Note: Descriptions are shown in the official language in which they were submitted.
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"Boreho]e fluid"
Background of the Invention
This invention concerns oil based fluids for borehole-operations and in
particular drill-
ing fluids for oil well drilling and completion etc.
A drilling fluid consists of a liquid, usually oil and/or water, with
different kinds of
additives. Drilling fluids are used to remove rock cuttings from the borehole
and bring
'them to the surface. The drilling fluid also helps to control subsurface
pressures and the
fluid has to provide a protective and stabilising coating to permeable
formations so that
the productivity of the reservoir is not hindered. The productivity of an oil
reservoir can
be adversely affected by solids from the drilling fluid penetrating and
blocking the flow,
channels in the formation. The productivity of a reservoir can also be reduced
by drilling
fluid filtrate causing hydrationand swelling of the formation clays.
The drilling fluid is pumped through a hollow drill string to the drill bit
cooling and
lubricating the drill string and bit. The fluid is recirculated. The
properties of the diill-
ing fluid areinonitored and adjusted during the operation. The density of the
fluid must
be high enough to control formation pressures, but low enough to permit the
fastest
possible drilling rate. At the surface the rock cuttings are removed from the
drilling fluid
by screening usually accompanied by the use of hydrocyclones and centrifuges.
Drill
cuttings are produced as a waste material..
Prior Art
At present oil-based. fluids are the fluids of choice for most deep drilling
operations on
the basis of the following criteria' : Drilling rate, lubricity, formation
stability,,ease of
' maintenance and cost. Therefore, current drilling technology is heavily
dependent on the
use of these fluids using diesel, mineral oils; olefins or esters as the-
external phase and
.30 halide brines as the internal phase. The drillinb fluid is formulated as
an emulsion to
accommodate additional water picked up during the drilling operation and the
salinity of
the, aqueous phase is controlled by the use of dissolved salts. Calsiurii
chloride is the
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2
most common salt used, although sodium chloride or various other brines have
been
used as well. The choice is dictated by both the formation location and
economics.
A typical continuous oil phase emulsion for drilling mud application
formulated by
conventional technology consists of 70-80 weight % of oil and 20-30 weight %
of brine.
In addition to oil and brine the emulsion contains an emulsifier. Such an
emulsion has
initially, before addition of weighting material, a density of approximately
0.9 kg/1. A
typical weighting agent is barite of density 4.2. Other additives in the
drilling fluid may
include filtration control additives, rheology modifiers, oil wetting
additives, corrosion
inhibitors, etc.
For offshore drilling the legislation is driving the industry to eliminate the
discharge
overboard of hydrocarbons, and this includes oil-contaminated cuttings from
wells
drilled with oil-based fluid. Disposal of the oil-contaminated drill cuttings
is severely
restricted in all sectors of the North Sea.
The unwanted cuttings can be re-injected back into the oil well as a ground up
slurry,
provided that the geological conditions down the wellbore are favourable, or
shipped to
shore and treated there to remove the oil contamination before disposal at
land-fill sites.
This can create problems in. bad weather if cuttings cannot be offloaded from
the rig.
The concept is difficult to apply to floating exploration rigs. In addition,
the extra energy
required to transport and handle the cuttings produces extra pollution, as
does the
disposal on land which usually involves a heat extraction or incineration
process.
The benefits shown by oil-based fluids in offshore drilling are equally
applicable to land
operations. The disposal of waste and cuttings from onshore drilling
operations using
oil-based muds' represents however a somewhat different problem, where the
dominat-
ing factor is the presence of environmentally harmful salts in the emulsified
brine. The
use of calcium chloride in the brine restricts the amount of drill cuttings
which can be
successfully landfarmed on a given area. Canadian Patent No. 2 101 884
(Flemming)
describes replacement of calsium choride in the brine phase with calsium
nitrate - the
objective being to minimise the environmental impact when wastes are spread to
land.
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By replacement of the chloride with nitrate the natural process of microbial
degradation
of the associated oil is also enhanced.
In the past the diesel and mineral oil-based fluids have offered good
technical perfonn-
ance, reasonable cost and operational flexibility for North Sea operations and
the
environmental acceptability of these fluids has gradually been enhanced in
terms of
reduced toxicity and improved biodegradability by introduction of olefins,
alkanes and
esters in place of the oils. The cost of the oil-based drilling fluids has
however increased
dramatically as operators have moved to these more sophisticated oils.
