Note: Descriptions are shown in the official language in which they were submitted.
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OIL BASED DRILLING FLUID AND COPOLYMER BASED ON STYRENE, LAURYL METHACRYLATE
AND (METH)ACRYLIC ACID
This invention relates to oil based drilling fluids and novel polymeric
products for use
therein particularly for improving the fluid loss properties.
It is standard practice to apply oil based drilling fluids during the drilling
of oil wells
and other bore holes in order to both lubricate the drill string and to
disperse and
carry cuttings away from the drill bit. It is known to use either water based
or oil
based drilling fluids for this purpose. Although water based drilling fluids
are widely
used they present certain disadvantages, which render them unsuitable in many
circumstances. These disadvantages include the tendency for such water based
fluids to disintegrate and disperse clay, dissolve salts and induce corrosion
of iron
components of equipment. In addition oil based drilling fluids offer improved
lubrication, higher boiling points and lower freezing points. It is desirable
that the oil
based drilling mud is formulated to achieve optimum performance.
The oil based drilling fluids may comprise as the oil component hydrocarbons
such
as diesel, alkyl benzenes, linear alpha olefins, internal olefins, linear
paraffins and
the like.
An oil based drilling fluid may contain water as a discontinuous phase in an
amount
up to 50% by weight. An oil based fluid can be formulated without water but
generally contains at least 10% water by weight. It is often preferred for the
oil
based drilling fluid to be formulated using 10 to 50% by weight water. In such
a fluid
the water would be emulsified throughout the oil continuous phase. Generally
oil
based drilling fluids containing dispersed water are known as invert emulsion
fluids.
Often the drilling fluid also comprises a theology modifying agent. This can
be a
synthetic or natural polymer that disperses and swells in the oil based fluid
thus
increasing its viscosity and enhancing the theological properties. It is
common
practice to include certain clays in oil based drilling fluids in order to
achieve the
appropriate theology for use.
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It is known to include surfactants, such as emulsifiers in oil based drilling
fluids. The
surfactant may be used to improve the stability of the dispersed water
droplets,
theology modification agent, other additives to the drilling fluid and to
disperse
cuttings and/or other extraneous solids that enter the drilling fluid during
the drilling
operation. Usually the main purpose of adding a surfactant is to stabilise the
dispersed water droplets.
It is common practice to apply other additives such a weighting agents. These
additives tend to be inorganic salts of high density, such as barium suphate
or other
barium salts. The function of weighting agents is to increase the density of
the
drilling fluid in order to ensure that the drilling fluid is not displaced by
aqueous fluids
entering the bore hole, thus ensuring that the fluid remains in the bore hole
around
the drill bit and drill string.
A problem that can occur during a drilling operation is the loss of the fluid
medium
from the drilling fluid. This is especially problematic when drilling through
rock with a
high degree of porosity. In such situations the liquid phase of the drilling
fluid filters
through porous zones in the rock depositing the dispersed solids of the
drilling fluid
as a filter cake. This is loss of fluid medium from a drilling fluid is
generally referred
to as fluid loss. Various fluid loss chemical additives have been proposed for
reducing this problem. For instance it is known to use a variety of styrenic
and
ethylenic polymer specifically for reducing fluid loss when using oil based
drilling
fluids. However, all of these known fluid loss additives do not significantly
reduce
fluid loss without adversely affecting the theology of the drilling fluid.
Therefore it
would be usual to apply fluid loss additives at dose that will achieve some
degree of
fluid loss without impairing the theology to the detriment of the drilling
operation.
It would therefore be desirable to provide an fluid loss additive for an oil
based
drilling fluid that provides a high degree of protection against fluid loss
and yet does
not significantly impair the theology of the fluid.
Thus in a first aspect of the present invention we provide an oil based
drilling fluid
comprising oil, a fluid loss additive, a theology modification agent,
optionally at least
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one surfactant, optionally water and optionally at least one weighting agent,
characterised in that the fluid loss additive is a polymer which is formed
from a
monomer blend comprising,
i) 50 to 90% by weight styrene,
ii) 10 to 50% by weight Coo-2o alkyl (meth)acrylate and
iii) 0 to 10% by weight (meth)acrylic acid.
