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Oil-Base Systems
نویسنده : رضا سپهوند - ساعت ۳:۱۳ ‎ب.ظ روز ۱۳٩٤/۸/۱۳
 
Oil-Base Systems
Oil-Base Systems 12.1 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
This chapter covers the specifics of the
VERSA oil-base systems. They are nonaqueous
systems as described in general
in the Non-Aqueous Emulsions
chapter. The oil in these systems functions
as a continuous external phase of
a water-in-oil-emulsion (invert) mud.
The VERSA systems are named according
to the base oil used and according
to special application (function). The
primary systems are:
Other VERSA names are used from
time to time for new or unique base
oils. Regardless of the base oil, these
systems can often use the same additives
and similar formulations. While
most VERSA systems use products from
the VERSA product line, some may
use additives from the NOVA system
product line, depending on the base
oil and environmental monitoring
and regulations.
Two special application systems are
VERSAPORTE and VERSACORE.E Each of
these systems can be formulated with
any base oil. VERSAPORT systems have
elevated Low-Shear-Rate Viscosity
(LSRV) and are formulated for highangle
and horizontal drilling. VERSACORE
systems are all-oil coring fluids designed
to produce minimal changes to the core.
VERSA systems differ from NOVA systems
in the kinds of base liquids used.
VERSA systems’ base oils either originate
from or are difficult to distinguish from
oils refined from crude oil. NOVA systems
base liquids are synthetic materials
and are easily distinguishable
from those oils refined from crude oil.
Regardless of the system name, there
are two general categories that can be
applied to all VERSA systems:
1. Conventional. Conventional VERSA
systems normally use VERSAMULT
emulsifier and VERSACOATT wetting
agent in the formulation, have low
filtration rates, and utilize lime to
form calcium-base soaps. These are
“tight” and very stable emulsions
that have zero API (100 psi) fluid
loss. They usually have high electrical
stability and a controlled High-
Temperature, High-Pressure (HTHP)
fluid loss of less than 10 cm3 at
500 psi and 300°F, with no water
in the filtrate.
2. Relaxed-filtrate. Relaxed-filtrate VERSA
systems normally use VERSACOAT as
the emulsifier and VERSAWETT as the
wetting agent in the formulation,
have high filtration rates, and rely
on “surfactant” chemistry to form
the emulsion (do not require lime
to form calcium soaps). These are
slightly less stable emulsions, purposefully
run with higher HTHP
filtrates than conventional invert
emulsion muds. It is normal for them
to have some water in the HTHP filtrate.
They may also have measurable
API (100 psi) filtrate. The emulsions
are loose and the electrical stability
will be lower than that of conventional
invert emulsion muds. Relaxedfiltrate
systems normally do not
contain fluid-loss-control additives.
Relaxed-filtrate systems are designed
for cost effectiveness and to increase
penetration rates. NOTE: A relaxedfiltrate
system can easily be converted
to a conventional system, but a conventional
system cannot be converted
to a relaxed system.
Introduction
System Name Base Oil
VERSADRILT Diesel
VERSACLEANT Mineral oil
VERSAVERTT Ultra low-tox mineral oil
VERSA systems
differ from
NOVA systems
in the kinds
of base
liquids used.
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.2 Revision No: A-0 / Revision Date: 03·31·98
The VERSA systems are invert-emulsion,
oil-base muds that can be formulated
and engineered to meet a wide range
of applications and requirements. The
following system descriptions and formulations
are presented as a basis and
guide for the wide range of fluids that
are possible with the VERSA oil-base
product line.
CONVENTIONAL VERSA SYSTEMS
Conventional VERSA systems are
tightly emulsified, temperature-stable,
invert-emulsion, oil-base drilling fluids.
Conventional systems can be formulated
for any oil mud application.
(Due to the large number of base oils
available, some areas use special system
names. Occasionally the base oil
will be named using the system name
followed by a “B” suffix, such as
VERSAVERT B.)
VERSAMUL is the primary emulsifier for
conventional VERSA systems. VERSAMUL
must react with lime to form a calcium
soap to act as an emulsifier. The system
must be kept alkaline at all times to
function properly. One pound of lime
should be added to the system for each
pound of VERSAMUL added. Additional
lime should be added as required to
maintain 3 lb/bbl of excess lime in the
system. (A similar product, VERSAVERT P,
is used in the North Sea and other areas.
The “P” suffix indicates that it is the
“primary” emulsifier.)
VERSACOAT is the primary wetting
agent for conventional VERSA systems
and provides secondary emulsification.
(A similar product, VERSAVERT S, is
used in formulations for the North Sea
and other areas instead of VERSACOAT.
The “S” suffix indicates that it is the
“secondary” emulsifier.)
VG-69T organophilic clay is used to
viscosify the fluid to support weight
material and provide gel strengths. A
number of other organophilic clays are
available, including VG-PLUS,E VG-HT,E
VERSAVERT VIS and others depending
on the formulation and requirements.
If additional viscosity is required,
VERSAMODE or VERSA-HRPT can be used.
Calcium chloride brine is normally
used as the internal phase of the invert
emulsion. The amount of brine, or
Oil:Water Ratio (OWR), will affect properties
and formulation. Any concentration
of calcium chloride up to 38% by
weight can be used.
The VERSA systems usually have a sufficiently
low fluid loss with the basic
formulation. However, if ultra-low fluid
loss is required, VERSATROLT I is the
preferred filtration control additive.
VERSALIGT can be used if asphalt and
gilsonite are not allowed. The VERSAVERT
systems use the filtration-control additive
VERSAVERT F, a resin copolymer. Pilot
testing should be performed to determine
the exact amount of VERSATROL I,
VERSALIG or VERSAVERT F to be used in a
particular formulation.
When mixing a conventional system,
the following order of addition
is recommended:
1. Oil.
2. Organophilic clay (VG-69).
3. VERSA-HRP or VERSAMOD.
4. Lime.
5. VERSAMUL.
6. VERSACOAT (allow to mix for
20 min).
7. CaCl2 brine (add slowly).
8. Weight material.
9. VERSATROL I (allow to mix for 30 to
60 min).
Systems
VERSA systems
are invertemulsion,
oil-base muds
that can be
formulated…
VERSAMUL is
the primary
emulsifier for
conventional
VERSAsystems.
Oil-Base Systems
Oil-Base Systems 12.3 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 1: Conventional system — barite formulations.
Oil:Water Ratio ® 70:30 80:20 90:10
VERSAMUL (lb/bbl) 6 - 8 8 - 10 8 - 10
VERSACOAT (lb/bbl) 1 - 2 1 - 2 2 - 3
Lime (lb/bbl) 6 - 8 8 - 10 8 - 10
VG-69 (lb/bbl) 2 - 4 2 - 3 1 - 1.5
VERSATROL I (lb/bbl) (if required) 4 - 6 6 - 8 8 - 10
Oil:Water Mud Weight Oil Water CaCl2 M-I BART
Ratio (lb/gal) (bbl) (bbl) (lb/bbl) (lb/bbl)
8.5 0.625 0.265 32.6 22.7
70:30
9 0.613 0.259 31.9 49.7
10 0.588 0.249 30.6 103.8
11 0.563 0.235 29.3 157.8
12 0.609 0.150 18.5 226.3
80:20
13 0.581 0.143 17.7 279.6
14 0.552 0.136 16.8 332.9
15 0.524 0.129 15.9 386.2
16 0.555 0.061 7.5 451.9
90:10 17 0.523 0.057 7.1 504.4
18 0.491 0.054 6.6 557.1
VERSA-HRP/VG-69 CALCULATION
Calculate the reduced VG-69 by multiplying
the amount of VG-69 listed in the table above
by 0.80:
VG-69 = VG-69 (lb/bbl) x 0.80
Calculate the amount of VERSA-HRP by multiplying
the amount of VG-69 listed in the
table above by 0.40:
VERSA-HRP = VG-69 (lb/bbl) x 0.40
Example:
Oil:water ratio 70:30
VG-69 (lb/bbl) 4 (from table above)
New VG-69 (lb/bbl) = 4 x 0.80 = 3.2
VERSA-HRP (lb/bbl) = 4 x 0.40 = 1.6
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CONVENTIONAL VERSA SYSTEM FORMULATION CHARTS
(25% BY WT CACL2 BRINE: 96% SALT PURITY)
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 2: Conventional system — FER-OX formulations.