Oil based drilling fluids designed especially for the purpose of being non-
polluting are
described, for instance, in European Patent 764 711 B I and 1029 908 A2. The
patents
describe the use of minimaiIy toxic drilling fluids which are based on
synthetic hydro-
carbons derived from alpha-olefinic monomers. The patent claims include
drilling fluid
emulsions containing up to about 70 % by volume of an aqueous phase. The
possibility
of formulation of a drilling mud containing such a high amount of aqueous
phase is not
demonstrated by the examples given in the patents. The emulsions described in
these
patents, containing the highest amount of water, will have a density of not
more than
approximately 1.0, before the weighting material is being added.
Summary of the Invention
The present invention provides a novel borehole fluid, especially a drilling
fluid, that could
provide the beneficial drilling properties of oil-based muds while generating
a minimal
environmental impact. The invention provides a water-in-oil emulsion of high
initial
density. The present invention provides an oil-continuous drilling fluid which
offers
economic benefits by reduction of the amount of ester, when ester is used as
the oil phase,
compared to the brine phase. The oil phase being the most expensive one in
this case.
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These and other aspects of the invention are
obtained with the borehole fluid as described below.
In one aspect, the invention provides a
water-in-oil emulsion borehole fluid comprising oil, brine
and emulsifier or a mixture of emulsifiers, and at least one
of the emulsifiers being of high molecular weight in the
range from 500 up to 5000 and having a HLB-value in the
range from 2 up to 8, in a concentration being 0.2-5.0 % by
weight of the oil and brine mixture, wherein the
oil-to-brine ratio by weight is between 10:90 and 30:70, the
hydrophobic part of the emulsifier molecule consists of a
long branched or bulky alkyl group and the hydrophilic polar
part of the molecule being polyoxyalkylene, polyol, amine or
amide, the aqueous brine phase is a concentrated solution of
a nitrate, nitrite, chloride, bromide, acetate, tungstate or
formate, or mixtures thereof.
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The invention thus concerns a water-in-oil emulsion borehole fluid, especially
a drilling
fluid. The fluid comprises an oil, a brine and emulsifier wherein the oil-to-
brine ratio by
weight is between 10:90 and 30:70. The lowest oil-to-brine ratio is preferably
15:85.
The emulsifier is preferentially of high molecular weight, the molecular
weight being in
the range from 500 up to 5000. The preferred HLB-value is in the range from 2
up to 8.
The emulsifier concentration is 0.2-5.0 % by weight active matter,
preferentially 0.4-3.0
weight % of the oil and brine mixture. It is preferred that the hydrophobic
part of the
emulsifier molecule consists of a long branched or bulky alkyl group and the
hydrophilic
polar part of the molecule being polyoxyalkylene, polyol, amine or amide.
Suitable
emulsifiers are for example a polyisobutenyl succinic anhydride derivative or
a polyal-
kylene glycol where the hydrophobic part consists of an oligomer of alfa-
olefines, the
hydrophilic part being a polyethylene glycol sequenee.
The aqueous phase should be a concentrated solution of a nitrate, nitrite,
chloiide,
bromide, formate, acetate, tungstate or mixtures thereof. Preferably the
aqueous phase is
a concentrated solution of a nitrate compound selected from the group
consisting of
alkali metal, alkaline-earth metal or ammonium nitrates, and hydrates and
complexes
thereof. The oil phase is a mineral oil-based fluid, a diesel oil, an olefin,
alkane or an
ester. It is preferred to use an ester of fatty acids. A preferred composition
comprises an
ester of a fatty acid, a concentrated solution of a nitrate compound or
mixtures of nitrate,
chloride and/ or bromide, 0.4-3.0 weight % of an emulsifier, and where the oil-
to-brine
ratio by weight is between 20:80 and 30:70. The fluids could in addition
comprise
suitable wetting agents, viscosifiers, weighting materials and fluid loss
additives used to
obtain the desired properties with respect to stability, rheology, filtration
control, density
etc.
By introducing an emulsifier of the specified type stable oil-continuous
emulsions can
be prepared with a low content of oil, the speed of mixing not being that
critical
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preferentially in combination with using an ester or an alpha-olefin as the
external phase
and a nitrate brine as the internal phase.
