American Oil and Gas Reporter - October 2015 - 82

SpecialReport: Drilling Fluids Technology
FIGURE 4
Wellbore Pressure Differential and Stability Issues

Major Kick
or Blowout

Oriented
Shear Failure
Wellbore Collapse

May produce
angular caving visible
in surface returns

Nonoriented
Splintering

Stable
Wellbore

Hole
Ballooning

Hydraulic
Fracturing

Well "breathes"
alternately producing
and taking fluids

Major fluid losses

Pore pressure "pops"
chips into wellbore

Nonaqueous Systems
All nonaqueous drilling fluids (NADFs) are based on some
type of liquid, which from a chemistry standpoint, is an oil (diesel,
mineral oil or synthetic). Environmentally acceptable, nonaqueous drilling fluids are the fluid of choice for deepwater drilling
operations around the world. The International Association of Oil
& Gas Producers (IOGP) classifies NADFs into three categories:
* Group 1 (diesel and conventional mineral oils);
* Group 2 (low-toxicity mineral oils); and
* Group 3 (synthetics and highly refined paraffins).
These three groups were developed based on the relative marine organism toxicity and the biodegradation of the fluids.
Whether a particular base fluid can be used is subject to local regulatory agencies, but according to IOGP statistics, 55 percent of
offshore operations in international waters use Group 3, 26 percent use Group 2, and 19 percent use Group 1 NADFs.
Diesel and conventional mineral oil are banned in U.S. federal waters and are tightly regulated elsewhere around the world
since they contain aromatic compounds that can be toxic to marine organisms. For the most part, they are relegated to onshore
work where hazardous waste sites are available or where regulations permit land farming for cuttings disposal.
Synthetic NADFs, and in some cases low-toxicity mineral oil,
are the fluid of choice for most offshore drilling. In deepwater
operations, drilling fluids are constructed exclusively with a synthetic-base fluid, typically an olefin with a carbon chain length
of 14-18. Another type of synthetic is esterified vegetable oil, primarily from palm oil. Synthetics are preferred because they can
eliminate wellbore stability problems relating to shale water-wetting, thereby minimizing NPT.
Wellbore Stability
There are many mechanisms involved in wellbore stability,
including unconsolidated formations, tectonic stresses, abnormal
pore pressures, and swelling clays in shales. In any case, the first
line of defense is to have the proper mud weight (Figure 4). The
vast majority of formation footage drilled is through some type
of shale containing various types of clays. Some clays are very
active (swell) and some are not. It is well established that NADFs
can eliminate wellbore instability caused by clay swelling. The
primary mechanism for this is the osmotic control of water flow
from the mud to the formation and vice versa. The correct mud
weight must also be in place.
One way an NADF enhances wellbore stability is by making
the wellbore completely oil-wet. A water-wet wellbore allows wa-

82 THE AMERICAN OIL & GAS REPORTER

ter invasion as a result of capillary action (Figure 5A). Calcium
chloride (CaCl2) is the most common additive for osmotic flow
control (Figure 5B) in NADFs. CaCl2 imparts a very low activity in the water phase, thereby producing the largest osmotic pressure driving force.
As noted, chlorides may cause additional waste management
costs, depending on local disposal rules. An ongoing line of research is to find materials with low water activities that can replace the chlorides. Additives that have been tried include formates and acetates, calcium nitrate, and many types of polyols.
Because of high rig day rates in deepwater drilling operations,
the practice is to use a synthetic oil to allow the cuttings to be
dropped overboard. NADFs can be up to five or six times the cost
per barrel of a water-based system, but their ability to improve
wellbore stability makes them cost-effective by eliminating NPT.
In applications where day rates are lower, high-performance
WBMs can be much more cost effective than NADFs.
NADF Mechanism
Although the newer generations of high-performance WBMs
have improved well stabilization compared with WBMs, they can
only slow the imbibition of water. By delaying clay swelling, however, these fluids can buy enough time so that casing can be set
to protect the wellbore. High-performance WBMs need a number of additives to minimize water imbibition into shales. In addition to having the proper mud weight, additives reduce the
FIGURE 5A
Water Invasion in Water-Wet Wellbore
Oil/synthetic-based mud

Wellbore
pressure

Water-based mud

Wellbore
pressure

Water

Pore pressure

Water

Pore pressure

FIGURE 5B
NADF Osmotic Flow Control using Calcium Chloride
Low-salinity water

High-salinity water

Water migration

Formation (low-salinity water)

Semipermeable membrane

Oil mud (high-salinity water phase)

Water migration

Emulsified
water
Semipermeable membrane

Monomolecular emulsifier film

American Oil and Gas Reporter - October 2015

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