Oil:Water Mud Weight Oil Water CaCl2 FER-OXT
Ratio (lb/gal) (bbl) (bbl) (lb/bbl) (lb/bbl)
8.5 0.627 0.265 32.7 21.1
9 0.617 0.261 32.1 46.8
70:30
10 0.597 0.252 31.1 98.3
11 0.578 0.244 30.1 149.7
12 0.558 0.236 29.1 201.2
13 0.538 0.228 28.0 252.6
14 0.588 0.145 17.9 316.6
80:20 15 0.566 0.140 17.2 367.5
16 0.544 0.134 16.5 418.4
90:10
17 0.584 0.064 7.9 482.3
18 0.558 0.061 7.5 532.7
(NOTE: VERSA-HRP or VERSAMOD may be needed for additional viscosity and gels depending on the base
oil used. VERSA-HRP is particularly applicable prior to transporting muds to the rig. Use the calculation
listed below to determine the amount of VERSA-HRP and to adjust the amount of VG-69.)
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.4 Revision No: A-0 / Revision Date: 03·31·98
VERSAVERT SYSTEM
The VERSAVERT system and products are
widely used in Norway and the North
Sea. As mentioned above, the basic
products are VERSAVERT B (base oil),
VERSAVERT P (primary emulsifier) and
VERSAVERT S (secondary emulsifier). Other
products used in this system include
VERSAVERT F (fluid loss), VERSAVERT VIS
(organophilic clay) and VERSAVERT M
(rheology modifier).
Other products may be used in these
systems, including products named
with a “C” suffix (indicating “conditioner”),
an oil-wetting agent to help
prevent and correct water-wet solids,
and products named with a “T” suffix
(indicating “thinner”), an oil-base mud
dispersant similar to VERSATHIN.T
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 3: VERSAVERT system — barite formulation.
Additive Material Function Concentration
VERSAVERT B Ultra-low-tox mineral oil Continuous phase 65 - 95% (vol)
Freshwater Freshwater Water phase 5 - 35% (vol)
CaCl2 Calcium chloride Salinity and 100 - 250 kg/m3
water-phase activity
VERSAVERT P Tall oil polyamide Primary emulsifier 10 - 25 kg/m3
VERSAVERT S Polyamide Secondary emulsifier 6 - 20 kg/m3
and fluid loss
VERSAVERT F Resin copolymer Fluid loss 0 - 20 kg/m3
VERSAVERT VIS Organo - hectorite Viscosifier 20 - 30 kg/m3
VERSAVERT M Dimer fatty acid Rheology modifier 0 - 10 kg/m3
Lime Calcium hydroxide Alkalinity control 0 - 25 kg/m3
M-I BAR Barite Density control As required
Oil-Base Systems
Oil-Base Systems 12.5 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
RELAXED-FILTRATE VERSA SYSTEMS
Relaxed VERSA systems are less tightly
emulsified fluids resulting in higher
fluid loss that helps to maximize penetration
rates. These economical systems
combine the inhibitive properties inherent
in oil-base drilling fluids with high
penetration rates.
Relaxed-filtrate VERSA systems use
VERSACOAT as the primary emulsifier,
VERSAWET as the wetting agent and
VG-69 and VERSA-HRP as viscosifiers
and gelling agents. Calcium chloride
(CaCl2) brine at 25% by weight normally
comprises the internal phase,
but any desired percent by weight
up to 38%, may be used. Fluid-loss
additives are generally not used in
relaxed systems.
When mixing a relaxed system,
the following order of addition is
recommended:
1. Oil.
2. Organophilic clay (VG-69).
3. VERSA-HRP or VERSAMOD.
4. VERSACOAT.
5. VERSAWET.
6. Lime (allow to mix 20 min).
7. CaCl2 brine (add slowly).
8. Weight material (allow to mix for
30 to 60 min).
RELAXED VERSA SYSTEM BARITE FORMULATION CHART
(25% BY WT CACL2 BRINE: 96% SALT PURITY)
Oil:Water Ratio ® 75:25 80:20 85:15 90:10
VERSACOAT (lb/bbl) 2 - 3 3 - 4 4 - 5 5 - 6
VERSAWET (lb/bbl) 1 - 2 2 - 3 2 - 3 3 - 4
VG-69 (lb/bbl) 8 - 10 6 - 8 6 - 8 4 - 6
Lime (lb/bbl) 1 - 2 1 - 2 1 - 2 1 - 2
Oil:Water Mud Weight Oil Water CaCl2 M-I BAR
Ratio (lb/gal) (bbl) (bbl) (lb/bbl) (lb/bbl)
75:25
8.5 0.703 0.231 28.5 21.5
9 0.690 0.227 27.9 49.0
10 0.703 0.173 21.4 111.7
80:20
11 0.675 0.166 20.5 166.1
12 0.646 0.159 19.6 220.5
13 0.617 0.152 18.8 274.9
85:15
14 0.622 0.108 13.3 335.8
15 0.591 0.103 12.7 389.9
16 0.590 0.065 8.0 449.8
90:10 17 0.558 0.061 7.5 503.5
18 0.526 0.058 7.1 557.3
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 4: Relaxed systems — barite formulations.
VERSA-HRP/VG-69 CALCULATION
Calculate the reduced VG-69 by multiplying
the amount of VG-69 listed in the table above
by 0.80:
VG-69 = VG-69 (lb/bbl) x 0.80
Calculate the amount of VERSA-HRP by multiplying
the amount of VG-69 listed in the
table above by 0.40:
VERSA-HRP = VG-69 (lb/bbl) x 0.40
Example:
Oil:water ratio 75:25
VG-69 (lb/bbl) 10 (from table above)
New VG-69 (lb/bbl) = 10 x 0.80 = 8
VERSA-HRP (lb/bbl) = 10 x 0.40 = 4
(NOTE: VERSA-HRP or VERSAMOD may be needed for additional viscosity and gels depending on the base
oil used. VERSA-HRP is particularly applicable prior to transporting muds to the rig. Use the calculation
listed below to determine the amount of VERSA-HRP and to adjust the amount of VG-69.)
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Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.6 Revision No: A-0 / Revision Date: 03·31·98
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 5: Relaxed systems — FER-OX formulations.
