sequence using scaled down flowrates
Based on the results of the coreflooding combined with the section IV 19B testing, the low ECD OBM was selected as the perforating fluid for use on Gudrun. The perceived benefits of using the low ECD OBM were as follows.
Simplification: use of the same fluid for drilling and perforating the reservoir section.
Tangible cost savings in fluid cost and time savings of approximately 40M NOK ($5M).
Potentially increased productivity compared to cesium formate.
Improved standardization of the operational sequence.
Perforation modelling is described and comparison is made between this and the Section IV tests. Finally, the well start-up experiences and production data are presented demonstrating the effectiveness of the low ECD oil based mud as a perforation fluid.