It has been found that the fluid loss properties of oil based drilling fluids
are
surprisingly effective when the fluid loss additive contained therein is a
polymer
comprising these ratios of styrene, Coo-2o alkyl (meth)acrylate and optionally
(meth)acrylic acid. Particularly effective fluid loss properties are observed
in the oil
based drilling fluid when the polymer comprises 15 to 25% by weight, more
preferably 19 to 21 % by weight, most preferably 19 to 20% by weight C~o_2o
alkyl
(meth)acrylate. The preferred amount of styrene comprised in the polymer is 75
to
85% by weight, more preferably 79 to 81 % by weight, most preferably 79 to 80%
by
weight. The fluid loss additive can be a polymer that also comprises 0 to 10%,
preferably 0 to 5% by weight acrylic acid or methacrylic acid. Usually,
however, the
polymer according to this first aspect of the invention comprises 0 to about 1
(meth)acrylic acid. Preferably the polymer has an average molecular weight in
the
range 100,000 to 750,000, especially 150,000 to 500,000, most preferably
around
200,000.
Particularly improved fluid loss is achieved when the oil based drilling fluid
according
to the first aspect of the invention comprises a polymer that has been formed
from
styrene and lauryl methacrylate and optionally acrylic acid. This improvement
in fluid
loss is especially when the polymer comprises the lauryl methacrylate in an
amount
of 19 to 21 % most preferably 19 to 20% by weight, styrene in an amount of 79
to
81 % by weight, most preferably 79 to 80% by weight and acrylic acid in an
amount
of 0 to about 1 % by weight.
According to the first aspect of the invention the oil based drilling fluid
comprises the
polymer of styrene, C~o_2o alkyl (meth)acrylate and optionally (meth)acrylic
acid in a
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fluid loss reducing amount. Typically the amount of polymer would be the
minimum
amount to achieve maximum fluid loss properties. It would be possible to add
more
fluid loss reducing polymer than is specifically required to achieve maximum
fluid
loss prevention without being detrimental to the theology of the drilling
fluid.
However, excess fluid loss reducing polymer would not provide any additional
advantage to the fluid and thus would be wasteful.
Typically the oil based drilling fluid according to the first aspect of this
invention may
comprise as the fluid loss additive the polymer of styrene C~o_2o alkyl
(meth)acrylate
and optionally (meth)acrylic acid in an amount of up to 20 pounds polymer per
barrel
of drilling fluid (ppb) based on active polymer. Usually, however, the dose of
polymer
is not in excess of 10 ppb, preferably not more than 4 or 5 ppb and most
preferably
about 2 ppb.
It may be desired to include some water in the oil based drilling fluid in
order to
improve the theological properties. For instance the drilling fluid may
include 10 to
50% by weight water, preferably 20 to 30%. Where water is present in the oil
based
drilling fluid it preferably exists as small droplets which are uniformly
dispersed
throughout the oil based fluid.
It may also be desirable to add other chemical additives which enhance the
theology. For instance the oil based drilling fluid may comprise a theology
modifying
agent. This can be a synthetic or natural material, for instance a clay or
polymer that
disperses and swells in the oil based fluid. The theology modifying agent
would
normally work by increasing the viscosity and enhancing the theological
properties.
When a theology modifying agent is present it is preferably a clay which is
both oil
swellable and oil dispersible. Normally such a clay would be considered to be
organophilic in nature or at least rendered organophilic by chemical
treatment. Such
clays should be easily dispersible and swellable in the oil in order to
perform
effectively as a theology modification agent. Typically clay is an amine
modified clay
from the group of clays known as smectites, attapulgites or hectorites. The
theology
modifying agent may be added in an amount of up to 5% by weight of total
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composition. Preferably the amount of rheology modifying agent is 0.5 to 2% by
weight, more preferably 0.5 to 1 % by weight.
The oil based drilling fluid according to the first aspect of the invention
may
comprise a surfactant, for instance an emulsifier. The surfactant if used
would be
present in low concentrations but in sufficient quantities to assist in the
stabilisation
of the dispersed water droplets, rheology modification agent, other additives
to the
drilling fluid and to disperse cuttings and other extraneous solids that enter
the
drilling fluid during the drilling operation. Ideally the surfactant would
have a low
hydrophilic lipophilic balance (HLB) and should be soluble or miscible with
the oil
phase. The surfactant may be included in the drilling fluid in an amount up to
5% by
weight of total drilling fluid, preferably 0.5 to 3%, more preferably 1 to 2%.
A suitable
surfactant would tend to be a chemical compound that contains a large
lipophilic
moiety and a relatively smaller hydrophilic moiety. Typically any commercially
available surfactant which has a low HLB and is oil soluble or oil miscible
could be
used for this application.
It may be necessary to apply other chemical additives to the drilling fluid of
the first
aspect of the invention, such a weighting agents. Weighting agents tend to be
inorganic salts of high density, for instance barium sulphate or other barium
salts.