Oil:Water Ratio ® 70:30 80:20
VERSACOAT (lb/bbl) 3 - 8 2 - 6
VERSAWET (lb/bbl) 1 - 2 2 - 3
VG-69 (lb/bbl) 2 - 10* 2 - 8*
Lime (lb/bbl) 2 - 4 2 - 4
Oil:Water Mud Weight Oil Water CaCl2 VG-69 FER-OX
Ratio (lb/gal) (bbl) (bbl) (lb/bbl) (lb/bbl) (lb/bbl)
8.5 0.653 0.276 34.0 8 - 10 12.6
9 0.643 0.272 33.5 6 - 8 39.0
70:30
10 0.624 0.264 32.5 6 - 8 91.9
11 0.605 0.255 31.5 4 - 6 144.9
12 0.585 0.247 30.5 4 - 6 197.8
13 0.566 0.239 29.5 2 - 4 250.7
14 0.621 0.153 18.9 6 - 8 313.0
15 0.599 0.148 18.2 6 - 8 364.9
80:20 16 0.577 0.142 17.5 4 - 6 416.8
17 0.555 0.137 16.8 4 - 6 468.7
18 0.532 0.131 16.2 2 - 4 520.6
*See specific mud weights below.
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RELAXED VERSA SYSTEM FER-OX FORMULATION CHART
(25% BY WT CACL2 BRINE: 96% SALT PURITY)
(NOTE: VERSA-HRP or VERSAMOD may be needed for additional viscosity and gels depending on the base
oil used. VERSA-HRP is particularly applicable prior to transporting muds to the rig. Use the calculation
listed below to determine the amount of VERSA-HRP and to adjust the amount of VG-69.)
VERSA-HRP/VG-69 CALCULATION
Calculate the reduced VG-69 by multiplying
the amount of VG-69 listed in the table above
by 0.80:
VG-69 = VG-69 (lb/bbl) x 0.80
Calculate the amount of VERSA-HRP by multiplying
the amount of VG-69 listed in the
table above by 0.40:
VERSA-HRP = VG-69 (lb/bbl) x 0.40
Example:
Oil:water ratio 70:30
VG-69 (lb/bbl) 8 (from table above)
New VG-69 (lb/bbl) = 8 x 0.80 = 6.4
VERSA-HRP (lb/bbl) = 8 x 0.40 = 3.2
Oil-Base Systems
Oil-Base Systems 12.7 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
VERSAPORT SYSTEMS
VERSAPORT systems utilize elevated
low-shear-rate viscosities to provide
improved hole cleaning in high-angle
wells. A VERSAPORT system consists of
any VERSA system treated with an LSRV
rheology modifier. A VERSAPORT system
can be either a conventional or relaxedfiltrate
system. Either VERSAMOD or
VERSA-HRP can be used to modify the
LSRV of the conventional VERSA systems.
However, only VERSA-HRP is recommended
to modify the LSRV of
relaxed-filtrate VERSA systems. NOTE:
VERSAMOD is not used in relaxed systems
because it requires a high lime content to
be effective, and these systems do not
normally use a high lime content.
VERSAMOD is an organic gelling
agent that increases the LSRV and
gel strengths with minimal effect on
high-shear-rate viscosities. Increased
water content (lower OWR) improves
the performance of VERSAMOD, and the
concentration needed to achieve the
desired effect is lower. It also requires
the addition of a pound of lime for
each pound of VERSAMOD used to
achieve the desired effect. VERSAMOD
must be subjected to high shear
conditions or increased temperature
to fully yield its maximum effect.
Since most liquid mud plants do not
have the ability to expose VERSAMOD
to conditions that will fully activate
it, care must be taken not to overtreat
when mixing VERSAMOD or other fattyacid
additives at a mud plant. Once
on the rig, they will readily yield
when sheared through the bit and
exposed to temperature, producing
excessive rheological properties if
overtreated.
VERSA-HRP is the preferred viscosifier
to increase rheology for supporting
weight material prior to shipping the
mud to the rig. It yields better in the
mud plant and will produce a more
stable viscosity as the system is circulated
through the well. VERSA-HRP is a
polyamide gelling agent that increases
the yield point and gel strengths with
minimal effects on the plastic viscosity.
Unlike VERSAMOD, which interacts with
the emulsified water phase, VERSA-HRP
works on and requires active solids
(organophilic clay or drill solids) to
viscosify a fluid.
When engineering a VERSAPORT system,
a six-speed VG meter is required to
check the rheological properties. Tables
6 and 7 are formulation charts for conventional
VERSAPORT systems. The following
order of addition is recommended,
when mixing a VERSAPORT system:
1. Oil.
2. Organophilic clay (VG-69).
3. Lime.
4. VERSAMOD or VERSA-HRP.
5. VERSAMUL.
6. VERSACOAT (allow to mix for 20 min).
7. CaCl2 brine (add slowly).
8. Weight material.
9. VERSATROL I (allow to mix 30 to
60 min).
VERSAPORT
systems
utilize
elevated lowshear-
rate
viscosities…
When
engineering
a VERSAPORT
system, a
six-speed
VG meter is
required to
check the
rheological
properties.
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.8 Revision No: A-0 / Revision Date: 03·31·98
*At higher mud weights, it is often advantageous to use a combination of VERSA-HRP and VERSAMOD at the mud plant to avoid excessive viscosity
after the fluid is displaced and circulated.
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 6: VERSAPORT system – barite formulations.
Oil:Water Ratio ® 60:40 70:30 80:20
VERSAMUL (lb/bbl) 6 - 8 5 - 7 4 - 6
VERSACOAT (lb/bbl) 0 0 - 1 0 - 1
Lime (lb/bbl) 8 - 10 8 - 10 8 - 10
VG-69 (lb/bbl) 1 - 3 1 - 3 1 - 3
VERSATROL I (lb/bbl) 0 - 4 2 - 5 3 - 6
VERSAMOD* (lb/bbl) 1 - 2 2 - 4 3 - 5
Oil:Water Mud Weight Oil Water CaCl2 M-I BAR
Ratio (lb/gal) (bbl) (bbl) (lb/bbl) (lb/bbl)
8.5 0.549 0.361 44.6 10.0
60:40
9 0.539 0.354 43.7 37.4
10 0.517 0.340 41.9 92.2
11 0.496 0.324 40.2 147.0
12 0.550 0.233 28.7 217.9
70:30 13 0.525 0.222 27.4 272.0
14 0.500 0.211 26.0 326.0
80:20
15* 0.535 0.132 16.3 393.9
16* 0.506 0.125 15.4 447.2
*At higher mud weights, it is often advantageous to use a combination of VERSA-HRP and VERSAMOD at the mud plant to avoid excessive viscosity
after the fluid is displaced and circulated.
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 7: VERSAPORT system — FER-OX formulations.
Oil:Water Mud Weight Oil Water CaCl2 FER-OX
Ratio (lb/gal) (bbl) (bbl) (lb/bbl) (lb/bbl)
8.5 0.551 0.362 44.6 9.5
60:40
9.0 0.542 0.356 43.9 35.5
10 0.525 0.345 42.6 87.5
11 0.508 0.334 41.2 139.5
12 0.572 0.242 29.8 207.4
70:30 13 0.552 0.233 28.8 258.9
14 0.533 0.225 27.7 310.3
80:20
15* 0.580 0.143 17.6 376.0
16* 0.557 0.137 16.9 426.9
VERSAPORT SYSTEM FORMULATION CHARTS
(25% BY WT CACL2 BRINE: 96% SALT PURITY)
Oil-Base Systems
Oil-Base Systems 12.9 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
VERSACORE SYSTEMS
VERSACORE systems are all-oil or minimal
water-containing oil-base systems. These
systems are used most often for coring
operations where the invasion of drilling
fluid containing emulsified water or
changes in wettability from high concentrations
of emulsifiers and wetting
agents is undesirable. VERSACORE systems
can be formulated from any base oil,
using several approaches.
