Note: Descriptions are shown in the official language in which they were submitted.
CA 02684230 2009-10-30
WATER FLOODING METHOD FOR SECONDARY
HYDROCARBON RECOVERY
TECHNICAL FIELD
A method of secondary hydrocarbon recovery of the type which comprises passing
a
water flooding composition through a subterranean formation containing a
hydrocarbon deposit.
BACKGROUND OF THE INVENTION
The first stage of hydrocarbon production is known as primary hydrocarbon
recovery. In primary hydrocarbon recovery, energy (eg., pressure or potential
energy) within the
subterranean formation is utilized to displace hydrocarbons from the
subterranean formation, into a
production wellbore, and ultimately to the earth's surface. Primary
hydrocarbon recovery may be
assisted by artificial lift systems such as pumps or gas lift installations.
The second stage of hydrocarbon production is known as secondary hydrocarbon
recovery. In secondary hydrocarbon recovery, an external fluid in gas or
liquid form is injected
into the subterranean formation through one or more injection wells. The
external fluid typically
functions to displace hydrocarbons through the subterranean formation toward
one or more
production wellbores through which the hydrocarbons may be produced to the
earth's surface. The
external fluid may also assist in maintaining or increasing the pressure in
the subterranean
formation.
One form of secondary hydrocarbon recovery is water flooding. In water
flooding,
a water flooding composition is injected into the subterranean formation as
the external fluid. The
water flooding composition is typically comprised of water and one or more
other materials which
function to provide desirable properties to the water flooding composition.
Such other materials
may include a thickener to increase the viscosity of the water flooding
composition and thereby
decrease the mobility of the water flooding composition through the
subterranean formation.
Suitable thickeners for use in water flooding compositions are often polymers,
with the result that
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water flooding using a water flooding composition containing a thickener is
sometimes referred to
as polymer flooding.
Mobility of a fluid is defined as the ratio of permeability (of the medium
through
which the fluid is passed) to viscosity (of the fluid). Mobility is therefore
a function of both the
properties of the fluid and the properties of the environment in which the
fluid is located.
A hydrocarbon deposit in a subterranean formation may exhibit a relatively
high
viscosity and a relatively low mobility. If a water flooding composition has a
relatively lower
viscosity and a relatively higher mobility than the hydrocarbon deposit, the
water flooding
composition may tend to move through the hydrocarbon deposit or to bypass the
hydrocarbon
deposit so that the hydrocarbon deposit is not effectively displaced toward
the production wellbore
or wellbores by the water flooding composition.
This phenomenon is described as "fingering", and results in a reduction in the
"sweep efficiency" of the water flooding procedure. Sweep efficiency is
defined as the ratio of the
volume of the subterranean formation which is actually contacted by the water
flooding
composition during a water flooding procedure to the volume of the
subterranean formation which
is available to be contacted by the water flooding composition during the
water flooding procedure.
Fingering can be reduced and sweep efficiency can be increased by including a
thickener in the water flooding composition in order to increase its viscosity
and thus reduce its
mobility through the subterranean formation.
U.S. Patent No. 4,529,523 (Landoll) describes a water flooding method for
enhanced recovery of oil from a subterranean oil-containing formation using a
water flooding
medium which includes a thickener and which may also include a compatible
surfactant.
In Landoll, it is suggested that the problems which may limit the
effectiveness of
water flooding procedures include high mobility of the water flooding medium,
immiscibility of
the water flooding medium with oil, and lack of durability of the water
flooding medium when
exposed to salts/brine, shear forces, heat, and/or biological activity. These
problems are stated in
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Landoll to be overcome by the use of a water flooding medium which contains a
hydrophobically
modified, water-soluble polymer as the thickener.
In Landoll, the thickening polymer includes a polymer backbone which may be
any
nonionic, water soluble polymer including poly(acrylamide), a cellulose ether,
poly(ethylene
oxide), a natural polysaccharide gum, and poly(vinyl alcohol). The nonionic
character of the
backbone is stated to be important in promoting salt tolerance. Operable
polymers in Landoll have
molecular weights of about 50,000 to 1,000,000. Preferable molecular weights
in Landoll are from
about 150,000 to about 800,000.
The polymer backbone in Landoll is modified by the incorporation of small
amounts of long chain alkyl groups. It is stated in Landoll that in general,
the alkyl modifier
contains from about 8 to about 25 carbons, preferably from about 16 to about
25 carbons. The
alkyl modifier is stated to be present in an amount from about 0.2 percent by
weight to the amount
which makes the polymer less than 1 percent soluble in water, or from about
0.2 to about 2.0
percent by weight of the polymer.
The concentration of the polymer in the water flooding medium is stated in
Landoll
to be from about 0.01 to 2.0 percent by weight, preferably from about 0.1 to
0.5 percent by weight.
In Landoll, the preferred polymer is hydrophobically modified hydroxyethyl
cellulose where the alkyl chain modifier is from about 8 to about 25 carbon
atoms in length.
The nonionic, hydrophobically modified, water-soluble polymers in Landoll are
described in Landoll as being especially well suited for use in polymer-water
flooding media
because they possess surface activity which may reduce or eliminate the use of
a separate
surfactant in water flooding procedures.
However, in addition to providing a water flooding composition which exhibits
a
suitable viscosity and mobility, another goal in formulating a water flooding
composition is to
provide a water flooding composition which is injectable through the
subterranean formation. A
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water flooding composition is injectable if it can pass through the
subterranean formation without
causing significant plugging of the subterranean formation.
A water flooding composition which exhibits a suitable viscosity and mobility
may
be unsuitable for use as a water flooding composition if it does not exhibit
sufficient injectability.
Injectability of the water flooding compositions is not a consideration which
appears to be
addressed in Landoll.
