World First: Drilling with Casing and Riserless Mud Recovery

This paper discusses the planning and operational aspects of drilling with casing (DWC) combined with riserless mud recovery (RMR), along with the benefits experienced compared to conventional drilling. The process of identifying and mitigating risk when applying new technologies to ensure a first time success will also be discussed. The Dampier Sub-Basin offshore Australia presents major challenges to tophole operations when drilling through the problematic ‘Bare Sands.’ The formation is composed of highly porous sand in which lost circulation is often encountered. Severe losses prevent sufficient hole cleaning, leading to packoffs around the bottomhole assembly (BHA) and stuck pipe. The formation is also interbedded with soft sand and hard cemented layers, causing severe torsional vibration and twistoffs in the BHA. Well designs and operations had been altered, incorporating larger hole sizes and control drilling to reduce packoff risks. Multiple bit runs were also optimised for each formation. This method reduced the occurrence of junked wells at the cost of individual well performance. DWC promised to enhance performance by drilling the interval in a single run and reducing the number of trips. However, as applications of new technology encompass risk and uncertainty, the operator performed extensive analysis to quantify the likelihood and cost of failure modes and the expected reward. A key finding of the analysis was a necessity to mitigate stuck pipe. RMR offered a cost-effective method of enhancing hole cleaning to avoid this hazard. The operation was performed from a semi-submersible rig, reaching a world record depth in technical limit time without hole condition problems. The methods discussed ensured the successful application of multiple new technologies, achieving a world record and technical limit performance. DWC combined with RMR is a world-first, and offers an effective solution for tophole drilling in the basin, and for similar basins worldwide.