An Integrated Geomechanical Approach for Successful Drilling through Coal in Peninsular Malaysia and Offshore Vietnam

Abstract

Past drilling of vertical and moderately deviated development wells in offshore Peninsular Malaysia and Vietnam has proven to be challenging. Drilling experience in the development wells highlighted the issue of wellbore instability in the respective areas, particularly though coal seams. Numerous lost-time incidents related to wellbore instability-related problems were experienced, ranging from tight hole (remedied by reaming) to overpull, pack-off followed by stuck pipe, fill on-bottom to difficulties in running casing, and coal cavings to high gas associated with drilling breaks. These problems were observed particularly when drilling through the weak shales with interbedded unstable coals. Coal instability often does not respond to the same remediation used in shale, where we usually simply raise the mud weight to reduce the compressional hoop stress below the strength of the rock. Borehole collapse or breakout may still occur when the coal cleats and natural fractures allow the drilling fluid filtrate to invade despite using an optimum mud weight. This results in pressurization of the near-wellbore region and loss of effective mud weight support for coal stability. All the wells in the respective areas were drilled with WBM and so drilling performance benchmarking with other drilling fluids was not possible in the study area. Faced with continual NPT, a geomechanical study was initiated to mitigate the coal related wellbore instability problems. The recommendations arising from the comprehensive geomechanical and drilling experience analyses have been implemented to improve performance during subsequent development drilling. This paper highlights the importance of integrating geomechanics with proper drilling practices when developing strategies to mitigate unstable hole problems, especially related to coals. A full-scale geomechanical model was developed, validated and updated using logs and drilling data from wells in both the areas. The drilling experiences, rock mechanical properties, in-situ stresses and formation pressure in both the areas are presented and discussed in detail. A detailed recommendation on the drilling strategy through coal and the associated uncertainties was implemented. Utilization of geomechanical results from the two studies and the approach adopted in the development of drilling strategies helped to determine recommended optimal mud weight programs for the future wells. Subsequent drilling campaigns have all been successful by incorporation of the entire risk and mitigation plan. This included a generalized road map with a protocol for drilling through coal, tripping and back reaming during drilling, pre-drilling and post-drilling. This effort, together with optimized drilling fluid design and the correct mud weight based on previous drilling experience as per recommended wellbore stability assessments, helped in two recently drilled wells in different areas with reduced NPT.