Page 248 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 224

[ ] Well Programming
2.5.5

has to be treated out to protect the PHPA. This would not be necessary
if PAC were used instead of PHPA.
Inhibitive KCl-Glycol mud. This system is based upon the potas-
sium chloride polymer mud, but with the addition of a polyglycol.
Certain glycols can impart many of the properties of diesel and miner-
al oils but without the toxicity. Most glycols, unlike diesel or mineral
oils, are miscible in water at normal temperatures. There are several
theories as to why glycol inhibits shale hydration; with a TAME mud
(thermally activated mud emulsion) the glycol solution is designed so
that glycol precipitates out of the filtrate when it heats up after enter-
ing the shale, which blocks further filtrate entry. This requires the mud
to exhibit a cloud point behavior, whereby at a temperature between
the bottom hole circulating temperature and the (higher) formation
temperature, the glycol starts to de-emulsify. Glycol systems are also
formulated without cloud point behavior.
Glycol coats steel with a hydrophobic film that repels water. This
helps to prevent bit and BHA balling and gives good lubricity, reduc-
ing drags and torques. If balling is suspected, a drum or two of glycol
+ surfactant pumped downhole may remove the balling. Glycol can
also be used in noninhibited water-based systems for lubrication and
prevention of balling. This makes the use of PDC bits more effective
in WBM, as any balling of PDCs causes rapid heat buildup that leads
to cutter damage.
Glycol improves filter cake quality, reduces filtrate loss, and
reduces cake thickness while improving filter cake lubricity, therefore
reducing differential sticking tendencies. There is also some evidence
to show that glycol improves tolerance to drilled solids, reducing the
need for dilution.
Silicate muds—general. Refer to the discussion on wellbore sta-
bility in Section 2.9.1 for background information on reaction of shales
with drilling fluids.
Silicate muds provide pore pressure isolation (see Section 2.5) and
prevent hydration by blocking off the pore throats of the shales while
drilling. Silicates are also effective at encapsulating cuttings (assisting
removal at the surface and preventing solids content buildup) and pre-
venting dispersion (of limestones as well as shales, due to the high
pH). Silicate muds are currently one of the most effective water-based
fluids for drilling shales, limestones, and sands.
Groups of silicate molecules are small enough to penetrate into the
tiny pores in the shale surface. In the mud, filtrate pH is kept high—

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