The difficulty with these all-oil systems
is obtaining adequate viscosity, just
as with the early oil-base muds. One
solution is to use a very high concentration,
15 to 30 lb/bbl, of asphaltic additives
such as VERSATROL I or STABIL HOLE.T
Another approach involves using the
combination of VERSA-HRP with an
organophilic clay and a lesser amount
of VERSATROL I. A third approach uses
the combination of some asphalt, some
organophilic clay and an oil-viscosifying
polymer. A number of such polymers
exist, and they require specific pilot testing
to identify an appropriate formulation.
Polymeric viscosifiers can be used
to further enhance the viscosity of the
basic VERSACORE system. Regardless of
the actual formulation, VERSACORE systems
develop more viscosity when special
high-yielding organophilic clays are
used, such as VG-HT or VERSAVERT VIS.
These systems can be formulated
with minimal amounts of emulsifier
and wetting agent because they do
not contain added water. In addition,
the selection of an emulsifier and wetting
agent is less important. In fact,
the selection of a powerful emulsifier
and wetting agent (such as VERSAMUL
and VERSAWET as is normally used
in other oil-base systems) may be
undesirable due to their ability to change
wettability. Systems can be easily formulated
with just 1 lb/bbl VERSAMOD
and 1 lb/bbl VERSACOAT so that core
wettability is affected less. Although
no water is added to the system, they
usually pick up water from the pits
during the displacement and while
drilling so that actual water contents
are in the 3 to 5% range.
Low HTHP values are a very good
indicator of the ability of a coring fluid
to minimize fluid invasion. One advantage
to using a high concentration of
VERSATROL I is the low HTHP values.
Bridging agents are extremely important
in minimizing core invasion in
addition to low HTHP values. Barite
and ground calcium carbonate (such as
LO-WATEE or SAFE-CARBE) are excellent
bridging agents. The quantity and the
particle size distribution of the bridging
agent are important. As a general rule of
thumb, 15 to 30 lb/bbl of a bridging
agent with a median particle size onehalf
to one-third the largest pore-throat
diameter is needed to initiate bridging.
Table 8 gives VERSACORE formulations
using LO-WATE (calcium carbonate) as a
bridging agent and M-I BAR for density.
When mixing a VERSACORE system,
the following order of addition is
recommended:
1. Oil.
2. Organophilic clay.
3. VERSA-HRP.
4. Lime.
5. Emulsifier or wetting agent: VERSACOAT,
VERSAMOD, VERSAWET, VERSAMUL, etc.
(allow to mix for 20 min).
6. VERSATROL I (allow to mix for 30 to
60 min).
7. Weight materials.
VERSACORE
systems are
all-oil or
minimal
watercontaining
oil-base
systems.
Low HTHP
values are
a very good
indicator of
the ability
of a coring
fluid to
minimize
fluid
invasion.
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.10 Revision No: A-0 / Revision Date: 03·31·98
*VG-HT, VERSAVERT VIS or similar high-yielding organophilic clay.
**1 lb/bbl VERSACOAT and 1 lb/bbl VERSAMOD are recommended.
NOTE: 1 lb/bbl is equal to 2.85 kg/m3.
Table 8: Generic VERSACORE formulations.
Emulsifier
Mud Organo- and Wetting
Weight Oil clay* VERSA-HRP Agent** Lime LO-WATE VERSATROL I M-I BAR
(lb/gal) (bbl) (lb/bbl) (lb/bbl) (lb/bbl) (lb/bbl) (lb/bbl) (lb/bbl) (lb/bbl)
7.5 0.91 11.4 3.8 2 - 4 1 - 2 16.3 16.3 9
8.0 0.89 11.0 3.7 2 - 4 1 - 2 16.0 16.0 35
9.0 0.86 10.2 3.4 2 - 4 1 - 2 15.5 15.5 88
10.0 0.83 9.4 3.1 2 - 4 1 - 2 15.0 15.0 141
11.0 0.79 8.6 2.9 2 - 4 1 - 2 14.5 14.5 194
12.0 0.76 7.8 2.6 2 - 4 1 - 2 14.0 14.0 247
13.0 0.73 7.0 2.3 2 - 4 1 - 2 13.5 13.5 300
14.0 0.70 6.2 2.1 2 - 4 1 - 2 13.0 13.0 353
15.0 0.66 5.4 1.8 2 - 4 1 - 2 12.5 12.5 406
16.0 0.63 4.6 1.5 2 - 4 1 - 2 12.0 12.0 459
17.0 0.60 3.8 1.3 2 - 4 1 - 2 11.5 11.5 512
18.0 0.57 3.0 1.0 2 - 4 1 - 2 11.0 11.0 565
VERSACORE System Formulation Chart
LO-WATE and M-I BAR
Oil-Base Systems
Oil-Base Systems 12.11 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
BASE OILS
This section describes the typical properties
of some base oils used for VERSA
systems that are listed in Table 9.
Diesel oil composition may differ
somewhat from one refiner to another,
but most #2-grade diesel is acceptable
for use in oil-base muds without requiring
changes in mud formulations. Some
refiners add pour-point suppressants to
their diesel oils in cold climates (and
change the grades they sell) during the
winter months. This may affect the performance
of mud additives. The diesel
oil should be pilot tested if this is
suspected to be a problem.
Mineral oils vary widely in composition
and properties depending on the
crude oil, refining process and “cut.”
The properties of mineral oil from one
company are usually consistent, but
the properties of mineral oils from different
companies vary widely. One
method used to compare mineral oils is
the aromatic content. Various methods
exist for measuring and reporting the
aromatic content of oils. One proposed
standard is called the Polycyclic (or
polynuclear) Aromatic Hydrocarbon
(PAH) content reported as phenanthrene.
These phenanthrene aromatic
values are approximately 1⁄10 of normal
reporting values, considerably less than
the values normally used to express
aromatic content. Using this PAH measure,
standard mineral oils such as used
in VERSACLEAN systems contain ~0.35%
PAH as phenanthrene.
Ultra-low-toxicity systems, such as the
VERSAVERT system, use base mineral oils
that may be classified as Enhanced
Mineral Oil (EMO). These are highly
purified materials having lower PAH
content. One proposed standard for
EMOs is to have a PAH content of
about 0.001% as phenanthrene.
Products
System VERSADRIL VERSACLEAN VERSAVERT
Base oil #2 diesel Mineral oil Enhanced mineral oil
Density (SG) 0.83 - 0.86 0.80 - 0.86 0.81
Viscosity
(cSt* at 106°F) 3 - 4 2 - 3 3 - 4
Flash point (°F) 150 (130 min.) 212 (150 min.) >239
Pour point (°F) 14 -0.4 -74
Aniline point (°F) 149 (135 min.) 169 (150 min.) 194
Aromatics (normal
reporting units) 18 - 30% 1 - 15% nil
Aromatics PAH
(as phenanthrene) ~3% ~0.35% ~0.001%
ADDITIVES
VERSAMUL is a blend of liquid emulsifiers,
wetting agents, gellants and fluidstabilizing
agents. It is used as the
primary emulsifier in the conventional
VERSA systems and can often be used as
the only product needed to form the
basic oil-in-water emulsion. VERSAMUL
reacts with lime to form calcium
soap. This calcium soap acts as the
emulsifier in the tightly emulsified
conventional low-filtrate systems.
