Page 321 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 297

2.9.1
Drilling Problems--Avoidance Planning [ ]

When a good mud cake is formed, the mud pressure does not affect
the pore pressure distribution. This is most advantageous from a bore-
hole stability point of view. A good mud cake is usually formed if the
permeability exceeds 1 md. If formation permeability is lower than 1
md, mud cakes do not form. They would be ineffective anyway since
mud cake permeability is approximately 1 md. Field indications are that
borehole instability is much less prolific in formations exhibiting rea-
sonable permeability. Gauge holes have been drilled in weakly consoli-
dated, highly permeable sandstones. Most drilling problems are
observed in shales. One of the contributing factors is that shales have
very low permeability, typically between 10 -12 to 10 darcy and usually
-6
between 10 -10 to 10 darcy. This makes mud cakes formation ineffective.
-8
A further mechanism for the abundant drilling problems in shales
is the water adsorption potential of its clay components. The intake of
water causes shales to swell, weaken, and fracture leading to hole fail-
ure. The sensitivity to water is large for smectite (montmorillonite),
medium for illite, and small for kaolonite and chlorite. This is associ-
ated with the clay surface area that controls the amount and effects of
water adsorption.
Fractured limestones and coal seams can also cause serious prob-
lems. Any fractured rock can be further destabilized by the drilling
practices employed.

In-situ stresses.
Collapse. The loads acting on the borehole region consist of the far
field in-situ stresses, the wellbore mud pressure, and the formation
pore pressure. The onset and severity of borehole collapse is deter-
mined by the magnitude of the in-situ effective stresses (i.e., total
stress, pore pressure) and the mud overbalance relative to rock
strength. Borehole collapse increases as the effective stress increases.
Fracturing. Borehole fracturing becomes less likely as the mini-
mum in-situ stress increases. Borehole fracturing increases as the total
minimum in-situ stress decreases and/or mud weight exceeds the
breakdown pressures for intact rock, or mud weight exceeds the frac-
ture extension pressures for fractured rocks.

Drilling practices.
Drilling practices can have a dramatic effect on wellbore stability.
Consider the following points. Ensure that precautions are designed

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