The weighting agent may be included in an amount of up to 50 or 60% by weight
of
total drilling fluid, preferably 10 to 45% by weight, more preferably 25 to
40%, most
preferably around 30 to 35% by weight.
In some applications the drilling fluid may be exposed to temperatures below
freezing, for instance as low as -4°C or -8°C or below, and it
is important in this
situation the fluid loss additive remains liquid and stable such that it can
still be
conveniently added to the drilling fluid.
In some situations the drilling operation may encounter high temperature
zones, for
instance temperatures of up to at least 400°F. In such circumstance it
would be
important that the drilling fluid still remains sufficiently fluid. This can
be particularly
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problematic when the bore hole also penetrates porous formations, since the
drilling
fluid would be required to function with no significant impairment of the
fluid loss
properties.
Typically the drilling fluid according to this first aspect of the invention
can be
prepared by mixing together the base hydrocarbon oil, optionally water,
optionally
emulsifiers, optionally rheology modifiers, optionally, weighting agents and
the
polymer of Cio-2o alkyl (meth)acrylate, styrene and optionally acrylic acid
polymeric
fluid loss additive. The fluid would normally be rendered homogenous by a
suitable
homogenising equipment, such as high speed mixers or passing the fluid mixture
through one or more fluid screens. Typically the mixing process may be carried
out
using mixing speeds of for instance up to 8,000 rpm, preferably 2,000 to 6,000
rpm.
The mixing process may be as long as 20 or 30 minutes, but can be shorter
times,
for instance 5 or 10 or even 15 minutes.
The second aspect of the invention is directed to the use of a polymer which
is
formed from a monomer blend comprising,
i) 50 to 90% by weight styrene,
ii) 10 to 50% by weight C~o_2o alkyl (meth)acrylate
iii) 0 to 10% by weight (meth)acrylic acid,
as a fluid loss additive in an oil based drilling fluid. The polymer may be
provided in
any convenient form by polymerisation of the monomers in the monomer blend.
For
instance it may be provided as a low solids solution in a suitable solvent by
polymerisation of a solution of the monomer blend in said solvent. Preferably
the
polymer is provided as an aqueous emulsion, with the polymer present in the
dispersed phase. Typically the aqueous emulsion can be prepared by well know
aqueous emulsion polymerisation techniques known in the art. For instance this
is
could be achieved by emulsifying the monomer blend into water containing a
suitable emulsifier and introducing initiators in order to effect
polymerisation.
Preferably the polymer has an average molecular weight in the range 100,000 to
750,000, especially 150,000 to 500,000, most preferably around 200,000.
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A further enhancement of the second aspect of the invention is the use of a
polymer
as fluid loss additive in an oil based drilling fluid, in which the polymer is
in the form
of a polymer in water emulsion and wherein the polymer is formed from a
monomer
blend comprising,
i) 50 to 90% by weight styrene,
ii) 10 to 50% by weight Coo-2o alkyl (meth)acrylate
iii) 0 to 10% by weight (meth)acrylic acid,
In one preferred form of this second aspect of the invention the polymer in
water
emulsion has a pour point of below -4°C, preferably -8°C or
less. Thus in this form
the emulsion further comprises a pour point depressing substance, for instance
glycol, diethylene glycol and the like. The pour point depressing substance if
present is included in the aqueous phase of the polymer in water emulsion, in
an
amount of up to 15% by weight of emulsion, preferably between 5 and 10%.
In a further form of the second aspect of the invention, the polymer in water
emulsion may be used in an oil based drilling fluid used at elevated
temperatures,
for instance up to up to 400°F. Thus in this form the performance of
the fluid loss
additive is maintained when drilling through high temperature subterranean
environments.
Preferably the second aspect of the invention is directed to the use as a
fluid loss
additive in an oil based drilling fluid a polymer that has been formed from
styrene
and lauryl methacrylate and optionally acrylic acid. More preferably is the
use of a
polymer comprising lauryl methacrylate in an amount of about 19 to 21 % by
weight,
preferably 19 to 20%, styrene in an amount of about 79 to 81 % by weight,
preferably 79 to 80% by weight and acrylic acid in an amount of 0 to about 1 %
by
weight.
The third aspect of the invention is directed to a polymer which is formed
from a
monomer blend comprising,
WO 00/78891 CA 02376585 2001-12-04 pCT/EP00/05174
i) 75 to 85% by weight styrene,
ii) 15 to 25% by weight lauryl methacrylate
iii) 0 to 5% by weight (meth)acrylic acid.