Initial system formulations require
4 to 10 lb/bbl (11.4 to 28.5 kg/m3),
depending on the properties desired
and other components in the system.
For VERSAMUL to function effectively,
one pound of lime must be added for
every pound of product. An excess
lime content of 3 lb/bbl must be
*cSt = centistokes.
Table 9: Typical base oil properties.
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Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.12 Revision No: A-0 / Revision Date: 03·31·98
maintained. VERSAMUL forms an
extremely tight emulsion and is
stable at high temperatures.
VERSACOAT surfactant is a multi-functional
liquid additive used as a wetting
agent for the conventional VERSA systems
and as a primary emulsifier in the
relaxed VERSA systems. Secondary benefits
include improved thermal stability
and HTHP filtration control. The product
is effective over a wide temperature
range and in the presence of contaminants.
VERSACOAT also reduces the
adverse effects of water contamination.
Initial system formulations require
from 1 to 8 lb/bbl (2.85 to 22.8 kg/m3),
depending on desired properties and
other components in the system. This
polyamide-base material is a versatile
and economical additive.
VERSAWET surfactant is a concentrated
liquid and powerful oil-wetting
agent used in relaxed-filtrate systems
where the excess lime content is less
than 2 lb/bbl. It is an excellent wetting
agent that is especially effective in
systems using difficult-to-wet FER-OX
(hematite). Initial system formulations
require 1 to 4 lb/bbl VERSAWET (2.85 to
11.4 kg/m3). It is also effective at oilwetting
barite and drill solids, and at
reducing the adverse effects of water
contamination. VERSAWET is an oil-base
mud thinner and wetting agent at low
alkalinity but acts more like an emulsifier
at high alkalinity. Overtreatment
with VERSAWET will thin the mud at
low alkalinity and viscosify it at high
alkalinity.
VG-69 organophilic clay is the primary
viscosifier and gelling agent used
in most oil-base systems. VG-69 is an
amine-treated bentonite, which provides
viscosity and gel structure to
increase carrying capacity and suspension
properties, providing support for
weight materials and improved cuttings
removal. VG-69 also aids in filtercake
formation and filtration control.
Typical concentrations range from
2 to 10 lb/bbl (5.7 to 28.5 kg/m3).
Depending on the base oil, higher concentrations
of VG-69 may be needed to
have the same rheological properties as
a comparative diesel-oil mud. In addition,
VG-69 does not yield as rapidly
in some base oils and when mixing
new fluids in mud plants. Care must
be taken not to overtreat because when
it is exposed to shear and temperature in
the well it will fully yield. Water acts as
a polar activator in these systems, and
the performance of VG-69 is enhanced
by lower oil-to-water ratios (higher
water content).
VG-PLUS organophilic clay is an
improved viscosifier and gelling additive
for all non-aqueous fluids, including
NOVA synthetic-base and VERSA oil-base
systems. VG-PLUS is an amine-treated
bentonite that improves the carrying
capacity, gel strength and suspension
of weight material. It will also assist in
improving filter-cake quality and filtration
control. VG-PLUS has particular
application in mixing plants and when
building new fluids, to provide viscosity
for fluids that have not been exposed to
shear and temperature. Typical concentrations
range from 2 to 10 lb/bbl (5.7 to
28.5 kg/m3). Water acts as a polar activator
in these systems and the performance
of VG-PLUS is enhanced by lower
oil-to-water ratios (higher water content).
VG-HT organophilic clay is a premium
viscosifier and gelling agent
for use in VERSA oil-base/pseudo-oilbase
and NOVA synthetic-base systems
exposed to high temperatures. This
high-quality, amine-treated hectorite is
used to increase carrying capacity and
suspension properties, providing support
for weight materials and improved
drill cuttings removal in high-temperature
wells. VG-HT also aids in filtercake
formation and filtration control.
Typical concentrations range from 2
to 10 lb/bbl (5.7 to 28.5 kg/m3). Water
acts as a polar activator in these systems
and the performance of VG-HT
VERSACOAT
also reduces
the adverse
effects of
water contamination.
Oil-Base Systems
Oil-Base Systems 12.13 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
is enhanced by lower oil-to-water
ratios (higher water content).
Calcium chloride (CaCl2) salt is used
in most oil-base mud systems to reduce
the activity (Aw) of the mud for shale
inhibition. High-purity (95 to 98%
purity) granular or powdered calcium
chloride is preferred to tech grade (77
to 80% purity) or flaked products. Care
should be taken to identify the grade
of CaCl2 that is being used when preparing
VERSADRIL and VERSACLEAN systems.
Granular or powdered CaCl2 should
be used instead of flakes or pellets,
especially when it is being added to
an existing mud. Stock 11.6 lb/gal calcium
chloride brine used for workovers
and completions can be diluted and
used instead of sacked materials. See
the appropriate salt table for activity
vs. percent-by-weight salt correlation
in the Non-Aqueous Emulsion chapter.
Lime (hydrated or slaked lime —
Ca(OH)2) is used in all oil-base muds for
alkalinity control to increase the POM
and maintain some excess lime. In conventional
systems, it is used in higher
concentrations as a source of calcium
for forming calcium soaps with the primary
emulsifiers. It is used in all oil-base
mud systems as a source of alkalinity
when drilling acid gases (CO2 and H2S).
Quick lime (CaO) is sometimes used
as a source of calcium and alkalinity
in oil-base muds. In humid or wet
(rainy) environments, hydrated lime
should be used instead of quick lime.
In conventional systems, quick lime
will react with the emulsifiers to form
calcium soaps. Quick lime reacts with
water to evolve heat and form calcium
hydroxide (lime, Ca(OH)2). The evolution
of heat may be helpful in building
emulsions. Quick lime can be used
in oil-base mud systems as a source
of alkalinity when drilling acid gases
(CO2 and H2S). CAUTION: Quick lime
is a highly reactive chemical and should
not be used in situations where it might
come into contact with water, such as
high humidity and rainy climates! When
it gets wet, heat is generated that can
cause fire or injury. Quick lime is a strong
irritant and precautions should be taken
to prevent inhalation and skin exposure.
VERSA-HRP, a polyamide liquid,
increases the yield point and gel
strengths of both conventional and
relaxed filtrate systems with minimal
effects on the plastic viscosity. The primary
application of VERSA-HRP is the
mixing of new VERSA systems, but it can
be used with any type of oil to increase
the carrying capacity and improve its
shear-thinning characteristics. The recommended
concentration of VERSA-HRP
for the initial makeup of new fluids is
1 to 4.5 lb/bbl (2.85 to 12.83 kg/m3)
of VERSA-HRP in combination with 4
to 12 lb/bbl (11.4 to 34.2 kg/m3) of
organophilic clay. VERSA-HRP can also
be used in sweeps and viscosified spacers.
VERSA-HRP does not viscosify oil. It
requires active solids (organophilic clay
or drill solids) to viscosify. NOTE:
VERSA-HRP should be pilot tested before
it is added to a mud system.