Conventional drilling fluids produce cuttings that are coated in oil, and
disposal of these
5 cuttings is a problem. Disposal to sea bed results in the formation of a
cuttings pile
coated in oil which biodegrades extremely slowly as the oil has to be broken
down
under anaerobic conditions. When applied on offshore drilling locations this
fluid
formulation should significantly reduce the amount of oil being discharged
with the
cuttings to sea bed or to other recipients. In the preferred formulation the
presence of
nitrates will introduce an alternative terminal electron acceptor for
microbial respiration
thereby enhancing the rate of biodegradation of the oil by encouraging anoxic
microbial
respiration. Under anoxic denitrifying conditions oils will degrade more
rapidly than
under anaerobic conditions as the process is more thermodynamically efficient
and more
complex molecules can serve as carbon sources.
It is also possible to provide an oil-continuous drilling fluid which offers
economic
benefits by reduction of the amount of oil, especially when alpha olefin or
ester is used
as the oil phase, compared to the brine phase. The oil phase being the most
expensive
one. This should apply to brines containing chlorides, bromides and nitrates
and
mixtures there of.
It is also important that it is possible to produce water-in-oil emulsions
having a high
initial density for drilling fluid application. Typically the density for such
an emulsion
will be 1.25-1.35 or even higher. In such a drilling fluid the requirement for
addition of
weighting material will be less than normal. The drilling rate, all other
things being
equal, is a function of the percentage of solids in the fluid and the plastic
viscosity. The
proposed formulation should offer improved drilling rates due to the lower
solids
content. Problems with barite sag will be less. This should represent a
considerable
improvement over the technology of today also where diesel or mineral oil are
used.
Calcium nitrate based solutions have for some time been considered for use as
drilling
fluids. Due to the tendency of inducing stress corrosion cracking (SCC) on
carbon steel
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(unalloyed steel), there has been hesitations in applying these solutions. By
using an oil
continuous emulsion the steel surface will be wetted by the oil and not by the
aqueous
phase. The nitrate will not be in contact with the surface and we will not
have any corro-
sion. This has been experimentally verified by testing U-bend specimens of
steel quality
Q 125 for 4 weeks at 100 C in well-formulated nitrate emulsions.
The technology of emulsification which makes it possible to prepare stable oil
continu-
ous emulsions with low content of oil, will be applicable to compositions
containing
diesel oils, mineral oils, olefins or esters and a long range of brine
compositions. The
essential point is to apply a surface active substance which has a high
affinity with
respect to the oil-water interface and a low mobility at the interface. A more
stable
emulsion will be the result. Various emulsifiers of relatively high molecular
weight and
of the right hydrophilic-lipophilicbalance (HLB) may be applied.
Description of the Preferred Embodiment
The preferred emulsifiers for this application are of high molecular weight -
the molecu-
lar weight being in the range from 500 up to 5000 - and the preferred
emulsifiers will
have a HLB-value in the range from 2 up to approximately 8. The hydrophobic
part of
the emulsifier molecule consisting of a long branched or bulky alkyl group.
The hydro-
philic, polar group of the molecule having an amine, amid, polyol or
polyoxyalkylene
functionality. The hydrophobic part of such an emulsifier will occupy a larger
volume
than the hydrophilic part. The result of this will be a preferred curvature of
the oil-water
interface being convex towards the oil phase and the emulsifier will therefore
be
suitable for stabilizing water-in-oil emulsions. Mixtures of emulsifiers may
also be used
to obtain optimal performance.
Examples of emulsifiers suitable for this application are polyisobutenyl
succinic
anhydride derivatives and polyalkylene glycols of various composition. The
hydropho-
bic part of these emulsifiers consists of either a fairly long polyisobutylene
sequence or
a bulky oligomer of alfa-olefines. The hydrophilic part will either be a
derivative of
succinic acid or a polyethylene glycol sequence.
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Other types of complex, high molecular weight emulsifiers may be used for the
purpose
described, provided that they have a lipophilic-hydrophilic balance suitable
for stabilis-
ing water-in-oil emulsions. Poly glycerol esters of different kinds i.e.
esters with inter-
esterifed ricinoleic acid or esters of dimerised soya bean oil will give
stable emulsions.