SUMMARY OF THE INVENTION
References in this document to dimensions, to orientations, to operating
parameters,
to ranges, to lower limits of ranges, and to upper limits of ranges are not
intended to provide strict
boundaries for the scope of the invention, but should be construed to mean
"approximately" or
"about" or "substantially", within the scope of the teachings of this
document, unless expressly
stated otherwise.
The present invention is directed at methods of secondary hydrocarbon recovery
of
a type which comprises passing a water flooding composition through a
subterranean formation
containing a hydrocarbon deposit. The present invention is more specifically
directed at
improvements in the composition of the water flooding composition.
The water flooding compositions of the invention are comprised of water and a
thickening polymer, wherein the thickening polymer is comprised of a
hydroxyethyl cellulose
backbone polymer having a molecular weight of between about 1,000,000 and
about 2,000,000
and a hydrophobic modifier comprised of an alkyl hydrocarbon based material.
The hydroxyethyl cellulose backbone polymer may be described as "HEC". The
thickening polymer may be described as hydrophobically modified HEC or as
"HMHEC".
The hydroxyethyl cellulose backbone polymer has a hydroxyethyl molar
substitution or "MS", which is the average number of moles of hydroxyethyl
which are
incorporated in the polymer per anhydroglucose unit of the cellulose. In some
embodiments, the
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MS of the hydroxyethyl cellulose backbone polymer is at least about 0.5. In
some embodiments,
the MS of the hydroxyethyl cellulose backbone polymer is at least about 1. In
some embodiments,
the MS of the hydroxyethyl cellulose backbone polymer is at least about 2. In
some embodiments,
the MS of the hydroxyethyl cellulose backbone polymer is between about 2 and
about 2.5. In some
embodiments, the MS of the hydroxyethyl cellulose backbone polymer is about
2.5.
In some embodiments, the hydroxyethyl cellulose backbone polymer may be
comprised of a combination of different hydroxyethyl cellulose (HEC) polymers.
The alkyl hydrocarbon based material of the hydrophobic modifier may be
comprised of any alkyl group and/or substituted alkyl group or any combination
of alkyl groups
and/or substituted alkyl groups. In some embodiments, the alkyl hydrocarbon
based material of the
hydrophobic modifier may be comprised of one or more alkyl groups and/or
substituted alkyl
groups which contain between about 10 and about 24 unsubstituted carbon atoms
per group. In
some embodiments, the alkyl hydrocarbon based material of the hydrophobic
modifier may be
comprised of one or more alkyl groups and/or substituted alkyl groups which
contain between
about 12 and about 18 unsubstituted carbon atoms per group.
As used herein, "unsubstituted carbon atom" means a carbon atom which is
directly
bonded only with hydrogen and/or carbon.
The water flooding compositions of the invention may be further comprised of
other substances in addition to water and the thickening polymer. As non-
limiting examples, the
water in the water flooding composition may be present in brine form (i.e.,
containing up to about
10 percent sodium chloride and/or other equivalent monovalent metal salts), as
hard brine (i.e.,
brine containing up to about 0.4 percent divalent and/or polyvalent metal ions
such as calcium or
magnesium), and/or may contain other substances and/or impurities. The water
flooding
composition may also be further comprised of other materials for enhancing the
properties of the
water flooding composition or the effectiveness of the water flooding
procedure.
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The water flooding compositions of the invention are formulated to have a
viscosity
and/or mobility which is compatible with the viscosity and/or mobility of the
hydrocarbon deposit
which is intended to be produced using the water flooding method.
As used herein, "viscosity" means dynamic viscosity and is expressed in pascal-
second (Pa.s) units at a shear of between about 7/s and about 10/s. As used
herein, permeability is
expressed in darcy (D) units. As used herein, mobility is the ratio of
permeability to viscosity,
where permeability is expressed in darcy (D) units and viscosity is expressed
in pascal-second
(Pa.s) units at a shear of between about 7/s and about 10/s.
In some embodiments, the viscosity of a water flooding composition may be
considered to be compatible with the viscosity of a hydrocarbon deposit if the
viscosity of the
water flooding composition is between about 2 mPa.s and about 100 mPa.s. In
some
embodiments, the viscosity of a water flooding composition may be considered
to be compatible
with the viscosity of a hydrocarbon deposit if the viscosity of the water
flooding composition is
between about 5 mPa.s and about 50 mPa.s. In some embodiments, the viscosity
of a water
flooding composition may be considered to be compatible with the viscosity of
a hydrocarbon
deposit if the viscosity of the water flooding composition is between about 5
mPa.s and about 40
mPa.s.
In some embodiments, the mobility of a water flooding composition may be
considered to be compatible with the mobility of a hydrocarbon deposit if a
ratio of the mobility of
the water flooding composition to the mobility of the hydrocarbon deposit is
no greater than about
100:1. In some embodiments, the mobility of a water flooding composition may
be considered to
be compatible with the mobility of a hydrocarbon deposit if a ratio of the
mobility of the water
flooding composition to the mobility of the hydrocarbon deposit is no greater
than about 50:1. In
some embodiments, the mobility of a water flooding composition may be
considered to be
compatible with the mobility of a hydrocarbon deposit if a ratio of the
mobility of the water
flooding composition to the mobility of the hydrocarbon deposit is no greater
than about 10:1. In
some embodiments, the mobility of a water flooding composition may be
considered to be
compatible with the mobility of a hydrocarbon deposit if the ratio of the
mobility of the water
flooding composition to the mobility of the hydrocarbon deposit is no greater
than about 2:1.
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The water flooding compositions of the invention are also formulated to be
injectable into the subterranean formation in which the water flooding method
is to be performed.
Generally, a water flooding composition may be considered to be injectable
into the subterranean
formation if it can pass through the subterranean formation without causing
significant plugging of
the subterranean formation. Plugging of the subterranean formation may result
when one or more
constituents of the water flooding composition become separated from the water
flooding
composition during the water flooding procedure and remain in the subterranean
formation after
the water flooding composition has passed through the subterranean formation.
