BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
Stabilization
The swelling activity is controlled by adjusting the electrolyte content of the fluid to minimize
expansion of the clays. Lime and gypsum-treated fluids have long been used for this purpose, but
basically do not satisfy the inhibition needs of the more troublesome shales. The potassium ion is
an effective swelling inhibitor and is used extensively for this purpose through the addition of
potassium salts. The concentration of potassium utilized ranges from several hundred mg/L to over
60,000 mg/L. The most cost-effective potassium concentration, however, appears to be in the 3000
to 7000 mg/L range. In marine environments, seawater or seawater with additional sodium
chloride, are good alternates to potassium. The tendency for the shale to disperse or subdivide into
discrete particles appears to be best controlled by the addition of an adsorptive, high molecular
weight water-soluble polymer.
One of the more effective polymers for this purpose is the partially hydrolyzed polyacrylamide
with molecular weight in excess of 15 million. These anionic polymers adsorb on to the surface of
the cuttings and borehole wall to act as a binder or elastic coating to retard dispersion.
Another approach to stabilizing the hard brittle shales that contain natural or induced microfractures
is the use of water-dispersible asphalt or asphaltines. The addition of these type materials
appear to plug the micro-fractures, or perhaps even surface pore throats to retard further fluid
invasion. This action has proven very effective in certain shales and can be applied in most waterbase
fluids.
Hole Cleaning
Cleaning cuttings from a vertical hole has been covered extensively by numerous investigators.
Hole cleaning in a vertical hole is essentially a matter of overcoming the slip velocity of the
cuttings with sufficient annular velocity and carrying capacity of the drilling fluid. Although any
velocity greater than the settling velocity of the largest cutting will theoretically lift all the cuttings
to surface eventually, too low an annular velocity will lead to an undesirable high concentration of
cuttings in the annulus. Experience has shown that cuttings concentrations more than 5% by
volume cause tight hole or stuck pipe when circulation is stopped.
Flow Characteristics
Theoretically, turbulent flow is more effective than laminar flow, not only because of the higher
annular velocities required to reach this flow regime, but it also provides a flatter velocity profile
that reduces edge-wise slip of the cuttings. On the other hand, turbulent flow may cause erosion
and hole enlargement. Although a competent formation may not erode even if the flow is turbulent,
others will erode because of weakening upon exposure to the drilling fluid, or by having been
fractured by tectonic movements. It may become impractical or impossible with the existing rig
equipment to maintain turbulent flow in any particular well, especially in washed-out or enlarged
sections. Also, certain fluid motors have limits which will not allow the fluid to be pumped fast
enough to reach turbulence. For this reason, it may be more desirable or necessary to increase the
viscosity of the fluid to improve carrying capacity.
Fluid Viscosity
The structural viscosity of the fluid can be increased to improve lift without greatly affecting rate
of penetration. This type of fluid is referred to as a shear thinning fluid and its behavior is
illustrated in Figure 11–5, which describes shear rate to the various flow regimes in the wellbore.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-11