VERSA SWA, an amphoteric surfactant
for all oil-base muds, is a powerful supplemental
wetting agent that aids in
oil-wetting solids. It can reverse waterwetting,
even in badly contaminated
muds. It is particularly useful when
complex salts are drilled or water flows
are encountered. Small treatments (usually
less than 1 lb/bbl) are adequate.
The product is a supplemental wetting
agent intended only to be used in conjunction
with the primary wetting
agent, and is often kept in inventory
as a contingency item. Pilot testing is
recommended before treatment.
VERSAMOD, an organic gelling agent, is
a liquid rheology profile modifier for oilbase
mud systems. It increases the LSRV
and gel strengths with minimal effect
on its high-shear-rate viscosities. Its primary
application is in large-diameter
directional wells where improved hole
cleaning is needed. Water improves the
Granular or
powdered
CaCl2 should
be used
instead
of flakes
or pellets…
Quick lime
is a highly
reactive
chemical and
should not
be used in
situations
where it
might come
into contact
with water…
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.14 Revision No: A-0 / Revision Date: 03·31·98
performance of VERSAMOD and the concentration
needed to achieve the desired
effect is less at low oil:water ratios.
Normal concentrations range from
1 to 4 lb/bbl (2.85 to 11.4 kg/m3) of
VERSAMOD, depending on the brine
content. The VERSAPORT system uses
VERSAMOD to obtain its rheology profile.
It requires the addition of a pound of
lime for each pound of VERSAMOD used,
to achieve the desired effect. Because of
the lime requirement, VERSAMOD is not
as effective in relaxed-filtrate systems as
it is in conventional systems.
VERSATROL I gilsonite is a temperaturestable,
naturally occurring weathered
asphalt. It is an effective filtrationcontrol
additive and plugging agent
that can be used at bottom-hole temperatures
up to and in excess of 400°F.
VERSATROL I enhances emulsion stability
and imparts minimum viscosity
increases. Treatments range from
2 to 8 lb/bbl (5.7 to 22.8 kg/m3) of
VERSATROL I as a fluid-loss-control
agent in most systems. Pilot testing
should be conducted to determine the
actual concentration needed for each
application. At high concentrations,
VERSATROL I can be used to increase
the viscosity of the system. Difficultto-
viscosify fluids like VERSACORE will
use between 15 and 30 lb/bbl (43 to
86 kg/m3) of VERSATROL I to obtain
adequate viscosity.
VERSALIG amine-treated lignite is
used as a fluid-loss-control agent.
VERSALIG is an alternative to the commonly
used gilsonite/asphalt base
fluid-loss agents like VERSATROL I or
STABIL HOLE. Recommended treatments
range from 2 to 12 lb/bbl (5.7 to 34.2
kg/m3) for most applications. Pilot
testing should be conducted to determine
the concentration for each
application.
VERSATHIN, a liquid, oil-base mud dispersant,
is designed to reduce the yield
point and gel strengths. Additions of
VERSATHIN result in a less-viscous fluid
without the need for dilution or changing
the oil:water ratio. Recommended
treatment levels range from 1 to 2 lb/bbl
(2.85 to 5.7kg/m3) ofVERSATHIN.VERSATHIN
tends to work best in high solids-muds
as it tends to disperse aggregating solids.
NOTE: VERSATHIN must be pilot tested
before being added to a mud system.
VERSAVERT P primary emulsifier blend
is based on polyamides and modified
fatty acids designed for use in the
VERSAVERT system. It is a primary emulsifier
or “basic package” and can be used
alone to form tight water-in-oil emulsions.
It is particularly effective when
used in conjunction with VERSAVERT S.
This product is similar in function to
VERSAMUL (in a conventional system)
and has application in many systems.
VERSAVERT S secondary emulsifier is
used in the VERSAVERT system to provide
high emulsion stability and solids
wetting. It is primarily a secondary
emulsifier. Although it can be used
alone to form a water-in-oil emulsion,
it is more effective when used in conjunction
with VERSAVERT P. This product
is similar in function to VERSACOAT
(in a conventional system) and has
application in many systems.
VERSAVERT F filtration additive is a
resin copolymer used in the VERSAVERT
system. It is used to provide supplementary
fluid-loss control and has
application in many systems.
The
VERSAPORT
system uses
VERSAMOD
to obtain
its rheology
profile.
VERSATHIN
tends to work
best in high
solids-muds…
Oil-Base Systems
Oil-Base Systems 12.15 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
It is difficult to specify exact ranges for
mud properties such as the plastic viscosity,
yield point and gel strengths due
to the wide range of applications. Many
variables affect the value of these properties
including the base oil’s properties;
temperature; the type, size and concentration
of solids; oil:water ratio; brine
concentration; and the overall stability
of the mud. Determining whether these
properties are in the correct range for a
given mud weight depends heavily on
the fluid properties needed for the well
conditions. For example, a high yield
point and gel strengths are needed for
carrying capacity in large-diameter
holes, but these properties may not be
desirable in small-diameter holes with
mud of the same weight.
Plastic viscosity should be maintained
at minimum values to optimize bit
hydraulics and penetration rates. If the
plastic viscosity trends upward over a
period of time without increases in the
mud weight, it usually indicates that
fine solids are building up in the mud.
Increases in the volume percent solids
even from weight material will increase
the plastic viscosity. Decreases in the
oil:water ratio (higher water content)
will increase the plastic viscosity.
Yield point and gel strengths are governed
by two requirements. The first is
the need to maintain sufficient thixotropy
(gel structure) to suspend weight
material and cuttings, plus provide carrying
capacity. The second requirement
is to minimize annular pressure losses
and Equivalent Circulating Densities
(ECDs). The yield point and gel strengths
can be increased with additions of
VG-69, VERSAMOD or VERSA-HRP. They
can be reduced with additions of
VERSATHIN or the base oil.
The allowable solids content depends
on the oil:water ratio, the water-phase
density and the volume and specific
gravity of the solids. Solids are abrasive,
and they increase the cake thickness,
plastic viscosity, pressure losses, the
need for chemical treatments and the
likelihood of water wetting the solids.
The low-gravity solids should be kept
as low as economically possible with
solids-control equipment.
The alkalinity (POM or VSA) of an oilbase
mud is an indication of the excess
lime in the mud. The POM of a conventional
controlled filtrate system should
be maintained above 2.5 cm3 of 0.1 N
sulfuric acid. The emulsion may become
unstable if the POM of a conventional
system falls below 2.5 for an extended
period of time. The POM is normally
maintained at 1 to 2 cm3 of 0.1 N sulfuric
acid in relaxed filtrate systems to
buffer against acid gases. NOTE: M-I bases
all recommendations concerning alkalinity
treatments on the API VSA (POM) method. If
the operator desires, M-I will determine the
POM by both the API method and the “Back
Titration” method. However, all treatment
decisions will be made exclusively based on
the API POM (direct) method.
The HTHP filtrate (300°F and 500
psi) of conventional systems is usually
less than 10 cm3. Low filtrates reduce
the loss of expensive fluids to the formation
and reduce the likelihood of
differential sticking in highly permeable
formations. Relaxed systems normally
do not use a filtration-control
additive, and may contain some water
in the HTHP filtrate.
Properties
Plastic
viscosity
should be
maintained
at minimum
values to
optimize bit
hydraulics
and penetration
rates.
