Barite "sag" is known to reduce drilling performance. This experimental study, which used full scale coring bits on large sandstone blocks, shows that the recrushed barite and cuttings can be smeared on the borehole wall to create a low permeability coating. In at least one mud type this coating may be difficult to remove. Drilling fluids are often tailored to block anticipated pore throat distributions with components that can be removed during clean up. In these experiments the grains lining the wellbore were shattered by cutter impacts to a depth of up to 2 mm. This changed the pore throat size distributions in this thin zone, and crushed grain debris was a major component blocking pore throats in this zone. If these factors apply in the subsurface, then drilling fluid may not behave as planned during spurt loss, and successfull clean up may prove more difficult than anticipated. Depth of filtrate invasion is known to be controlled in part by formation permeability, by degree of static mud weight overpressure and by drilling fluid perfomance. In this study, the duration of dynamic interactions between bit, rock and mud is an additional control on filtrate invasion. This suggests that petrophysical analyses of log responses should not ignore ROP.

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