Geomechanical Evaluations Coupled with Optimised Drilling Practices and Fluid Design Helped in Successful Drilling Through Coal in Offshore Peninsular Malaysia and Vietnam
Authors: Abdullah M. Baihaky B. , Ariffin M. Syafiq B. M. , Chatterjee Avirup , Ghosh Amitava , Bordoloi Sanjeev 
Published: March 2018

Two studies were showcased on the use of geomechanical evaluations with optimised drilling practices and fit for purpose fluid design to mitigate instability problems in coals which eventually helped to reduce drilling risks and costs in development wells in Offshore Peninsular Malaysia and Vietnam. The major risks anticipated for drilling the wells were borehole collapse due to pressure transmission across the mud-rock interface and loss circulation attributed to pressurized coal cleat and fracture system.

A full-scale geomechanical model was developed, validated and updated using logs and drilling data from the wells. The drilling experiences, rock mechanical properties, in-situ stresses and formation pressure are modelled. Recommendations on the drilling strategy through coal and the associated uncertainties were implemented. This included a road map with protocol for drilling through coal, tripping and back reaming during drilling, pre- and post-drilling. This was combined with tailor made drilling fluid design to overcome drilling challenges in coal. Utilization of geomechanical results and the approach adopted in the drilling strategies helped to determine recommended mud weight programs for the wells.

The insitu stress state indicates the area was found to be a normal stress regime (SV, SHMAX, Shmin). The geomechanical model was verified at the offset wells using the actual mud weight used to drill the offset wells and the drilling experiences.

It was observed that coal instability does not respond to same remediation used in shale, which is simply to raise the mud weight to reduce the compression hoop stress below the strength of the rock. Borehole collapse occurred even with higher mud weight due to invasion of drilling fluid across mud-rock interface with pressurization of the near-wellbore region and loss of effective mud weight support. Loss circulations were observed with pressurised cleats and fractures in coal.

An optimum mud weight window was estimated based on geomechanical evaluation. A quantitative risk assessment was performed to understand the uncertainties in the input parameters used in the geomechanical model and its implication on the mud weight. A mud weight sensitivity was carried out to understand the development of predicted breakout. The wells were drilled successfully with proper drilling practices and fit for purpose drilling fluid design to counteract mechanical borehole stability with the ability to seal the cleat system of the coals.

The integration of geomechanical modeling with effective drilling operation and fit for purpose drilling fluid design helped the well planning team to design the well in the planning phase, enabling a more proactive approach for flawless execution during the drilling phase. The well was successfully drilled with reduced NPT to the desired depth using an optimum mud weight while maintaining a balance between wellbore breakouts and loss circulation.