BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
BOREHOLE PROBLEMS
Chapter 7
Wellbore problems are the result of many circumstances, not the least of which is
misinformation resulting from inadequate planning. Drilling fluids are not the sole cause of
wellbore problems; they are however, the first line of defense to correct them.
HOLE STABILITY PROBLEMS
Introduction
The maintenance of wellbore stability is one of the most critical considerations in any drilling
operation. As a minimum, an unstable wellbore will reduce drilling performance and in the
worst case could result in the loss of the hole through borehole collapse.
Wellbore instability can occur as a result of mechanical effects, chemical effects, or a
combination of both. In simple terms, mechanical effects are usually related to drilling fluid
density (drilling fluid density too high or too low) or drilling practices (rate of penetration,
inadequate hole cleaning, vibration effects, torque and drag and frequency of trips), whereas
chemical effects are drilling fluid type related (inappropriate drilling fluid type or inhibition
level for the formation being drilled). The following sections give more detail on this, and
provide a guide to minimize wellbore instability in the planning, implementation and drilling
phases.
Common Misconceptions
Before detailing aspects of hole stability problems some popular misconceptions must be
dispelled.
• Many in the industry believe that well control purposes alone dictate the required mud
weight. They also assume that hole collapse is simply a result of drilling with
insufficient drilling fluid density. Thus, the perception is that drilling with a nominal
overbalance for well control will also ensure hole stability. The logic behind these
beliefs is flawed. Weak formations may need overbalance in excess of 1000 psi to
prevent hole collapse, whereas some mudstones can be drilled problem free in
“underbalanced” conditions.
• It is common to assume that increasing drilling fluid density is always the answer to hole
instability problems. This is not always correct. Increasing drilling fluid density can
amplify problems. For instance, more rapid failure could occur in fractured rocks,
whereas in some porous formations the resulting higher filtration rates and thicker
filter cakes could promote differential sticking.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-1