Abstract
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.
