I am sorry to be a pain on this matter.This is purely for my...

I am sorry to be a pain on this matter.
This is purely for my general knowledge or better for my general confusion....
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The sidetrack well (Mukuyu-1 ST-1) will be drilled to a slight updip location to the north of
Mukuyu-1, with a lower mud weight and solids content mud system (approximately 1,500psi
reduction in overbalance), which will improve the logging conditions and chances of
successfully obtaining a fluid sample.
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....I fail to understand the reference of (approximately 1,500psireduction in overbalance)
having something to do with a lower mud weight and solids content mud system.
Is it not the density of mud something different from the pressure or the mud system?