Plugging of the subterranean formation may be indicated by a decrease in the
viscosity of the water flooding composition as it passes through the
subterranean formation and/or
by a decrease in the concentration of the thickening polymer in the water
flooding composition as
the water flooding composition passes through the subterranean formation.
Plugging of the
subterranean formation may also be indicated by a decrease in the permeability
of the subterranean
formation as the water flooding composition passes through the subterranean
formation.
In some embodiments, the injectability of a water flooding composition may be
evaluated by passing the water flooding composition through a permeable test
medium. In some
embodiments, the injectability of a water flooding composition may be
evaluated by considering
the properties of the water flooding composition before and after the water
flooding composition
has been passed through the permeable test medium. In some embodiments, the
injectability of a
water flooding composition may be evaluated by considering the properties of
the water flooding
composition at an upstream end of the permeable test medium and a downstream
end of the
permeable test medium. In some embodiments, the injectability of a water
flooding composition
may be evaluated by considering the permeability of the permeable test medium
before, during
and/or after the water flooding composition has been passed through the
permeable test medium.
The water flooding composition may have an initial viscosity at an upstream
end of
the permeable test medium and a final viscosity at a downstream end of the
permeable test
medium. In some embodiments, the injectability of the water flooding
composition may be
evaluated having regard to the initial viscosity of the water flooding
composition and the final
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viscosity of the water flooding composition. In some embodiments, the water
flooding
composition may be considered to be injectable if the final viscosity of the
water flooding
composition is greater than ninety percent of the initial viscosity of the
water flooding
composition.
The thickening polymer may have an initial concentration in the water flooding
composition at the upstream end of the permeable test medium and a final
concentration in the
water flooding composition at the downstream end of the permeable test medium.
In some
embodiments, the injectability of the water flooding composition may be
evaluated having regard
to the initial concentration of the thickening polymer and the final
concentration of the thickening
polymer. In some embodiments, the water flooding composition may be considered
to be
injectable if the final concentration of the thickening polymer in the water
flooding composition is
greater than ninety percent of the initial concentration of the thickening
polymer in the water
flooding composition.
In some embodiments, the properties of the permeable test medium may be
selected
to provide a reasonable simulation of the properties of the subterranean
formation in which the
water flooding composition may be used and/or may be selected so that they may
be correlated
with the properties of the subterranean formation empirically or in some other
manner.
In some embodiments, the permeable test medium may have specific dimensions
and/or properties. For example, in some embodiments, the permeable test medium
may have an
initial permeability of less than 10 darcies. For example, in some
embodiments, the permeable test
medium may have a length between the upstream end and the downstream end of
about ten
centimeters.
As indicated, the water flooding compositions of the invention are formulated
to
have a viscosity and/or mobility which is compatible with the viscosity and/or
mobility of the
hydrocarbon deposit which is intended to be produced using the water flooding
method, and are
formulated to be injectable into the subterranean formation in which the water
flooding method is
to be performed.
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The formulation of the water flooding compositions to achieve compatibility
with
the hydrocarbon deposit and injectability into the subterranean formation has
been determined to
be dependent upon one or more of the molecular weight of the hydroxyethyl
cellulose backbone
polymer, the composition of the hydrophobic modifier, the substitution level
of the hydrophobic
modifier in the thickening polymer, and the concentration of the thickening
polymer in the water
flooding composition.
It has been observed that the viscosity of a water flooding composition
according to
the invention tends to increase and the mobility of a water flooding
composition tends to decrease
as the molecular weight of the hydroxyethyl cellulose backbone polymer
increases. It has also
been observed that the injectability of a water flooding composition according
to the invention
does not appear to depend significantly upon the molecular weight of the
hydroxyethyl cellulose
backbone polymer.
It has been observed that the viscosity of a water flooding composition
according to
the invention tends to increase and the mobility of a water flooding
composition tends to decrease
as the number of unsubstituted carbon atoms in the hydrophobic modifier
increases. It has also
been observed that the injectability of a water flooding composition according
to the invention
tends to decrease as the number of unsubstituted carbon atoms in the
hydrophobic modifier
increases.
It has been observed that the viscosity of a water flooding composition
according to
the invention tends to increase and the mobility of a water flooding
composition tends to decrease
as the substitution level of the hydrophobic modifier in the thickening
polymer increases. It has
also been observed that the injectability of a water flooding composition
according to the invention
tends to decrease as the substitution level of the hydrophobic modifier in the
thickening polymer
increases.
It has been observed that the viscosity of a water flooding composition
according to
the invention tends to increase and the mobility of a water flooding
composition tends to decrease
as the concentration of the thickening polymer in the water flooding
composition increases. It has
also been observed that the injectability of a water flooding composition
according to the invention
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tends to decrease as the concentration of the thickening polymer in the water
flooding composition
increases.
The presence of substances other than water and the thickening polymer in the
water flooding compositions of the invention may also affect the mobility of
the water flooding
compositions and their injectability. By way of non-limiting examples, the
water in the water
flooding compositions of the invention may be present in brine form and/or as
hard brine.
It has been observed that the viscosity of a water flooding composition
according to
the invention tends to be higher and the mobility of a water flooding
composition according to the
invention tends to be lower if the water in the water flooding composition is
present in brine form
and/or as hard brine than if the water in the water flooding composition is
relatively pure. This
phenomenon is believed to be attributable to the enhancement of intramolecular
and intermolecular
association of the thickening polymers of the invention (as described below),
due to the presence
of ions in the water of the water flooding composition.
The thickening polymers of the invention may be described generally as
cellulosic
associating polymers. In such polymers, viscosity/mobility and injectability
are believed to be
dependent upon at least two different mechanisms of action. A first mechanism
of action is
"entanglement", which is believed to be attributable primarily to the
molecular weight of the
backbone polymer and which increases as the molecular weight (and thus the
length) of the
backbone polymer increases.
