American Oil and Gas Reporter - October 2015 - 85

SpecialReport: Drilling Fluids Technology
FIGURE 6
Clay Fixation using K+ Cation

K+

NH+4

Mg+2
Na+

Ca+2

Close-up of void space
between clay platelets showing
relative sizes of available cations

shale's permeability by plugging the tiny pore openings in shale
formations. Additives also may be needed to plug any natural fractures in steeply dipped formations.
A generic list of some of the additives used includes products
for osmotic control, clay fixation, accretion control, membrane/film
formation, and pore/fracture plugging. Osmotic control is not as
critical in WBMs as in NADFs, but some water activity control
is needed, and it usually is accomplished by using either organic or inorganic brines.
Clay fixation usually is done by the cation associated with the
brine. The most effective cation is the potassium ion (K+). The
hydrated size of the K+ ion seems to be the best size to fit into
the pore openings of most shales (Figure 6).
Accretion control additives are surfactants similar to the materials used to control bit balling. They are normally co-polymers
of amid or amine compounds with fatty acids. These materials
physically attach to the clays, blocking free water from the clays.
NADFs work so well because they form a semipermeable
membrane on the wellbore, allowing for the osmotic transfer of
water. The fluids industry has not yet been able to construct this
type of membrane or film formation in a WBM. Instead, WBMs
form a "leaky" membrane, which is still important because it reduces the shale's pore openings and minimizes the capillary action that occurs when water contacts a shale (Figure 7).
Most drilling fluid polymers are film formers to some extent,
but partially-hydrolyzed-polyacrylamide is the most common. Another set of chemicals used for film forming are polyols and multihydroxy alcohols, which are very successful and work by hydrogen bonding with the shales.
Plugging materials plug openings in the shales to slow water penetration. Some of the materials used are asphalt products, gilsonites,
lignites, graphite, and fibrous cellulose products. A relatively new
line of research is focusing on using nanoparticles.
Lost Circulation
A joint industry project initiated in the 1990s targeting lost circulation and wellbore ballooning has resulted in a much greater
understanding of those problems and how to use drilling fluids
to strengthen the borehole. Researchers learned there is a difference between WBMs and NADFs in the pressures needed to prop-

agate a fracture once it is initiated. Wellbore ballooning describes
the effect of creating a series of microfractures in the wellbore
that expand and close in response to ECD (Figure 8A).
The solution is to add bridging agents to the mud system. These
small particulates enter a microfracture as it opens, acting to prop
it open and plug the microfracture. Then, when pumping stops
and ECD is removed, the additives act similar to the keystone of
an arch, imparting increased hoop stresses around the wellbore.
Figure 8B shows this so-called stress cage theory of wellbore
strengthening.
The materials used for wellbore strengthening include deformable graphite, small-sized calcium carbonate, small nut shells,
micronized cellulose, and fibers. The materials must be small
enough to pass through shale shaker screens since they are carried continuously in the drilling fluid.

The Future of Drilling Fluids
Going forward, drilling fluids research and development likely will focus on several key areas, including:
* High-performance WBMs;
* Solids-free WBMs;
* High-temperature/high-pressure drilling applications;
* Mud testing automation;
* Optimized drilling fluids engineering; and
* Competency assurance.
Work certainly will continue on developing high-performance
water-based systems capable of eliminating the use of NADFs.
From an operational standpoint, there also will be more use of
high-density brines as the base fluids to WBM construction. This
likely means increased use of organic brines, such as sodium and
potassium formates.
In regard to HP/HT drilling, predictions indicate that pressures
to 30,000 psi and temperatures to 600 degrees Fahrenheit will be
encountered soon, particularly in subsalt plays such as the LowFIGURE 7
'Leaky' Membrane Formed with WBM

Mud

1-Polymer macromolecule; 2-Forming micelle; 3-Forming ultralow permeability film

OCTOBER 2015 85

American Oil and Gas Reporter - October 2015

Table of Contents for the Digital Edition of American Oil and Gas Reporter - October 2015