Innovative Riserless Drilling Technique to Mitigate Low Risk Shallow Hazard in Ultra-Deepwater Well West Africa
Authors: Toha Rozaidi , Zainal Abidin Abdullah , Aziz M Azam , Ahmad Shafei M Syazwan , M Sofian Anas 
Published: October 2022

Drilling in ultra-deepwater well poses prominent challenge especially in the top section, normally associated with shallow hazard risks. Among common risks are shallow gas and shallow water flow (SWF) which are identified by shallow seismic geohazard assessment. The best approach in dealing with shallow hazard is to shift the well location to a lesser risk area. However, there are cases where this is unavoidable. The assessment for this well highlighted two low probability intervals of shallow water flow at depths 2617 - 2752m Below Sea Level (BSL) and 2752 - 2934m BSL due to presence of mass transport deposits, with water depth 2105m and subsequent section total depth (TD) at 3305m BSL. This paper highlights Operator's engineering design and experience in drilling the SWF intervals using the innovative riserless drilling technique.

The conventional approach requires preparing huge volume of water-based mud (WBM) using Dynamic Kill Drilling (DKD) method, estimated around 19,000bbls of 16ppg density. This causes logistical challenge with limitation of mixing and storage at the onshore liquid mud plant, coupled with longer vessels turnaround. The innovative technique is by drilling the riserless section (24 in) with seawater only and achieving high rate of penetration (ROP) to generate sufficient equivalent circulating density (ECD) to suppress any potential SWF. Subsequently, this permits the delay of DKD activation, ensuring TD is achieved and enabling seamless transition to dual-derrick operations. A comprehensive decision tree is specifically developed top-down to section TD addressing multiple challenges such as SWF, borehole instability, pack-off and potential stuck pipe event.

The riserless section was completed in 3.36 days, from drill ahead 24in hole until landing high-pressure wellhead housing (HPWHH) as compared to 8.66 days conventionally. Additionally, only 4,000bbls of WBM was consumed. Faster net ROP was achieved compared to offset wells by 100% with higher ECD generated than the pre-drill roadmap. 24in hole section drilling commenced with seawater and achieving ROP more than 80 meter per hour (m/hr) for an interval around 1000m until section TD. The section was displaced with WBM mud for the last 180m by diluting from 16ppg to 9.7ppg. Average slip-to-slip connection time was also reduced from 6.9 minutes to 4.5 minutes to further minimize static time. Other parameters monitored were standpipe pressure, torque, and drag for any indication of wellbore pack-off.

This paper contributes to novelty in the ultra-deepwater region of West Africa for engineering design and execution of mitigating low risk shallow water flow via unconventional riserless drilling technique. This would reduce the volume of WBM required, essentially simplify the logistic challenge for riserless ultra-deepwater operations, enable the riserless section to be completed swiftly, highly beneficial for drillship operations with high spread rate.