Oil-continuous emulsions of high stability and of high density can be prepared
contain-
ing as little as 6 weight % of oil, i.e. an oil to brine ratio of 6:94 by
weight. The aqueous
phase being..a concentrated solution of nitrate, choride, bromide, tungstate.
acetate or
formate, or mixtures thereof. To be used in a drilling fluid formulation a
water-in-oil
emulsion will contain an oil-to-brine ratio between 10:90 and 30:70,
preferentially a
ratio between 15:85 and 30:70 by weight. The emulsifier concentration in such
an
emulsion will be 0.2-5.0 % by weight active matter, preferentially 0.4-3.0
weight % of
the oil and brine mixture.
The following examples are submitted for the purpose of illustrating the
invention. The
properties of the different l:inds of oil being used in the examples are
listed in Table 1
below:
Table 1.
Oil Density at 20 C Viscosity at
(kg/dm3) 50 C being
Newtonian
(mPas)
Base oil (Sipdrill 4/OTM) 0.82 3
Jafa-ester 2000 DFTM 0.86 5.5
Tecluiical white oil (Bayol 85TM) 0.85 12.5
Alpha olefin (Novatec BTM) 0.78 1.8
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EXAMPLE 1. - For comparison.
For the purpose of comparison an ordinary oil-based drilling fluid was
formulated based
on an emulsion containing an oil to brine ratio of approximately 70:30 by
weight. The
drilling fluid had the following composition : 286 g of base oil, 12 +6 g of
the emulsifi-
TM TM
ers Versavert PE and Versavert SE, 116 g of a calcium chloride brine of
density 1.18, 2
TM TM TM
g of Versavert F, 4 g of Versamod and 8 g of Bentone 128. The emulsifier
mixture was
dissolved in the oil by low speed stirring. The emulsion was then prepared at
room
temperature in a 1000 ml beaker by applying a turbo-mixer at 2000 rpm. The
total
mixing time was approximately 12 minutes.
The density of the emulsion was 0.89 measured at 20 C (Sample 1A). The
viscosity
TM
measured on a Bohlin CS Rheometer was shear thinning. The viscosity was
decreasing
from approximately 120 to 25 mPas as the shear rate was increased from 10 to
100 sec'
at 50 C. After addition of 13 g lime and 438 g of barite the density of the
drilling fluid
was 1.50 at 20 C (Sample 1B). The viscosity at 50 C was shear thinning from
approxi-
mately 250 to 60 mPas at shear rates 10-100 sec'. This drilling fluid (1B)
contains
approximately 50 % by weight of the weighting material (barite).
The storage stability of the drilling fluids were evaluated as excess oil
separating out on
top of the nuxtures at 20 and 80 C and is shown in Table 2:
Table 2.
Sample Excess oil after I day
(% of mixture)
20 C so C
1A 5 10-15
IB 2-4 10
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TM
Sample 1B was then treated for 10 minutes at 6000 rpm in a Silverson high
speed mixer.
TM
Measured on a Fann Rheometer at 50 C the drilling fluid had the following
characteris-
tics as shown in Table 3:
Table 3.
Plastic viscosity 30
Yield Point 9
Viscosity at 3 RPM 12
EXAMPLE 2. - Low oil emulsion containing different kinds of emulsifier.
A series of samples were prepared with low oil content to illustrate the
difference in
emulsion stability using different emulsifiers. The samples were prepared with
an oil to
brine ratio of 40:60 and 20:80 by weight. The oil being used in these samples
was an
isopropyl-ester of fatty acids (Jafa-ester 2000 DF) and the aqueous phase was
a mixed
nitrate brine of density 1.61. The emulsions were prepared at room temperature
in a
1000 ml beaker by applying a turbo-mixer at 1100 rpm for 3 minutes for
dispersion of
the brine in the oil phase. Emulsifiers of "low molecular weight" are
Versavert PE-SE
TM
and Span 80. Span 80 has a molecular weight of approximately 410. The
molecular
weight of the Versavert emulsifiers is estimated to be somewhat higher than
Span 80.
The molecular weight of the other 3 emulsifiers used in sample no. 2.5, 2.6
and 2.7 are
in the range of approximately 1000-4000 and all are illustrated in Table 4.
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Table 4.
