BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
These classifications obviously overlap, with the soft, sticky shales exhibiting some dispersion and
the dispersive shales showing a tendency to ball on the bit and bottom-hole assembly. In general,
however, these descriptions can be used to differentiate the behavioral characteristics of shale.
• The sticky composition, often referred to as gumbo shale, is characteristic of many young
marine shales and offers a challenge to the directional driller to gain and maintain angle if the
balling cannot be controlled.
• The dispersive shales manifest their behavior by being easily subdivided into their integral
parts, resulting in excessive fluid solids, high fluid viscosities, and borehole enlargement.
• The hard, brittle shales exhibit the classic behavior of spalling and sloughing with the
associated problems – hole cleaning, hole fill, hole wash-out, bridges, and pack off.
Extreme differences in behavior exist between the hard shale and the softer rock upon exposure to
a freshwater clay-base fluid. The soft shale balls into a plastic mass while the hard shale
disintegrates into slivers.
An interesting point to consider is that once the hole has become essentially horizontal in the
production zone, whether it be sandstone or carbonate rock, less borehole stability problems might
be expected. Since the horizontal section of the borehole is being contained in one particular body
of rock, there should be fewer discontinuities than going from one formation to another, and there
would be fewer zones of weakness, unless of course, the production zone is highly fractured or
faulted.
Drilling Fluid Selection
The proper selection and composition of the drilling fluid will depend upon the type or types of
shale to be drilled. Invert emulsion fluids (oil or synthetic external-phase emulsions) can be
prepared with the internal water phase containing sufficient electrolyte to show zero or even
negative water absorption. Preventing water absorption by the formation effectively controls
swelling and maintains the virgin cohesive strength between formation particles. Thus, oil fluids
with the proper salinity of the internal water phase are extremely effective in stabilizing
troublesome shale formations.
Unfortunately, oil-base fluids have the disadvantage of being prone to loss of circulation and more
difficult to regain circulation with than water-base systems. Nance reported that the density of the
oil fluid needs to be maintained 2 lb m /gal below the lowest calculated fracture pressure for the
exposed hole to minimize the probability of loss of circulation. Since the fluid density range
narrows with increasing hole angle, this requirement could become a limiting factor for oil fluids in
horizontal wells. The environmental aspect of oil fluids is also a deterrent to their use in some
areas.
Therefore, if the horizontal well is to be drilled in a troublesome shale area, there could be a need
to select a water-base fluid that provides the most stable environment possible. Unlike the
controlled activity oil-base fluids, water-base fluids expose the formation to water wetting
regardless of the electrolyte content of the fluid. To counteract this effect requires a fluid that not
only controls the swelling of the expandable clays in the shale, but also is capable of retarding
dispersion as discussed in shale classification above. Although these two phenomena are related,
they must be controlled by different mechanisms.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-10