Local rock mechanical knowledge improves drilling performance in fractured formations at the Heidrun field

Abstract

Despite the use of inhibitive water-based mud (KCl), high pump rates to obtain good hole cleaning and relatively high mudweight, problems with cavings, pack-offs and lost circulation persisted when drilling Tertiary shale formations in the Heidrun field offshore Mid-Norway. A study was therefore initiated, including extensive data collection (logging and coring of the shale) and subsequent core testing, data analysis and evaluation of field experience. The study showed that the major problem zone was not the low-density zone as anticipated. However, the core revealed a fractured and crushed zone, which was eventually found to be the main source of the problem. The study has resulted in a new strategy. An improved borehole stability model has been established, and the mud and operational strategies have been revised. The mudweight has been reduced, the salt content of the mud has been reduced and good hole cleaning is obtained through a low-viscosity mud, which is kept close to turbulent flow. Furthermore, if possible, wells are now planned without steering in these formations. If steering cannot be omitted, 3-D rotary steerable systems are utilized. An economic evaluation of the study was also performed, showing a substantial benefit/cost ratio. So far, the cost reduction for an average well is close to 20 MNOK (ca. 2.5 million USD). With more than 50 wells left to drill in the field, the potential for cost-saving is large. Additionally, there is a huge and non-quantified effect from accelerated production.