In the Dutch sector of the North Sea the Ommelanden Chalk formation commonly contains large amounts of chert, a very hard nodular rock. High friction exacerbates the harsh drilling conditions leading to mechanical dysfunction of the drilling process: severe downhole shocks, large torsional vibrations, bit and bottom hole assembly (BHA) wear and loss of directional steerability. Acceptance of these drilling conditions have long prevented drilling teams in the Netherlands to make significant drilling performance improvements.
This case study shows a step-by-step optimization approach, where we evaluated various drilling systems and their interdependence at each step. Separately optimizing the individual drilling sub-systems: bit, BHA, fluids and the rig led to little overall improvements. The initial focus was on the bit and drive system, but their successful utilization was limited by the harsh drilling environment. Close examination and various attempts proved that a combination of non-aqueous based mud (NABM), lubricant and a rotary drive control system could deliver a stable drilling environment. This allowed us to select different bits and drive systems. After multiple attempts with new technology PDC bits we proved drilling the entire chalk interval in one run was possible. In addition, rotary steerable systems (RSS) enabled directional steering in any part of the 12 1/4-in section.
The improvements led to fewer bit runs. The average progress rate through this section has been increased from 100 m/day to 300 m/day, with estimated savings of 10 to 15 rig days per well. In addition, the downside risk of equipment damage, inability to maintain trajectory control and related cost has been reduced significantly.
Pursuing drilling optimization can take considerable time and effort. It is often not a single-step process, but requires a multidisciplinary approach. This case study demonstrates important process improvements and technology applications, which reduced operating risk and cost.