A second mechanism of action is "association", which is attributable to the
presence and hydrophobicity of the hydrophobic modifier. Without intending to
be bound by
theory, it is believed that the hydrophobic modifier groups interact or
"associate", both within a
single molecule of a thickening polymer and between adjacent molecules of a
thickening polymer.
The degree of association of a water flooding composition according to the
invention increases as the number of unsubstituted carbon atoms provided by
the hydrophobic
modifier increases. Increasing the number of unsubstituted carbon atoms may be
achieved by
increasing the "size" of the hydrophobic modifier, by increasing the
substitution level of the
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hydrophobic modifier in the thickening polymer, and/or by increasing the
concentration of the
thickening polymer in the water flooding composition. As the degree of
association increases, the
viscosity of a water flooding composition increases, the mobility of the water
flooding composition
decreases, and the injectability of the water flooding composition decreases.
As noted above, the molecular weight of the hydroxyethyl cellulose backbone
polymer does not appear to significantly affect the injectability of the water
flooding composition.
As a result, it would appear that while the viscosity and mobility of a water
flooding
composition according to the invention is dependent upon both the molecular
weight of the
hydroxyethyl cellulose backbone polymer and upon the degree of association of
the water flooding
composition, the injectability of a water flooding composition according to
the invention is
dependent primarily upon the degree of association of the water flooding
composition.
These phenomena facilitate the formulation of the water flooding compositions
of
the invention which provide an appropriate viscosity and/or mobility of the
water flooding
compositions while maintaining injectability of the water flooding
compositions.
For example, in comparison with the teachings of U.S. Patent No. 4,529,523
(Landoll), the water flooding compositions of the invention utilize relatively
higher molecular
weight backbone polymers having a molecular weight of between 1,000,000 and
2,000,000 (in
stark contrast with the molecular weight range of 50,000 to 1,000,000
specified in Landoll) to
increase the viscosity and reduce the mobility of the water flooding
composition, while utilizing a
modest degree of association derived from the hydrophobic modifier to provide
the beneficial
effects of the presence of the hydrophobic modifier without unduly
compromising the injectability
of the water flooding composition.
As a result of the above, in some embodiments, the invention relates to a
method of
secondary hydrocarbon recovery of a type which comprises passing a water
flooding composition
through a subterranean formation containing a hydrocarbon deposit, in which
the method is
characterized by the water flooding composition being comprised of water and a
thickening
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polymer, the thickening polymer having a concentration of between about 0.01
percent and about 1
percent by weight of the water flooding composition, the thickening polymer
comprising:
(a) a hydroxyethyl cellulose backbone polymer having a molecular weight of
between
about 1,000,000 and about 2,000,000; and
(b) a hydrophobic modifier in a substitution level in the thickening polymer
of between
about 0.1 percent and about 2 percent by weight of the thickening polymer,
wherein
the hydrophobic modifier is comprised of an alkyl hydrocarbon based material
containing between about 10 and about 24 unsubstituted carbon atoms per group;
wherein the water flooding composition is formulated to have a viscosity of
between 2 mPa.s and
100 mPa.s and to be injectable into the subterranean formation.
As a result of the above, in some embodiments, the invention relates to a
method of
preparing a water flooding composition for use in a method of secondary
hydrocarbon recovery of
a type which comprises passing the water flooding composition through a
subterranean formation
containing a hydrocarbon deposit, the method comprising:
(a) selecting a hydroxyethyl cellulose backbone polymer having a molecular
weight of
between 1,000,000 and 2,000,000;
(b) selecting a hydrophobic modifier comprised of an alkyl hydrocarbon based
material
containing between 10 and 24 unsubstituted carbon atoms per group;
(c) providing a thickening polymer comprising the hydroxyethyl cellulose
backbone
polymer and the hydrophobic modifier in a substitution level in the thickening
polymer of between 0.1 percent and 2 percent by weight of the thickening
polymer;
and
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(d) combining the thickening polymer with water to provide the water flooding
composition, wherein the thickening polymer has a concentration of between 0.1
percent and 1 percent by weight of the water flooding composition;
wherein the water flooding composition has a viscosity of between 2 mPa.s and
100 mPa.s and is
injectable into the subterranean formation.
In some embodiments, the molecular weight of the hydroxyethyl cellulose
backbone
polymer may be about 1,300,000.
In some embodiments, the concentration of the thickening polymer in the water
flooding composition may be between about 0.05 percent and 0.25 percent by
weight of the water
flooding composition. In some embodiments, the concentration of the thickening
polymer in the
water flooding composition may be between about 0.05 percent and about 0.2
percent by weight.
In some embodiments, the concentration of the thickening polymer in the water
flooding
composition may be about 0.2 percent.
In some embodiments, the alkyl hydrocarbon based material of the hydrophobic
modifier may contain between about 12 and about 18 unsubstituted carbon atoms
per group. In
some embodiments, the alkyl hydrocarbon based material of the hydrophobic
modifier may be
comprised of a plurality of materials. In some embodiments, the alkyl
hydrocarbon based material
of the hydrophobic modifier may be comprised of dodecyl tetradecyl glycidyl
ether.
As used herein, the substitution level of the hydrophobic modifier in the
thickening
polymer may be calculated by acetylating hydroxyl groups in the thickening
polymer with acetic
anhydride, analyzing the reaction products using proton nuclear magnetic
resonance (proton NMR
or H NMR) techniques, and integrating the acetyl CH3 and hydrophobic modifier
CH2 peaks from
the NMR spectra. The ratio of these two peaks indicates the level of
substitution of the
hydrophobic modifier in the thickening polymer. Higher molecular weight
thickening polymers
may be partially hydrolyzed by sonication prior to acetylation in order to
reduce their molecular
weights and their ultimate viscosity in deuterated chloroform (CDC13) for the
NMR analysis.