Low filtrates
reduce
the loss of
expensive
fluids to the
formation…
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.16 Revision No: A-0 / Revision Date: 03·31·98
EMULSION STABILITY
The Electrical Stability (ES) is a relative
indication of emulsion stability. It is
a measure of the voltage required to
break down the emulsion and allow the
emulsified water droplets to connect
(i.e., coalesce) allowing electrical current
to flow. Strong emulsions require high
voltages to coalesce the water droplets
and break down the emulsion. The
electrical stability is recorded in volts.
There are several main factors that
affect electrical stability:
• Water content. As the water content
increases, the distance between the
water droplets decreases, allowing
for easier electrical circuit completion
through coalescence and a
reduction in the electrical stability.
• Water-wet solids. A water-wet solid
has a thin film of water on its surface
that functions to conduct electricity
like a water droplet. Solids in an invert
emulsion reduce electrical stability
when they become water-wet.
• Emulsification. The degree of emulsification
affects water droplet size.
Droplets are normally larger in new
and unstable mud systems, resulting
in low emulsion-stability values.
Increased shear and temperature
exposure will form smaller droplets
and a better emulsion. This increases
electrical stability values as does
increased emulsifier and wetting
agent concentration.
• Temperature. The temperature at
which the electrical stability measurement
is made will change the
value obtained. This temperature
should always be recorded with the
electrical stability value. For trend
analysis, the same temperature
should be used.
• Type of solids. The type of solids in
the mud will influence the electrical
stability. For example, FER-OX (hematite)
and other iron oxide materials
may reduce the electrical stability of
an invert emulsion mud.
Electrical stability is an important
indicator of emulsion stability, but it
should not be used as an absolute value
or indication of its condition. A mud
with a high but declining electrical stability
may not be as stable as a mud
with a lower but stable electrical stability.
Muds with extremely low emulsion
stability will have filtrate and rheological
indications as well as low and
declining electrical stability values. Low
electrical stability may be a cause for
concern, but an established trend of
declining electrical stability values is
more serious and requires immediate
action. The electrical stability values are
relative to the system from which they
are recorded. A well-defined downward
trend or a rapid drop indicates the
emulsion is weakening.
Electrical stability measurements
should be made and recorded routinely.
These values should be plotted
so trends can be easily seen. Trends
upward or downward indicate changes
in the system. An analysis of sequential
mud checks will indicate possible
causes of the change.
SALINITY AND CONTROLLED ACTIVITY
Calcium chloride (CaCl2) content
should be tested by titration and compared
with the AW of the cuttings
when running a controlled activity
mud. The CaCl2 content of the mud
should be maintained at a concentration
that will balance or be equal to
the AW of the formation. CaCl2 concentrations
above 38% are not recommended
due the near saturation of the
brine, which can cause fluid instability.
Salt crystallization from supersaturated
solutions heating and cooling
can produce water-wet solids and
unstable emulsions. Sodium chloride
and complex blends of magnesium,
Strong
emulsions
require high
voltages to
coalesce
the water
droplets
and break
down the
emulsion.
The CaCl2
content of
the mud
should be
maintained
at a concentration
that
will balance
or be equal
to the AW
of the
formation.
Oil-Base Systems
Oil-Base Systems 12.17 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
potassium, calcium and sodium chloride
brines can also be used in the internal
phase. A variety of other organic
non-chloride materials can be used to
decrease the activity of the water phase
in addition to the inorganic chloride
salts like sodium and calcium chloride.
Water activity (AW) is a measure of
the chemical potential for water to be
transferred between mud and shales.
Activity is measured using the vapor
pressure (relative humidity) of shale or
mud, or it can be estimated based on
the chemical composition of the brine
(salinity). Pure water has an AW of 1.0.
Calcium chloride brines used in most
non-aqueous emulsion muds have an
AW between 0.8 (22% wt) and 0.55
(34% wt). Lower values for activity
are more inhibitive.
Clay-containing formations swell
and are weakened by the adsorption
of water. The possibility exists that the
water from an emulsion mud can alter
shales if the AW of the shale is lower
than the activity of the mud. The key
to inhibition is to “balance” the activity
of the mud with that of the shale so
that the adsorption of water onto the
shales is theoretically reduced to zero.
The transfer of water between an
emulsified brine and a shale is often
compared to osmosis. In osmosis, a
solvent (water) diffuses through a semipermeable
membrane from a low concentration
of solute or salt, to a high
concentration of solute or salt, to equalize
the concentrations. The theory of
controlled activity describes the oil-base
or synthetic fluid and emulsifiers surrounding
the water droplets as a semipermeable
membrane. Shale control
using this controlled (or balanced) activity
is limited mainly to oil-base and synthetic
emulsion muds. Water-base muds
containing additives (such as glycols
and silicates) exhibit only weak semipermeable
membrane characteristics.
CaCl2 is normally used to obtain
activities from 1.0 to 0.40. Sodium chloride
(NaCl) may be used to obtain activities
from 1.0 to 0.75 (saturated NaCl).
A wide variety of alternative internalphase
chemicals can be used to reduce
activity. However, many alternative
materials may not provide sufficiently
low activity to achieve adequate inhibition.
Most shales were formed in
marine environments containing complex
salts with calcium chloride, magnesium
chloride and sodium chloride
the most common salts present. These
complex salts often have a greater affinity
for water than sodium chloride
brine, even when saturated. Calcium
chloride brines are used as the internal
phase of most oil-base muds as they
can balance the formation salinity of
most formations.
When CaCl2 is added to a saturated
NaCl brine, the activity is reduced but
the effect is not cumulative. Activity is
based on mutual solubility. Since CaCl2
has a greater solubility than NaCl,
sodium chloride will precipitate as fine
solids at conditions above saturation.
The activity of mud and shale samples
is measured with a hygrometer.
The sample being checked is placed in
a flask and tightly sealed with a stopper
containing the hygrometer probe. The
sample is given time to equalize the
moisture content of the air space in the
flask. The percent relative humidity,
corrected for temperature, is recorded
as the “Activity” (decimal value) of
the sample.
VERSA systems can be formulated with
either CaCl2 or NaCl brines. The use of
these salts in combination is not recommended,
as the solubility of NaCl
is limited in the presence of CaCl2.
AW is a
measure of
the chemical
potential for
water to be
transferred
between mud
and shales.
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.18 Revision No: A-0 / Revision Date: 03·31·98
The following section applies to displacing
an existing mud with an oilbase
mud. Many of the principles used
here also apply to displacing an oilbase
mud with cement or a water-base
mud. However, the spacers used would
be different.
SUMMARY
11. Have a predisplacement meeting
with tool pusher, company man
and mud engineer to discuss displacement
procedure and coordinate
sequence of events.
12. Drill out and perform leak-off or
formation-integrity tests.
13. Prior to the displacement, condition
existing wellbore fluid to
obtain the minimum acceptable
viscosity and gel strengths.
14. Have all oil-base mud on location
prior to displacement.
15. Have bit on bottom or close to
bottom as the oil-base mud clears
the bit.
16. Use large-mesh screen on shale
shaker during displacement and
1 to 2 circulations afterward.
17. Spacers generally should be 200 to
500 ft in length.
a) Water (water-base in hole).
b) Viscosified oil or viscous oil mud.
18. Use pump rates to obtain
turbulent flow.
19. Do not stop or slow pumps for
any reason.
10. Reciprocate and rotate drill pipe
during displacement.
