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Borehole instability and stuck pipe while drilling coal can lead to significant non-productive time. This paper describes a drilling fluid developed to stabilize coals in a program that was previously unable to achieve all extended reach objectives. The fluid design principles are believed to apply broadly in stabilizing coals, fractured shale, and other cleated formations. The rock mechanics that govern instability in coals is identical to shale. However, coal instability often does not respond to the same remediation used in shale, which is to simply raise the mud weight to reduce the compression hoop stress to below the strength of the rock. Despite using an optimum mud weight, hole breakout or borehole collapse may still occur when the coal cleats and natural fractures of the coal allow the drilling fluid filtrate to invade. This leads to a pressurization of the near- wellbore region and loss of the effectiveness of mud weight support for coal stability. A fluid was developed based on the belief that the wellbore is stabilized with increased mud weight if the additional pressure acts specifically on the face of the borehole. This stabilization effect can be achieved by preventing pressure penetration into the near-wellbore region through coal cleats or natural fractures. The fluid design developed included bridging particles with a size distribution based on analysis of the coal cleat apertures, as well as filtration control material to effectively reduce the permeability of the bridge. Sealing alone does not provide stability and an analysis of the coal strength and in-situ stresses were conducted to select a mud weight that would stabilize the very weak coals. The paper discusses the rock mechanics concepts, fluid design criteria for determining the allowed leakage rate when designing the bridging process, and the operational learnings from implementation. The use of the coal stabilization fluid and stability mud weight allowed the objectives to be achieved and contributed to record performance in this a narrow-margin drilling environment in Australia.
Borehole instability and stuck pipe while drilling coal can lead to significant non-productive time. This paper describes a drilling fluid developed to stabilize coals in a program that was previously unable to achieve all extended reach objectives. The fluid design principles are believed to apply broadly in stabilizing coals, fractured shale, and other cleated formations. The rock mechanics that govern instability in coals is identical to shale. However, coal instability often does not respond to the same remediation used in shale, which is to simply raise the mud weight to reduce the compression hoop stress to below the strength of the rock. Despite using an optimum mud weight, hole breakout or borehole collapse may still occur when the coal cleats and natural fractures of the coal allow the drilling fluid filtrate to invade. This leads to a pressurization of the near- wellbore region and loss of the effectiveness of mud weight support for coal stability. A fluid was developed based on the belief that the wellbore is stabilized with increased mud weight if the additional pressure acts specifically on the face of the borehole. This stabilization effect can be achieved by preventing pressure penetration into the near-wellbore region through coal cleats or natural fractures. The fluid design developed included bridging particles with a size distribution based on analysis of the coal cleat apertures, as well as filtration control material to effectively reduce the permeability of the bridge. Sealing alone does not provide stability and an analysis of the coal strength and in-situ stresses were conducted to select a mud weight that would stabilize the very weak coals. The paper discusses the rock mechanics concepts, fluid design criteria for determining the allowed leakage rate when designing the bridging process, and the operational learnings from implementation. The use of the coal stabilization fluid and stability mud weight allowed the objectives to be achieved and contributed to record performance in this a narrow-margin drilling environment in Australia.
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.
Summary The rapidly increasing global oil/gas demand and gradual depletion of shallow reservoirs require the development of deep oil/gas reservoirs and geothermal reservoirs. However, deep drilling suffers from drilling-fluid failures under ultrahigh temperature, which cause serious accidents such as wellbore collapse, stuck pipe, and even blowouts. In this study, we revealed the role of polymeric additives in improving the ultrahigh-temperature tolerance of bentonite-based drilling fluids, aiming to provide practical and efficient solutions to the failure of drilling fluids in severe conditions. By adding poly(sodium 4-styrenesulfonate) (PSS) to the original drilling fluid containing bentonite, significant fluid loss—as a consequence of bentonite-particle flocculation causing drilling-fluid shear-stress reduction and high-permeability mud—is successfully suppressed even at temperature as high as 200°C. This drilling fluid containing PSS was applied in the drilling of high-temperature deep wells in Xinjiang province, China, and exhibited high effectiveness in controlling accidents including overflow and leakage. NOTE: A supplementary file is available in the Supporting Information section.
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