Page 320 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 296

[ ] Well Programming
2.9.1

on the formation face (e.g., invert oil emulsion mud) then the
salinity of the mud-water phase can be adjusted to change the net
osmotic force. Briefly, if the mud-water phase salinity is different
from the pore fluid salinity, then water will tend to move to the area
of higher salinity. If the pore fluid is less saline, then a force to
move water out of the pores is present. Refer to “Properties of the
brine phase—osmosis” in Section 2.5.8.
Electrical potential—research indicates that this has some effect
but practical recommendations have yet to be made. The forces
involved are probably very small compared to the first two.

Rock properties. The strength of rock material is described by
strength parameters such as the unconfined compressive strength
and/or triaxial compressive strength. However, strength is not the only
rock property controlling stability.
First, strong intact rock may behave weak when fractured. The
rock strength is determined by the (weak) fractures rather than the
(strong) intact material. They can be natural or drilling induced (e.g.,
drillstring vibrations). Second, borehole stability also depends on the
ductility (the degree to which rock can plastically deform without los-
ing load bearing capacity). Plastic deformation results in delay of fail-
ure due to transfer of excess load to rock located away from the bore-
hole wall. A ductile rock remains intact under more severe loading
than a brittle material of similar strength. The mode of borehole insta-
bility is related to the ductility of the formation. A brittle rock can lead
to hole enlargement. The borehole wall material desegregates and
detaches, i.e., sloughs into the hole as soon as its strength is exceeded.
A ductile formation, on the other hand, may experience substantial
plastic deformation leading to load distribution to the adjacent intact
rock. Hole enlargement is observed in ductile formations as well due to
erosion of weakened or damaged material. The strength and ductility
of formation material depends on lithology.
The most important lithological parameters are mineral composi-
tion and porosity. Typically, high porosity material has a low strength
and some ductility, whereas low porosity rock has high strength and
low ductility. Given similar porosity, shales behave more ductile than
sandstone. The strength and ductility of the shales is also influenced by
the water content. In general, the lower the water content the higher
the strength and vice versa.

296