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In some embodiments, the substitution level of the hydrophobic modifier in the
thickening polymer may be between about 0.1 percent and about 1.5 percent by
weight of the
thickening polymer. In some embodiments, the substitution level of the
hydrophobic modifier in
the thickening polymer may be between about 0.4 percent and about 1.2 percent
by weight of the
thickening polymer. In some embodiments, the substitution level of the
hydrophobic modifier in
the thickening polymer may be between about 0.7 percent and about 1.2 percent
by weight of the
thickening polymer.
In some embodiments, the water of the water flooding composition may be
comprised of a brine solution. In some embodiments, the brine solution may be
comprised of
sodium chloride. In some particular embodiments, the concentration of the
brine solution may be
about 1 % by weight of the brine solution.
In a particular embodiment, the molecular weight of the hydroxyethyl cellulose
backbone polymer may be about 1,3000,000, the alkyl hydrocarbon based material
of the
hydrophobic modifier may be comprised of dodecyl tetradecyl glycidyl ether,
the substitution level
of the hydrophobic modifier may be between about 0.7 percent and about I
percent by weight of
the thickening polymer, and the concentration of the thickening polymer in the
water flooding
composition may be about 0.2 percent by weight of the water flooding
composition. In the
particular embodiment, the water of the water flooding composition may be
comprised of a brine
solution. In the particular embodiment, the brine solution may be comprised of
1 % sodium
chloride by weight of the brine solution.
BRIEF DESCRIPTION OF DRAWINGS
Embodiments of the invention will now be described with reference to the
accompanying drawings, in which:
Figure 1 is a table summarizing the compositions and properties of various
thickening polymers and water flooding compositions.
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Figure 2 is a table summarizing filtration test results for the water flooding
compositions in Figure 1, for filtration through both a Whatman # 1 filter and
a sandpack.
Figure 3 is a table summarizing the properties of sandpack cores which were
used
to conduct coreflood tests for selected water flooding compositions from
Figure 1.
Figure 4 is a table summarizing the properties of the oil which was used to
perform
coreflood tests for selected water flooding compositions from Figure 1.
Figure 5 is a table summarizing results of coreflood tests conducted using
selected
water flooding compositions from Figure 1.
Figure 6 is a schematic drawing of the apparatus used to conduct the sandpack
filtration tests which are summarized in Figure 2.
Figure 7 is a schematic drawing of the apparatus used to conduct the coreflood
tests
which are summarized in Figure 5.
Figure 8 is a graph depicting data obtained from coreflood tests comparing
injection
pressure in kPa as a function of throughput in pore volumes (PV) for the HPAM
and HMHEC
1206 water flooding compositions from Figure 1.
Figure 9 is a graph depicting data obtained from coreflood tests comparing
injection
pressure in kPa as a function of throughput in pore volumes (PV) for the HPAM
and HMHEC
0603 water flooding compositions from Figure 1.
Figure 10 is a graph depicting data obtained from coreflood tests comparing
oil
recovery as a percentage of original oil in place (OOIP) as a function of
throughput in pore
volumes (PV) for the HPAM, HMHEC 1206 and HMHEC 0603 water flooding
compositions from
Figure 1.
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Figure 11 is a graph depicting data obtained from sandpack filtration tests
comparing effective viscosity in the sandpack in mPa.s as a function of linear
velocity in feet per
day for selected water flooding compositions from Figure 1, in which the
aqueous component of
the water flooding compositions is comprised of I % NaCl.
Figure 12 is a graph depicting data obtained from sandpack filtration tests
comparing effective viscosity in the sandpack in mPa.s as a function of linear
velocity in feet per
day for selected water flooding compositions from Figure 1, in which the
aqueous component of
the water flooding compositions is comprised of either I % NaCl or hard brine.
DETAILED DESCRIPTION
The present invention is directed at a method of secondary hydrocarbon
recovery of
a type which comprises passing a water flooding composition through a
subterranean formation
containing a hydrocarbon deposit. One purpose of passing the water flooding
composition through
the subterranean formation is to displace the hydrocarbon deposit toward one
or more production
wellbores which are in fluid communication with the subterranean formation.
As a result, the method typically involves injecting the water flooding
composition
into one or more injection wellbores which are in fluid communication with the
subterranean
formation and which are separated from the production wellbores so that the
water flooding
composition can displace the hydrocarbon deposit toward the production
wellbores as it passes
through the subterranean formation.
The method may further comprise additional steps or procedures which are
performed before and/or after the water flooding composition is passed through
the subterranean
formation.
The invention is particularly directed at formulations for the water flooding
composition which result in the water flooding composition having a viscosity
and/or mobility
which is compatible with the viscosity and/or mobility of the hydrocarbon
deposit which is
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intended to be produced from the subterranean formation, and which result in
the water flooding
composition being injectable into the subterranean formation.
The water flooding compositions of the invention are comprised of water and a
thickening polymer. The thickening polymer is comprised of a hydroxyethyl
cellulose backbone
polymer and a hydrophobic modifier. The hydrophobic modifier is comprised of
an alkyl
hydrocarbon based material. The water flooding compositions of the invention
may be further
comprised of other materials and/or substances.
The formulations for water flooding compositions of the invention are based
upon a
number of considerations.
First, the formulations for water flooding compositions of the invention are
based
upon a consideration of the hydrocarbon deposit which is intended to be
produced from the
subterranean formation and upon the mobility of a water flooding composition
which must be
achieved in order for the mobility of the water flooding composition to be
compatible with the
mobility of the hydrocarbon deposit.
Generally, the ratio of the mobility of the water flooding composition to the
mobility of the hydrocarbon deposit is preferably no greater than about 100:1,
more preferably no
greater than about 50:1, more preferably no greater than about 10:1, or even
more preferably no
greater than about 2:1.
