Page 329 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 305

2.9.2
Drilling Problems--Avoidance Planning [ ]

Cement blocks. Where a large pocket exists under a casing shoe,
cement in the pocket after cementing the casing may fracture, as
drilling progresses, and fall into the hole. Do not program in a pocket
any larger than necessary. Consider adding fibers (such as Dowell
D094) to the tail slurry.
Soft cement. It is possible to run into cement that is not complete-
ly set, then find it impossible to pull back out or to circulate. Monitor
surface cement samples (preferably kept at bottom hole circulating
temperature) to ensure they are hard before approaching bottom and
run in the last couple of singles slowly and with the pumps on.
Stuck pipe: differential. The following four conditions are consid-
ered necessary for stuck pipe to occur.
Permeable zone covered with wall cake. Where such a zone is iden-
tified, the wall cake characteristics can be optimized with a good mud
program. Additives can be used to make the cake thinner and less
sticky or oil mud will form very little cake.
The desirability of low fluid loss is generally related to reducing
contamination of formation fluids, as discussed at length under
“Formation damage due to the liquid fraction” in Section 2.5.6,
“Formation Damage with Water-Based Muds (and Cements.)”
However, low solid content/high fluid loss muds tend to give increased
rate of penetration in permeable formations. The fluid loss also affects
wellbore clay hydration. Therefore, the desirable level of fluid loss is
often related to whether or not damage to the permeable formations
needs to be minimized.
Thick filter cakes are undesirable. Differential sticking becomes
much more likely with a thick cake. Cement-formation bonding
requires removal of the cake, which may be difficult. There are even
cases of thick cake causing mechanical sticking by reducing hole diam-
eter above the BHA. For reduced differential sticking, the API HPHT
fluid loss in oil mud should be kept below 5 cc, and in water mud 5-8
cc is desirable. Generally, higher fluid loss increases sticking. Filtrate
from oil muds should be 100% oil; the presence of water is a sign of
reduced emulsion stability.
When drilling the final (payzone) hole section it may be possible
to change to low solids, nondamaging drilling in fluid in a well-known
development area if the formations allow. This will be especially useful
if drilling in a depleted or low-pressure zone.

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