Qualification and Implementation of Oil-Based Mud as a Cost-Effective and Low-Damaging Perforation Fluid for High-Pressure/High-Temperature Fields: Gudrun Case History

Abstract

Summary With an increasing focus on identifying cost-effective solutions to well design with a minimal impact on productivity, this paper will focus on an alternative to cesium (Cs) formate as the perforation fluid in the high-pressure/high-temperature (HP/HT) Gudrun Field operated by Equinor. Cs formate has been used with success for drilling and perforating many HP/HT wells. However, because of the significant cost of this fluid coupled with low oil prices, Equinor wanted to perform testing to assess the performance of an alternative oil-based mud (OBM) as a perforation fluid. In this paper we describe the extensive qualification testing that we have conducted, which includes coreflooding using representative plugs from Gudrun Field under downhole temperature and pressure conditions. In addition, eight API RP19B (2014) Section IV perforation tests have been conducted to compare the performance of the Cs formate with the OBM. 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 a reservoir pressure and a laboratory-testing temperature of approximately 100°C Simulating an extended shut-in period after perforation Undertaking a cleanup sequence using scaled-down flow rates On the basis of the results of the coreflooding combined with the API RP19B (2014) Section IV testing, the OBM was selected as the perforating fluid for use on Gudrun Field. The perceived benefits of using the OBM were as follows: Simplification, by using the same fluid for drilling and perforating the reservoir section Tangible time savings and cost savings in the fluid cost of approximately 40 million NOK (USD 5 million) Potentially increased productivity compared with the Cs formate Improved standardization of the operational sequence Perforation modeling is described, and a comparison is made between this and the API RP19B (2014) Section IV tests. Finally, the well-startup experiences and the production data are presented, demonstrating the effectiveness of the OBM as a perforation fluid.