BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
Lignosulfonate additions are often unnecessary. However, UNI-CAL may be used in NEW-
DRILL systems when excess solids cannot be mechanically removed or diluted. Avoid excessive
thinning with lignosulfonate to insure effective hole cleaning and to prevent mechanical erosion
of the wellbore.
Maintain pH values from 9.0 (freshwater) to 10.5 (seawater) when using lignosulfonate. This
increases the solubility of UNI-CAL while limiting destabilization of shales from the hydroxide
ion.
Deepwater Drilling Fluid Systems
Drilling off the outer continental shelf has become more common in an effort to find significant
hydrocarbon deposits. Water depths range from 1,000 to 8,000+ ft in deepwater wells.
A special consideration related to deepwater exploration is gas hydrates. Gas hydrates are icelike
crystalline solids formed by the physical reaction of gas and water under pressure. They can
form in aqueous systems at temperatures well above the freezing point of water if the pressure is
sufficiently high. Low temperatures, when coupled with hydrostatic pressure or pressures
encountered during well control operations, create an environment conducive to the formation of
gas hydrates. NF2 ® and NF3 are gas hydrate suppressors specifically designed for use in
water-base drilling fluids.
In addition to gas hydrates, other potential problems must be considered when designing a
drilling fluid system for deepwater. Hole cleaning becomes critical in the surface casing due to
the necessity for a large inside diameter (I.D.) riser. A fluid with a high effective viscosity in the
riser, yet shear thinning for good hydraulics, is desirable. Boosting the riser with a third fluid
pump is a common practice to aid in hole cleaning.
Deepwater projects utilize large circulating volumes. The drilling fluid system of choice should
be easily mixed and maintained. It is advantageous to minimize the number of additives due to
logistics and storage space
A growing trend is the use of riserless drilling techniques, using the Dynamic Kill Drilling (DKD)
process with water-based mud. The DKD process uses large volumes of weighted water-based
mud blended “on the fly” to achieve a specific density while dumping the returns to the seabed.
This technique is used to reduce the overall hydrostatic pressure exerted by the mud column on
deepwater formations. The fracture pressure integrity of deepwater formations is low compared
to onshore and shelf formations. This is compounded by the depths of the water column in
deepwater (> 1000 feet). The riser is eliminated because the hydrostatic pressure exerted by the
mud column (within the riser), combined with the great depths (> 1000 feet), leads to hydrostatic
pressures greater than those of the fracture gradient of the formation, which in turn can lead to
lost circulation. Since the density of seawater is less than that of drilling fluid, the total
hydrostatic pressure “felt” by the formation is reduced because the hydrostatic pressure exerted is
a combination of the density of seawater, together with a component from the drilling fluid in the
annulus.
PYRO-DRILL®, High-Temperature Drilling Fluid
Introduction
The PYRO-DRILL ® system is a very flexible drilling fluid system that is used when temperature,
contaminants, and/or borehole instability make conventional systems impractical or
uneconomical. Components of the system have been used in geothermal wells with bottom hole
temperatures (BHT) in excess of 600°F. This high temperature system has been formulated in
Reference Manual
Baker Hughes Drilling Fluids
3-62 Revised 2006