BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
• A complex containing two polyol layers is formed when potassium is absent. This complex
is less stable in water.
• Studies on other polyols conclude an additional contribution comes from interactions
between polyol molecules at the clay surface.
Polyol fluids are effective in most shale types, particularly in young or relatively non-compacted
shales with high clay contents. Compared with, for example, KCl/polymer fluid, polyol systems
give improved wellbore conditions and produce firmer cuttings that do not readily disperse into
the fluid. These attributes frequently combine to give faster drilling rates, and reduced fluid
volumes that translate into reduced drilling costs.
Above the Cloud Point
Above the cloud point, glycols are present as emulsions while the separate phases are continually
intermixed by the circulating system, particularly at the bit. These emulsions block pores in the
formation, preventing fluid invasion and consequent instability in water-sensitive formations.
The AQUA-DRILL system can be engineered so drill cuttings at the bottom of the hole are
initially above the system’s cloud point. A protective glycol layer forms around the cuttings as a
result of glycol “clouding out” on its surface. This prevents the cuttings from reacting with water
until they have risen to a point in the wellbore where the fluid temperature drops below the cloud
point, allowing the glycol to re-dissolve into the fluid. This explains both the increased inhibition
present when using the AQUA-DRILL system in reactive shales and the low glycol depletion
rates seen in the field.
Below the Cloud Point
Glycols still provide enhanced performance below the cloud point. Field evidence has shown that
glycols below the cloud point deliver improved shale stability. Also, laboratory studies have
shown that shale inhibition with glycols is more effective when water-soluble, rather than waterinsoluble,
glycols are employed. One explanation is that the glycols adsorb onto shale surfaces
via oxygen molecules present in the glycol chain. Once the water-soluble glycols enter the
formation, the higher formation temperature causes the glycol/water solution to phase separate insitu,
forming an emulsion. The hydrophobic glycol droplets in the emulsion fill and block the
shale pores, preventing further fluid invasion and stabilizing the shale.
It can be concluded that shale inhibition and formation protection is achieved by:
• the polyol displacing water from adsorption sites on clay minerals present in shales, and
• blocking the formation pores from further ingress of invasive fluids by “clouding out.”
Formation Damage
Several operators have now carried out studies to evaluate the effect of polyols in drilling/coring
fluids on return permeability. These studies have shown no adverse effects on the formation
samples tested. Many suggest that the polyol actually protects the productive rock from
impairment by the drilling fluid provided it is above the cloud point.
As discussed earlier, the theory is that the polyol emulsion protects the formation from excessive
fluid invasion by pore plugging just inside the rock matrix as the material “clouds out” within the
hotter environment, thus sealing the pores against further ingress. At this point,
surfactant/polymer interactions and complexation of surfactant with monovalent ions (such as
potassium in solution) will occur more frequently. These reactions provide optimum benefit for
long term borehole protection by acting as a blocking layer which compliments polymer
additions. An additional benefit may be the inhibition of interstitial clays.
Reference Manual
Baker Hughes Drilling Fluids
3-76 Revised 2006