Emulsion Emulsifier % of the oil-brine
mixture
2.1-2.2 Versavert PE+SE 2+1 and 3+1.5
2.3-2.4 Span 80 1 and 2
2.5 Anfomu12500 1.2
2.6 Elfacos E 200 1.2
2.7 Grinsted PGPR 90 1.2
5
Examination 1 hour after mixing :
Oil to water ratio 40:60 : Excess oil fairly quickly separated out on top of
the samples
no. 2.1-2.4 (40:60). The emulsions are not stable enough for measurement of
viscosity.
10 : The sample no. 2.5-2.7 (40:60) were more stable. The viscosity of the
emulsions
measured at 50 C using a Bohlin CS Rheometer was Newtonian and approximately
30
mPas at shear rates 10-100 sec'.
Oil to water ratio 20:80 : The samples no. 2.1-2.4 (20:80) were unstable, but
a measure-
ment of the viscosity could be done just after mixing. The viscosity of these
emulsions
was shear thinning. The viscosity was decreasing from approximately 3000 to
approxi-
mately 500 mPas as the shear rate was increased from 10 to 100 sec I at 50 C.
The
samples no. 2.5-2.7 (20:80) were much more stable. The viscosity at 50 C was
found to
.
be shear thinning from 250 to 100 mPas at shear rates 10-100 sec-1
Further storage of the samples with an oil to brine ratio of 20:80 :
Samples 2.1-2.4 (20:80) : Almost all the oil separated out of the emulsions as
excess oil
some hours after mixing. The emulsion remaining on the bottom of these samples
were
jelly and were not easily remixed with the excess oil.
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Samples 2.5-2.6 (20:80) : The emulsions were still in good condition after 1
day of
storage at room temperature. Only a very small fraction of the oil separated
out on top of
the emulsions. A somewhat larger part of the oil separated out (because of the
lower
viscosity) at 80 C. The excess oil could easily be redispersed. More oil will
separate out
by further storage. The higher temperature the faster will this process
proceed.
Sample 2.7 (20:80) : The emulsion was stable at room temperature, but not at
80 C.
The emulsion was after a few days at 80 C separated into an oil and an
aqueous phase.
The reason for this may be a low heat tolerance of the emulsifier being used
in this case
(a polyglycerol ester).
The density of sample 2.5-2.7 (20:80) was approximately 1.37 at 20 C.
EXAMPLE 3. - Low oil emulsions containing different levels of oil.
A series of samples were prepared with low oil content. The samples were
prepared
with an oil to brine ratio in the range from 6:94 to 22:78 by weight. The oil
being used
in these samples was a technical white oil (Bayol 85) and the aqueous phase
was a
calcium nitrate brine of density 1.52. The emulsions were prepared at 80 C in
a 1000 ml
beaker by applying a turbo-rnixer at 1100 rpm for 3 minutes for dispersion of
the brine
TM
in the oil phase. The emulsifier used was Mobilad C 267, a poly-isobutylene-
succinic
amide derivative with a molecular weight estimated to be in the range of 1000-
2000.
The emulsifier concentration was 1% of the oil-brine mixture.
The viscosity was measured 1 hour after mixing and the storage stability of
the
emulsions was evaluated as excess oil separating out on top of the mixture at
20 and 80
C . The results are shown in Table 5:
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Table 5.
Emulsion Oil-water Viscosity at 50 Excess oil after 1 day
ratio C (rnPas) and (% of mixture)
share rates
100-100 sec-1
20 C 80 C
3.1 6:94 16000-3400 not detectable
3.2 10:90 4000-1100 not detectable
3.3 14:86 1600-550 n.d. 3-4
3.4 18:82 700-330 1-2 10-12
3.5 22:78 350-210 6-8 15
The excess oil could easily be redispersed.
EXAMPLE 4. - Low oil emulsions containing different kinds of brine.
A series of samples were prepared with low oil content using different brines.
The
samples were prepared with an oil to brine ratio of 20:80 by weight. The oil
being used
in these samples was a technical white oil (Bayol 85). The aqueous phase was
various
brines :
4.1. Ca-nitrate solution of density 1.52
4.2. Ca-bromide solution of density 1.72
4.3. Ca and K-nitrate solution of density 1.61
The emulsions were prepared at 10 C in a 1000 ml beaker by applying a turbo-
nlixer at
1100 rpm for 3 minutes for dispersion of the brine in the oil phase. The
emulsifier used
was Mobilad C 267. The emulsifier concentration was 1% of the oil-brine
mixture. The
density was measured at 20 C. The viscosity was measured 1 hour after mixing
and the
storage stability of the emulsions was evaluated as excess oil separating out
on top of
the mixture at 20 and 80 C . The results are shown in Table 6:
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Table 6.