DISPLACEMENT TECHNIQUES
The most efficient type of displacement
occurs when the total volume
of oil-base mud can be displaced in
one rapid, continuous operation without
stopping or slowing the pumps.
Regardless of the displacement technique
used, there are several factors
common to all displacements that
influence a good displacement and
prevent cross contamination.
• Density. It is desirable to have the displacing
fluid slightly heavier than the
fluid being displaced. Because the displacing
fluid is below the fluid being
displaced in the annulus, the heavier
density maintains segregation of the
two fluids (the lighter fluid tends to
float, the heavier fluid tends to sink).
If a lower-density, oil-base mud must
be used to displace a higher density
fluid, it may be advantageous to
reverse circulate.
• Spacers. The ideal spacer would thin
the fluid, maintain turbulence of the
fluid being displaced and viscosify the
displacing fluid. The difference in the
viscosity at the interface reduces the
tendency of the fluids to intermix.
Conditioning the existing fluid to
reduce the viscosity and yield point is
just as important as the spacer fluid.
Reducing viscosity, using a thinning
spacer and turbulent flow in the fluid
being displaced reduces channeling
and intermixing. Spacer volume is
usually selected based on some annular
length, with a 200- to 500-ft (61-
to 152-m) column in the annulus
being typical. These lengths should
be selected with well control and
other engineering factors considered.
Typical spacers are:
1. Water-base being displaced with
oil-base:
• Water or,
• Water, followed by viscosified oil
or viscous oil-base mud.
2. Oil-base being displaced with
water-base:
• Oil or,
• Oil, followed by viscosified water
or viscous water-base mud.
Displacements
It is desirable
to have the
displacing
fluid slightly
heavier
than the
fluid being
displaced.
Oil-Base Systems
Oil-Base Systems 12.19 Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
12
• Pipe position and movement. Drill
pipe is usually not positioned concentrically
even in a vertical well, and will
be close to the wall of the hole. This
eccentric annulus causes fluids to
channel up through the larger side of
the hole (just like the hole-cleaning
situation in a horizontal well). This
leaves a portion of the annular cross
section uncirculated so that old mud is
left behind the drill pipe on the narrow
side of the hole. For this reason,
the pipe should always be rotated and
reciprocated during all displacements.
Rotating the pipe forces the mud from
behind the pipe out into the flow
stream and moves the pipe around in
the annulus. This changes the flow
path and allows the entire cross section
to be circulated, producing a
more uniform displacement.
• Pump rate. Displacements should
always be done at a pump rate high
enough to ensure turbulent flow, if
possible. The velocity profile in turbulent
flow is flat and covers all of
the annulus, with only a small
boundary layer. This accomplishes
several things. It results in minimal
intermixing of the two fluids. It promotes
a more thorough displacement
of the mud in the hole by “scrubbing”
the hole with turbulence, and
it can clean wall cake better if an
open hole section is exposed.
• Contamination. Contamination of
some of the displacing fluid by the
fluid being displaced may occur. Any
fluid that is noticeably contaminated
with water-base mud or wall cake
should be discarded. The remaining
contamination should be treated with
emulsifier and/or wetting agent to
ensure that water becomes emulsified
and that the solids are in an oil-wet
condition. Pretreatment for contamination
is not recommended. The
most effective treatments can be
made after the displacement.
• Conditioning and stabilization.
Once an oil-base mud system has
been displaced, a period of circulating
and conditioning time is required
before it becomes fully stabilized. This
is especially evident in newly prepared
systems. Contamination from
the waterbase mud system during the
displacement can destabilize a system,
making the importance of a good
displacement even more important.
After displacement, it is usually necessary
to use higher than normal treatments
for a few days until the system
becomes stabilized. These increased
treatments may include emulsifiers,
wetting agents and viscosifiers.
• Completed displacement indicators.
In some instances, it is difficult
to identify when the displacing fluid
returns to the shale shaker, especially
when minimal intermixing occurs
so that a viscous interface is not
seen. Occasionally a few sacks of
Lost-Circulation Material (LCM) are
pumped in the leading edge of the
displacing fluid as a marker. While
pit volume and pump strokes are the
best measures to use, other indicators
listed below aid in determining
when the displacement is complete
or when to start returning mud to
the active pits:
1. Mud weight measurements, if the
weight of the two fluids differ.
2. Electrical stability measurements
or pH.
3. Change in viscosity.
4. Change in color or surface
appearance from grainy to
glossy or shiny.
5. Presence and subsequent elimination
of water-wet solids on the
shaker screens.
Drill pipe is
usually not
positioned
concentrically
even in a
vertical
well…
The velocity
profile in
turbulent
flow is flat
and covers
all of the
annulus,
with only
a small
boundary
layer.
Oil-Base Systems
CHAPTER
12
Oil-Base Systems 12.20 Revision No: A-0 / Revision Date: 03·31·98
Lost circulation with oil muds can
quickly become intolerable due to
the costs involved. In some instances,
the chances of losing returns increase
with oil-base drilling fluids due to the
viscosifying effect of pressure on oil
as compared to water. Consequently,
strict controls must be maintained to
minimize the viscosity and/or circulation
rate. This will reduce the annular
pressure losses and decrease the
risk of losing circulation.
Another reason for the increased risk
of losing returns with oil-base muds is
their low leak-off values. The properties
of oil make it an excellent fracture fluid,
thereby increasing the chance of breaking
down the formation. Its oil-wetting
character hinders the formation’s healing.
For this reason, oil muds are not
recommended for testing casing shoes
and fracture pressures.
Increasing pump rates too rapidly after
connections and trips can cause lost circulation
with oil-base fluids. Oil muds
thin with increasing temperatures generated
while circulating and thicken with
lower temperatures during periods of
quiescence. The failure to bring the
pumps up to speed slowly can put
much higher circulating pressures on
the formation. It is not uncommon
for circulating standpipe pressures to
decrease more than 100 psi as the mud
heats to circulating temperature.
The procedures to follow in the event
of lost circulation are similar to those
with a water-base mud system. The use
of LCM pills may be helpful under certain
conditions. From 30 to 50 lb/bbl
(86 to 143 kg/m3) of lost-circulation
material should be spotted at the thief
zone. It is recommended that medium
and/or fine grades of mica and/or
NUT PLUGT be used in these pills. A
blend of sized calcium carbonate particles
has been used successfully in some
areas. Fibrous, shredded materials such
as wood fiber, shredded newspaper,
etc. should be used with caution due
to their detrimental effects upon the
emulsion. In instances of severe lost
circulation, a specially formulated
high-fluid-loss diatomaceous earth
slurry squeeze (DiasealT M type), gunk
squeeze or a cement squeeze may be
the most practical approach.
In the most severe cases of lost circulation,
where procedures have failed
to regain total returns, the oil-base
system should be displaced with a
conventional water-base mud system.
Lost Circulation
Packer Muds
VERSA system fluids make excellent
packer fluids for leaving in the annulus
above a tubing packer after the well
is completed. An oil-base packer offers
the advantages of excellent temperature
stability over long periods of time,
excellent weight suspending characteristics
and lasting protection of the
metal goods from the effects of corrosion.
Few, if any, water-base mud
systems can offer these advantages
simultaneously. For a more thorough
discussion of this application see the
section on packer fluids in the chapter
on Non-Aqueous Emulsions.
Increasing
pump rates
too rapidly
after connections
and
trips can
cause lost
circulation
with oil-base
fluids.