Provide Borehole Stability

•Borehole instability is most often identified by a sloughing formation, which causes tight hole conditions, bridges and fill on trips.

•This means the well must be reamed and cleaned and in extreme cases re-drilled •Borehole stability is greatest when the hole maintains its original size and cylindrical shape. •Once the hole is eroded or enlarged in any way, it becomes weaker and more difficult to stabilize •Hole enlargement leads many problems–low annular velocity–poor hole cleaning–increased solids loading–fill–increased treating costs–poor formation evaluation–higher cementing costs and inadequate cement bonding •Hole enlargement through sand and sandstone formations –mechanical actions: –erosion most often being caused by hydraulic forces and excessive bit nozzle velocities –Need to reduce impact force and nozzle velocity –Weaker sands require a slight overbalenceand good quality filter cake containing bentonite •Hole enlargement through shale –Water based muds can penetrate shale making it swell and soften over time and slough in –Higher mud weights and chemical/polymer inhibitors can reduce or eliminate slough •Highly fractured shales are very unstable –Usually require mechanical methods to clean, they require higher mud weights to control or oil based muds •Extremely water sensitive shales require an oil based or synthetic based fluid to drill successfully –These fluids provide better shale inhibition than water based fluids –Clays and shales do not hydrate or swell in the presence of oil –Osmotic forces created by the emulsified brine phase prevent adsorption of water by the shales