Mud Weight as Explained by Dummies

lithostatic and hydrostatic pressure

Some of the freelancers we run across are highly skilled (to use LinkedIn's terminology) are the old CRP... copy-reword-paste. They are, unfortunately, hard to spot if one isn't well-versed in the topic they've chosen or been assigned; perhaps because they've honed their cribbing skills over many a post. Still, failure to actually understand what they're writing about can be both a clue to their ignorance and a problem for their readers. Take, as an example, the Paul Scott post "What Is Mud Weight?" at WiseGEEK niche site AboutMechanics.com.

We don't deny that Scott deftly reworded someone's definition of mud weight, albeit with a rather bizarre semi-synonym for "drilling":

"Mud weight is a quantitative term used to express the density of drilling fluids used in the sinking [sic] of wells..."

And yes, Paul managed to go into several hundred words of detail about the general purpose of drilling mud...

"Drilling fluids are used to cool drill bits, to remove drill cuttings from the wellbore, and to prevent casing [sic] collapse..."

...except that mud isn't used to prevent casing collapse; casing is used to prevent wellbore collapse. Paul even blathered through a couple of paragraphs about how mud weight is measured. The closest he came to the importance of mud weight (before veering off into how it's measured) was these two factoids:

"[Muds] also... control hydrostatic pressure within the well...  Incorrect mud weight values may cause several serious problems, such as circulation losses."

To which our staff petroleum geologist says, "So close, and yet so far!" What Paul glossed over here is the entire point of measuring and varying mud weight: the pressure resulting from the weight of the column of mud in the wellbore must be sufficient to offset the natural lithostatic pressure encountered downhole and the pressure of the fluids in a reservoir at depth. The fluids – water, oil natural gas – deep underground are under pressure because of all the rock above them, pressure that a well can alleviate and allow them to escape to the surface. The pressure exerted by the column of mud in the wellbore must be sufficient to keep fluids trapped at depth, otherwise a "serious problem" may occur.

That "serious problem"? Worst-case scenario: think Deepwater Horizon... a blowout.

No mention of fluid pressure or blowouts occurs in the post, which proves to us that Scott simply didn't understand the significance of mud weight. Instead of 120 words on mud scales and units of measure, a sentence or two about downhole pressure might have saved Paul from receiving a Dumbass of the Day award. But no...

SI - OIL