Emulsion Density Viscosity at 50 C Excess oil after I day
measured (mPas) and share rates (% of mixture)
at 20 C 10-100 sec-' 20 C 80 C
4.1 1.27 1 000 - 490 n.d. 5
4.2 1.37 450 - 260 2-4 10-12
4.3 1.32 800 - 410 1-2 8-10
The excess oil could easily be redispersed.
EXANIl'LE 5. - Low oil emulsions containing different kinds of brine.
A series of samples wpre prepared with low oil content using different brines.
The
samples were prepared with an oil to brine ratio of 20:80 by weight. The oil
being used
in these samples was an alpha olefin (Novatec B). The aqueous phase was
various
brines :
5.1. Ca-chloride (20%), density 1.18 measured at 20 C
5.2. Ca-chloride (40%), density 1.40
5.3. Ca-nitrate, density 1.52
5.4. Mixed Ca_(N03)2 (32.4%) and CaCI2 (19.5%), density 1.53
5.5. Mixed Ca(NQ3)a (46.1%) and KNO3 (15.1%), density 1.61
5.6. Mixed Ca(N03)2 (40%) and CaCb (18.9%), density 1.62
5.7. Mixed Ca(NO3)2 (31.2%)_ NaBr (10.6%) and CaBr2 (15.5%),
density 1.63
5.8. Ca-bromide, density 1.72
5.9. K-forrrtate,_ density 1 S 8
The emulsions were prepared at room temperature in a 1000 ml beaker by
applying a
turbo-mixer at 1100 rpm for 3 minutes for dispersion of the brine in the oil
phase. The
TM
emulsifier used was Anfomul 2500, a poly-isobutylene-succinic-acid derivative
(amide)
with a molecular weight around 1000. The emulsifier concentration (active
matter) was
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0.7 % of the oil-brine mixture. Included in the recipe was also 1 Io of
Bentone 128. The
organophilic clay was added during an additional time of mixing of 5 minutes.
The density and the viscosity was measured 1 hour after mixing and is shown in
Table
7:
Table 7.
Emulsion Density of emulsion Viscosity at 50 C (mPas)
measured at 20 C and share rates 10-100
sec l
5.1 1.06 920 - 310
5.2 1.18 500 - 200
5.3 1.24 230 - 105
5.4 1.26 360 - 150
5.5 1.31 300 - 150
5.6 1.31 200 - 100
5.7 1.31 270 - 145
5.8 1.34 250 - 135
5.9 1.25 600 - 150
As in the other examples a small fraction of the oil separated out on top of
the
emulsions. A somewhat larger part of the oil separated out at 80 C compared
with 20
C. The excess oil could easily be redispersed. More oil will separate- out by
further
storage. The rate of separation of excess oil will depend on the viscosity of
the emulsion
and the difference in density between the oil and the aqueous phase.
EXAMPLE 6. - Low oil emulsion formulated at a density of 1.5.
An oil-based drilling fluid based on an emulsion containing an oil to brine
ratio of
approximately 18:82 by weight, was prepared with the following composition :
108 g of
alpha olefin (Novatec B), 9.6 g of the emulsifier Anfomul 2500, 482.4 g of a
calcium
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nitrate brine of density 1.52, 1 g of Versavert F, 1 g of Versamod, 2 g of
Bentone 128,
0.5 g lime and 145 g of barite. The emulsifier was dissolved in the oil by low
speed
stirring. The emulsion was then prepared at 50 C in a 1000 ml beaker by
applying a
turbo-mixer at 2000 rpm. The total mixing time was approximately 16 minutes.
5
This drilling fluid contains 19.5 Wby weight of the weighting material
(barite). The
density was 1.49 at 20 C.
Measured on a Fann Rheometer at 80 C the drilling fluid had the following
characteris-
10 tics as shown in Table 8:
Table 8.