For many typical hydrocarbon deposits comprising oil, the viscosity of the
water
flooding composition is preferably between about 2 mPa.s and about 100 mPa.s,
more preferably
between about 5 mPa.s and about 50 mPa.s, or even more preferably between
about 5 mPa.s and
about 40 mPa.s.
Second, the formulations for water flooding compositions of the invention are
based
upon a consideration of the properties of the subterranean formation and upon
ensuring that a
water flooding composition will be injectable into the subterranean formation.
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Generally, a water flooding composition may be considered to be injectable if
it can
pass through the subterranean formation without causing significant plugging
of the subterranean
formation.
A water flooding composition may be assessed for injectability either during
performance of the water flooding method or by testing the water flooding
composition before it is
used in the performance of the water flooding method. In either case, indicia
of injectability or
lack of injectability may relate to changes in the composition or properties
of the water flooding
composition and/or the subterranean formation as the water flooding
composition is passed
therethrough.
One method for testing a water flooding composition before it is used in the
performance of the water flooding method comprises passing the water flooding
composition
through a permeable test medium.
One suitable permeable test medium is a sandpack having an upstream end and a
downstream end. One suitable test method is a sandpack filtration technique. A
sandpack used for
the sandpack filtration technique preferably has an initial permeability of
less than about 10 darcies
so that it is reasonably representative of a subterranean formation. In one
test configuration, a
sandpack has had an initial permeability of about 3 darcies. In one test
configuration, a sandpack
has had a length from the upstream end to the downstream end of about ten
centimeters.
The procedure for testing a water flooding composition in a sandpack comprises
passing the water flooding composition through the sandpack under constant or
varying conditions
of pressure and/or flowrate.
The water flooding composition will exhibit an initial concentration of the
thickening polymer at the upstream end of the sandpack and will exhibit a
final concentration of
the thickening polymer at the downstream end of the sandpack. If the final
concentration of the
thickening polymer is less than the initial concentration of the thickening
polymer, retention of the
thickening polymer in the sandpack, potential plugging of the sandpack, and a
lack of injectability
of the water flooding composition may be indicated.
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CA 02684230 2009-10-30
Generally, in order for a water flooding composition to be considered
injectable in
the sandpack test, the final concentration of the thickening polymer in the
water flooding
composition should be greater than ninety percent of the initial concentration
of the thickening
polymer in the water flooding composition.
The water flooding composition will exhibit an initial viscosity at the
upstream end
of the sandpack and will exhibit a final viscosity at the downstream end of
the sandpack. If the
final viscosity is less than the initial viscosity, retention of the
thickening polymer in the sandpack,
potential plugging of the sandpack, and a lack of injectability of the water
flooding composition
may be indicated.
Generally, in order for a water flooding composition to be considered
injectable in
the sandpack test, the final viscosity of the water flooding composition
should be greater than
ninety percent of the initial viscosity of the water flooding composition.
Third, the formulations for water flooding compositions of the invention are
based
upon a consideration of the effects of the following variables upon the
viscosity/mobility and the
injectability of a water flooding composition:
1. the molecular weight of the hydroxyethyl cellulose backbone polymer;
2. the composition of the hydrophobic modifier;
3. the substitution level of the hydrophobic modifier in the thickening
polymer; and
4. the concentration of the thickening polymer in the water flooding
composition.
Fourth, the formulations for water flooding compositions of the invention are
based
upon a consideration of the salt and/or brine conditions which the water
flooding compositions
may be exposed to, resulting either from the water from which the water
flooding compositions are
prepared or from the environment to which the water flooding compositions may
be exposed.
The thickening polymers of the invention may be described generally as
cellulosic
associating polymers. The formulations for water flooding compositions
according to the
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invention are based upon a consideration of theories relating to the
mechanisms of action upon
which the viscosity/mobility and injectability of a water flooding composition
may be dependent.
In particular these properties of water flooding compositions are believed to
be
dependent upon "entanglement" as a first mechanism of action and "association"
as a second
mechanism of action.
Entanglement is believed to be attributable primarily to the molecular weight
of the
hydroxyethyl cellulose backbone polymer and appears to affect only the
viscosity/mobility of a
water flooding composition. The degree of entanglement increases as the
molecular weight of the
hydroxyethyl cellulose backbone polymer increases, thereby resulting in an
increase in the
viscosity and a decrease in the mobility of a water flooding composition.
Association is believed to be attributable to the presence and hydrophobicity
of the
hydrophobic modifier and appears to affect both the viscosity/mobility of a
water flooding
composition and the injectability of a water flooding composition. The degree
of association
increases as the number of unsubstituted carbon atoms provided by the
hydrophobic modifier
increases. The number of unsubstituted carbon atoms provided by the
hydrophobic modifier may
be increased by increasing the "size" of the hydrophobic modifier, by
increasing the substitution
level of the hydrophobic modifier in the thickening polymer, and/or by
increasing the
concentration of the thickening polymer in the water flooding composition.
The goal in formulating the water flooding compositions of the invention is to
increase the viscosity and thus reduce the mobility of the water flooding
composition so that the
viscosity/mobility is compatible with the hydrocarbon deposit, while
simultaneously maintaining
an acceptable injectability of the water flooding composition in the
subterranean formation.
As noted above, an increase in viscosity/reduction in mobility of a water
flooding
composition can be achieved by increasing entanglement of the thickening
polymer and/or by
increasing the degree of association of the water flooding composition.
However, increasing
viscosity/reducing mobility of the water flooding composition by increasing
the degree of
association of the water flooding composition will simultaneously result in a
decrease in the
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CA 02684230 2009-10-30
injectability of the water flooding composition. Increasing the
viscosity/reducing mobility of the
water flooding composition by increasing the entanglement of the thickening
polymer appears to
have no significant effect upon the injectability of the water flooding
composition.