Preferably the polymer is formed from a monomer blend comprising 19 to 21 % by
weight, most preferably 19 to 20% by weight lauryl (meth)acrylate. The
preferred
amount of styrene comprised in the polymer is 79 to 81 % by weight, most
preferably
79 to 80% by weight. The fluid loss additive can be a polymer that also
comprises 0
to 5% by weight acrylic acid or methacrylic acid. Usually, however, the
polymer
according to this first aspect of the invention comprises 0 to about 1 %
(meth)acrylic
acid. More preferably is the use of a polymer comprising lauryl methacrylate
in an
amount of 19 to 21 % by weight, most preferably 19 to 20% by weight, styrene
in an
amount of 79 to 81 % by weight, most preferably 79 to 80% by weight and
acrylic
acid in an amount of 0 to about 1 % by weight. Preferably the polymer has an
average molecular weight in the range 100,000 to 750,000, especially 150,000
to
500,000, most preferably around 200,000. The polymer can be prepared by
subjecting the monomer blend to suitable polymerisation conditions selected
from
the group consisting of introducing thermal initiators and irradiation of the
monomer
One especially preferred form of this third aspect of the invention comprises
the
polymer in the form of a polymer in water emulsion. Thus according to this
preferred
form of the invention the polymer is prepared by aqueous emulsion
polymerisation.
Typically the polymer in water emulsion may be prepared by any standard
aqueous
emulsion polymerisation technique For instance the monomer blend would be
emulsified in an aqueous continuous phase containing emulsifier and then
subjected to polymerisation conditions.
In a preferable form the polymer of lauryl methacrylate, styrene with
optionally
acrylic acid in the form of a polymer in water emulsion is further
characterised by
having a pour point of below -4°C, preferably -8°C or less. In
this preferred form
the polymer in water emulsion further comprises a pour point depressant, for
instance glycol, diethylene glycol and the like. The pour point depressant may
be
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present in an amount up to 15% by weight based on total weight of emulsion,
preferably 5 to 15% by weight.
According to the fourth aspect of this invention a process is provided for
making a
polymer of styrene, lauryl methacrylate and optionally acrylic acid comprising
the
steps of,
a) forming a monomer blend comprising,
i) 75 to 85% by weight styrene,
ii) 15 to 25% by weight lauryl methacrylate
iii) 0 to 5% by weight (meth)acrylic acid.
b) subjecting the monomer blend of step a) to polymerisation conditions,
selected from the group consisting of introducing redox initiators,
introducing
thermal initiators and irradiation using ultraviolet radiation.
The process may also be applied to any of the preferred embodiments of the
third
aspect of the invention.
The polymer according to this fourth aspect of the invention may be prepared
by any
known convenient means. For instance the polymer may be prepared by forming a
solution for the monomer blend, comprising styrene and lauryl (meth)acrylate
and
optionally (meth)acrylic acid in a suitable solvent and polymerisation the
monomer
blend by the use of a suitable inititiator system, for instance by adding
redox and/or
thermal initiators or irradiation using ultraviolet radiation. Preferably the
polymer has
an average molecular weight in the range 100,000 to 750,000, especially
150,000 to
500,000, most preferably around 200,000.
Preferably the polymer is prepared as an aqueous emulsion polymer, by
emulsifying
the monomer blend, comprising styrene, lauryl (meth)acrylate and acrylic acid
into
water which contains a suitable emulsifying agent, for instance an alkyl
phenol
ethoxylate. Polymerisation may be initiated by the use of suitable redox
initiators,
thermal initiators or radiation, such as ultraviolet radiation. Alternatively
the
emulsified monomer and initiator system may be added slowly to a vessel
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containing water into which has been dissolved further initiator and/or
emulsifier.
When the polymer is prepared by aqueous emulsion polymerisation the polymer
may be present in an amount from 10 to 50%, by weight, preferably 20 to 45% by
weight, most preferably 30 to 40%. The resultant polymer emulsion polymer may
have an average particle size typically in the range of 80 to 150nm,
preferably
between 100 and 130nm, most preferably around 120nm.
Preferably the polymer of styrene, lauryl methacrylate and optionally acrylic
acid is
prepared by emulsifying the monomer blend into water and then subjecting it to
aqueous emulsion polymerisation. Preferably the process is carried out by
slowly
adding the emulsified monomer to a vessel containing water. A suitable
initiator
system may be used to initiate the polymerisation. For instance the water held
in the
vessel to which the emulsified monomer is being added is may contain dissolved
initiator. Preferably at least some of the initiator is added slowly to the
vessel
containing water simultaneously with the addition of emulsified monomer.
The following example serve to illustrate the invention.