Plastic viscosity 56
Yield Point 29,5
Viscosity at 3 RPM 9
EXAMPLE 7. - Low oil emulsion formulated at a density of 1.5.
An oil-based drilling fluid based on an emulsion containing an oil to brine
ratio of
19.5:80.5 by weight, was prepared with the following composition : 117 g of
alpha-
olefin (Novatec B), 6 g of the emulsifier Anfomul 2500, 483 g of a calcium
nitrate brine
of density 1.52, 1 g of Versavert F, 1 g of Versamod, 2 g of Bentone.128, 6 g
lime and
180 g of barite. The emulsifier was dissolved in the oil by low speed
stirring. The
emulsion was then prepared at 20 C in a 1000 ml beaker by applying a turbo-
mixer at
2000 rpm. The total mixing time on the turbo-mixer was approximately 30
minutes,
followed by 10 minutes on a Silverson high speed mixer at 6000 rpm.
This drilling fluid contains approximately 23 % by weight of the weighting
material
(barite). The density was 1.52 at 20 C.
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Measured on a Fann Rheometer at 50 and 80 C the drilling fluid had the
following
characteristics as shown in Table 9:
Table 9.
50 C 80 C
Plastic viscosity 58 38
Yield Point 15 7
Viscosity at 3 rpm 3 4
EXAMPLE 8. - Low oil emulsion formulated at a density of 1.5.
An oil-based drilling fluid based on an emulsion containing an oil to brine
ratio of
20.5:79.5 by weight, was prepared with the following composition : 123 g of
alpha-
olefin (Novatec B), 6 g of the emulsifier Anfomu12500, 477 g of a calcium
nitrate brine
of density 1.52, 1 g of Versavert F, 1 g of Versamod, 3 g of Bentone 128, 6 g
lime and
180 g of barite. The emulsifier was dissolved in the oil by low speed
stirring. The
emulsion was then prepared at 20 C in a 1000 ml beaker by applying a turbo-
mixer at
2000 rpm. The total mixing time on the turbo-mixer was approximately 30
minutes,
followed by 10 minutes on a Silverson high speed mixer at 6000 rpm.
This drilling fluid contains approximately 23 % by weight of the weighting
material
(barite). The density was 1.51 at 20 C.
The drilling fluid sample was stored at room temperature for 2 weeks and then
re-dispersed for 10 minutes on a Silverson mixer at 6000 rpm.
Measured on a Fann Rheometer at 50 and 80 C the drilling fluid had the
following
characteristics as shown in Table 10 :
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Table 10.
50 C 80 C
Plastic viscosity 64 42
Yield Point 12.5 9.6
Viscosity at 3 rpm 7 6
EXAMPLE 9. - Low oil emulsions containing different kinds of emulsifier.
3 oil-based drilling fluids were prepared containing an oil to brine ratio of
24:76 by
weight. The composition was as follows : 144 g of alpha olefin (Novatec B),
emulsifier
(specified below), 456 g of a calcium nitrate brine of density 1.52, 0.5 g of
Versavert F,
1 g of Versamod, 2 g of Bentone 128, 3 g lime and 180 g of barite. The
emulsifier was
dissolved in the oil by low speed stirring. The emulsion was then prepared at
20 C in a
1000 ml beaker by applying a turbo-mixer at 2000 rpm. The total mixing time on
the
turbo-mixer was approximately 30 minutes, followed by 10 minutes on a
Silverson high
speed mixer at 6000 rpm.
Emulsifier :
A 6 gram Span 80 - Molecular weight approximately 410
B 4.8 g Versavert PE + 2.4 g Versavert SE - Molecular weight 500-1000
C 6 gram Grinsted PGPR 90 - Molecular weight 2500-3500
The drilling fluids contain approximately 23 % by weight of the weighting
material
(barite).
The viscosity was measured 1 hour after mixing on a Bohlin CS Reometer.
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Table 11.
Emulsion Density of fluid measured Viscosity at 50 C (mPas)
at 20 C and share rates 10-100
sec-1
A 1.45 7000 - 300
B 1.41 2000 - 140
C 1.45 2000 - 180
Stability at 20 and 80 C :
A Crelled, unsatisfactory structure, especially at 80 C
B Emulsion stable over at least 3 weeks at 80 C
C Emulsion stable over at least 3 weeks at 80 C