As a result, a target viscosity/mobility of a water flooding composition can
be
achieved by a combination of the effects of entanglement and association. As
the degree of
entanglement increases, the degree of association may decrease in order to
achieve the target
viscosity/mobility. Conversely, as the degree of entanglement decreases, the
degree of association
must increase in order to achieve the target viscosity/mobility.
However, each particular hydrophobic modifier will exhibit a maximum degree of
association, above which the water flooding composition will not be
injectable. More particularly,
for any particular hydrophobic modifier, increasing the substitution level of
the hydrophobic
modifier in the thickening polymer and/or increasing the concentration of the
thickening polymer
in the water flooding composition beyond an association limit will result in
the water flooding
composition not being injectable.
As a result, the association limit of each particular hydrophobic modifier
will
determine the minimum amount of entanglement (i.e., the minimum molecular
weight of the
hydroxyethyl cellulose backbone polymer) which is required for achieving the
target
viscosity/mobility for the water flooding composition while simultaneously
maintaining the
injectability of the water flooding composition.
Fifth, the formulations for water flooding compositions of the invention are
based
upon considerations of cost and availability for different hydroxyethyl
cellulose backbone polymer
candidates and different hydrophobic modifier candidates.
Other or additional considerations may apply in particular circumstances
involving
the practice of the invention.
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CA 02684230 2009-10-30
Having regard to the considerations outlined above, it has been determined
that the
range of molecular weights for hydroxyethyl cellulose backbone polymers which
are suitable for
use in the invention is between about 1,000,000 and about 2,000,000.
If the molecular weight of the hydroxyethyl cellulose backbone polymer is less
than
about 1,000,000, the degree of association for the hydrophobic modifier which
is required to
achieve a viscosity/mobility which is compatible with the hydrocarbon deposit
may result in a
water flooding composition which is not injectable.
As the molecular weight of the hydroxyethyl cellulose backbone polymer
increases
from 1,000,000 and approaches 2,000,000, the backbone polymer may become more
difficult to
obtain and the cost of the backbone polymer may become prohibitive. As a
result, the practical
upper limit of the molecular weight of the hydroxyethyl cellulose backbone
polymer may be less
than 2,000,000. In some embodiments, depending upon availability and cost, the
preferred upper
limit of the molecular weight of the hydroxyethyl cellulose backbone polymer
may be about
1,500,000.
The following general guidelines have therefore been established for the
formulation of water flooding compositions which may have a viscosity/mobility
which is
compatible with a hydrocarbon deposit and which may be injectable into a
subterranean formation:
(a) the hydroxyethyl cellulose backbone polymer has a molecular weight of
between
about 1,000,000 and about 2,000,000;
(b) the hydrophobic modifier is comprised of an alkyl hydrocarbon based
material
containing between about 10 and about 24 unsubstituted carbon atoms per group,
or
more preferably between about 12 and about 18 unsubstituted carbon atoms per
group;
(c) the substitution level of the hydrophobic modifier in the thickening
polymer is
between about 0.1 percent and about 2 percent by weight of the thickening
polymer,
or preferably between about 0.1 percent and about 1.5 percent by weight of the
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thickening polymer, or even more preferably between about 0.4 percent and
about
1.2 percent by weight of the thickening polymer, or even more preferably
between
about 0.7 percent and about 1.2 percent by weight of the thickening polymer;
and
(d) the concentration of the thickening polymer in the water flooding
composition is
between about 0.01 percent and about 1 percent by weight of the water flooding
composition, or more preferably between about 0.05 percent and about 0.25
percent
by weight of the water flooding composition.
The water flooding compositions are preferably formulated within the ranges
set out
above to achieve a target viscosity of between about 2 mPa.s and about 100
mPa.s, more preferably
between about 5 mPa.s and about 50 mPa.s, or even more preferably between
about 5 mPa.s and
about 40 mPa.s and/or to achieve a ratio of the mobility of the water flooding
compositions to the
mobility of the hydrocarbon deposit of no greater than about 100:1, more
preferably no greater
than about 50:1, more preferably no greater than about 10:1, or even more
preferably no greater
than about 2:1.
The water flooding compositions are formulated within the ranges set out above
to
achieve injectable water flooding compositions, as assessed during performance
of the water
flooding method or by testing the water flooding compositions before they are
used in the water
flooding method.
In many applications of the invention, achieving injectability of the water
flooding
compositions may follow from formulating the water flooding compositions in
accordance with
the above ranges and target viscosities. In some applications of the
invention, achieving
injectability of the water flooding compositions may require some modification
of the formulations
of the water flooding compositions within the above ranges and target
viscosities.
The thickening polymers of the invention may be prepared by using any suitable
method, including the specific methods described in U.S. Patent No. 4,228,277
(Landoll), in U.S.
Patent No. 4,529,523, and other methods known in the art.
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The water flooding compositions of the invention may be prepared by mixing the
thickening polymer with water and with any other suitable materials and/or
substances. The water
may be present in relatively pure form, in brine form, as hard brine, and/or
may contain other
substances and/or impurities.
Example 1- Preparation of Thickening Polymers (HMHEC) from Hydroxyethyl
Cellulose (HEC)
A 500 mL round bottom flask was charged with 10 grams of hydroxyethyl
cellulose
(HEC) having a molecular weight of 1,300,000, followed by 1.25 grams of
dodecyl tetradecyl
glycidyl ether and 87.5 grams of isopropanol. The flask was then purged with
nitrogen gas.
The flask was fitted with a mechanical stirrer, and 87.5 grams of 1 percent
sodium
hydroxide (NaOH), pre purged with nitrogen gas was added to the flask while
stirring the flask.
The resulting viscous slurry was purged briefly with nitrogen gas and then
stirred
for five hours at 60 degrees Celsius. After five hours, 2 grams of acetic acid
and 100 milliliters of
acetone were added to the flask.