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Example 1
Preparation of Polymer
A monomer emulsion is prepared by mixing 304g Styrene , 76g Lauryl
Methacrylate and 3.8g Acrylic acid with 243.7g water into which has been
dissolved
34.2g Ethylan HA ( Nonyl Phenol 35 mole Ethoxylate ) using a Silverson mixer.
An initiator solution is prepared by dissolving 0.92g ammonium persulphate in
50g
deionised water.
The monomer emulsion and initiator solution are added over 3 hours
and 3.5 hours respectively to a 2 litre stirred reactor containing 339.6g
deionised
water, 3.8g Ethylan HA and 0.23g ammonium persulphate degassed with nitrogen.
The temperature is maintained at 85°C thoughout the addition period.
The reactor
contents are held for a further 1 hour at 85°C after which the contents
are cooled
and filtered. The resultant white emulsion polymer has a solids content of
40.0% , a
pH of 3.5 and a particle size of 120nm.
Example 2
Preparation of Bulk Drillings Fluid 1
The components of drilling fluid 1, shown in Table 1, are blended together
using a
Silverson mixer set at 6000 rpm using a square holed head.
Table 1
Components of Drilling Amount
Fluid 1
Ultidrill Base Fluid 818.4g
Emul HT ' 84.0g
Truvis HT 30.0g
Lime 72.0g
CaC12.2H20 148.7g
Water 269.78
Barite 2328.58
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Notes
1 Ultidrill Base Fluid is a commercially available base oil
2 Emul HT is a commercially available emulsifier
3 Truvis HT is a commercially available non-polar oil viscosifier
Preparation of Bulk drilling fluid 2
The components of drilling fluid 2, shown in Table 2, are blended together
using a
Silverson mixer set at 6000 rpm using a square holed head.
Table 2
Components of drilling Amount
fluid
2
Low toxicity low aromatic498.528
paraffinic mineral Oil
Kleemul '' 10.08
Kleemul 50 '' 30.08
Lime 24.08
Emul Vis" 8.0g
CaCl2 brine 474.968
Water 217.88
Emul Lift A ' 1.28
Barite 640.68
Notes
4 Kleemul is a commercially available emusifier
Kleemul 50 is a commercially available emulsifier
6 Emul Vis is a commercially available oil theology modification agent
7 Emul Lift is a commercially available emulsifier
Example 3
Testing of Drillings Fluid 1
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Four 350 ml aliquots of the bulk drilling fluid 1 are taken. The polymer
produced in
example 1 is applied to three of the aliquots at doses of 1, 2 and 4 pounds
per
barrel, based on the weight of active polymer. The polymer is mixed into the
drilling
fluid for a further 15 minutes.
The Fann rheologies of the unaged drilling fluids were measured at
120°F.
The drilling fluids are then hot rolled for 16 hours at 400°F under
250psi pressure
before cooling and remeasuring the Fann rheologies at 120°F.
The HTHP (High Temperature High Pressure) fluid loss measurements are then
taken at 350°F under 500psi differential pressure.
The results are shown in Table 3
Table 3
Dose of Rheology Rheology HTHP
before after
hot hot
rolling rolling
Polymer PV YP AV 10s PV YP AV 1 Fnid
A Gel Os joss
(ppb-active) Gel
0 28 20 38 10 33 3 34.5 4 45
1 31 20 41 10 28 4 30 4 10
2 32 19 41.5 10 49 12 55 6 9
3 31 19 40.5 10 36 6 39 4 8
4 33 21 43.5 11 63 14 70 7 8
PV is Plastic Viscosity
YP is Yield Point
AV is Apparent Viscosity
10s Gel is a measurement of gel strength after 10 seconds.
Testing of Drilling Fluid 2
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Five 350 ml aliquots of bulk drilling fluid 2 are taken. The polymer produced
in
example 1 is applied to four aliquots at doses of 1, 2, 3 and 4 pounds per
barrel,
based on the weight active polymer. The test of drilling fluid 1 is repeated
for each of
the aliquots of drilling fluid 2 except that the fluids are hot rolled at
250°F and the
HTHP fluid loss measurements are taken at 250°F. The results are shown
in Table
4.
Table 4
Dose of Rheology Rheology HTHP
before after
hot hot
rolling rolling
Polymer PV YP AV 10s PV YP AV 10s Fnid
A Gel Gel joss
(ppb-active) ~
0 23 9 27.5 4 29 18 38 8 5.2
1 28 9 32.5 5 29 16 37 9 1.6
2 30 10 35 5 35 17 43.5 8 1.2
4 32 10 37 6 38 16 46 8 0.8
The results show that the inclusion of Polymer A into the drilling fluids
improves fluid
loss without impairing the rheological performance of the fluids to any
significant
extent.