After 10-15 minutes, the contents of the flask was transferred to a one liter
beaker
and about 400 milliliters of acetone was added for precipitation with
stirring.
The resulting material was centrifuged and washed twice with 100 milliliter
acetone
washings. After air drying, 180 milliliters of water was added with stirring,
yielding a gel. An
additional 40 milliliters of water was added and mixed with a spatula
immediately prior to transfer
of the material to a dialysis tube. The removal of the sodium acetate salt was
confirmed using
Fourier transform infrared spectroscopy (FTIR).
The material was then freeze-dried and ground to yield 9 grams of a thickening
polymer, consisting of hydrophobically modified hydroxyethyl cellulose (HMHEC)
as a fluffy
white powder, which thickening polymer was designated as HMHEC 0603.
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A similar synthesis process was used to prepare other HMHEC thickening
polymers
from HEC having molecular weights of 720,000 or 1,000,000, also using dodecyl
tetradecyl
glycidyl ether as the hydrophobic modifier. These other HMHEC thickening
polymers were
designated as indicated in Figure 1.
Example 2 - Preparation of Water Flooding Compositions
A number of different water flooding compositions were prepared using the
HMHEC thickening polymers of Example 1. An additional water flooding
composition,
designated as HPAM, was prepared using FlopaamTM 3630 as the thickening
polymer. FlopaamTM
3630 is a polyacrylamide polymer produced by SNF Group of Andrezieux, France,
which is
commonly used as a thickening polymer in secondary hydrocarbon recovery.
The water flooding compositions were prepared from the thickening polymers by
mixing the thickening polymers with water. The water was provided as either
relatively pure
water, as a 1 percent brine (NaC1) solution, or as a hard brine (NaCI)
solution containing total
dissolved solids of 8.5 percent and a hardness of 0.38 percent.
The compositions and properties of the HMHEC and HPAM water flooding
compositions are summarized in Figure 1.
In Figure 1, it is noted that two different values for viscosity in 1 % NaCl
are
provided for the water flooding compositions containing HMHEC 1206 and HMHEC
0603. The
first (and lower) values for viscosity were obtained from the sandpack
filtration tests. The second
(and higher) values for viscosity were obtained from the coreflood tests.
Although the reason for
these discrepancies in the data is not known, it is possible that the HMHEC
1206 and HMHEC
0603 water flooding compositions which were used in the sandpack filtration
tests may actually
have contained a lower concentration of the thickening polymers than the water
flooding
compositions which were used in the coreflood tests.
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Example 3 - Water Flooding Compositions - Filtration Tests
The water flooding compositions of Example 2 and Figure 1 were filtered using
one
or both of two filtering techniques, both of which involved passing the water
flooding
compositions through a permeable test medium.
The first filtering technique comprised filtering the water flooding
compositions
through two Whatman #1 (11 gm) filters. A net pressure drop of 100 kPa using
compressed air
was placed across the filters in order to provide a pressure gradient.
The viscosities of the water flooding compositions were measured before and
after
the filtration to obtain an initial viscosity value and a final viscosity
value. A reduction in the
viscosity of the water flooding composition indicated that all of the
thickening polymer did not
pass through the filters. It is noted that a similar measurement could have
been made of the
concentration of the thickening polymer in the water flooding compositions to
obtain an initial
concentration value and a final concentration value.
The filterability of the water flooding compositions was also evaluated by the
filter
ratio, which compares the rate of filtration over different time intervals:
Filter Ratio = Time to Filter 300grams - Time to Filter 200 grams
Time to Filter 200 grams - Time to Filter 100 grams
A filter ratio greater than 1 indicated that the permeability of the filter
was
decreasing over time, suggesting that all of the thickening polymer did not
pass through the filter
and thus plugged the filter.
The second filtering technique comprised filtering the water flooding
compositions
through a compact sandpack filtration test core having a permeability of less
than about 10 darcies
and a length from an upstream end to a downstream end of about 10 centimeters.
A schematic drawing of the apparatus which was used to conduct the sandpack
filtration tests is provided in Figure 6.
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Referring to Figure 6, the filtration test apparatus (20) comprises a
filtration test
core (22). The filtration test core (22) has an upstream end (24) and a
downstream end (26). The
upstream end (24) of the filtration test core (22) is in fluid communication
with an injection fluid
vessel (28). A compressed air source (30) is in fluid communication with the
injection fluid vessel
(28). The compressed air source (30) provides a means for pressurizing fluid
which is contained
within the injection fluid vessel (28).
The downstream end (26) of the filtration test core (22) is in fluid
communication
with an effluent collection vessel (32). The weight of the effluent collection
vessel (32) is
measured with a balance scale (34) in order to determine the weight of
effluent fluid which exits
the downstream end (26) of the filtration test core (22). Data from the
balance scale (34) is
transferred to a computer (36) for recordation and analysis.
In the second filtering technique, the propagation of the water flooding
compositions through the filtration test core (22) was measured under a series
of net pressure drops
from 3.5 kPa to 100 kPa. The weight of the effluent water flooding composition
exiting the
downstream end (26) of the filtration test core (22) was measured by the
balance scale (34) and
recorded by the computer (36) as a function of time.
The water flooding compositions were sampled at the upstream end (24) of
the filtration test core (22) and evaluated with a rheometer in order to
obtain initial viscosity values
for the water flooding compositions. The water flooding compositions were
sampled at the
downstream end (26) of the filtration test core (22) and evaluated with a
rheometer in order to
obtain final viscosity values for the water flooding compositions.
As with the first filtering technique, a reduction in the viscosity of the
water
flooding composition indicated that all of the thickening polymer did not pass
through the filtration
test core (22). As with the first filtering technique, a similar measurement
could have been made
of the concentrations of the thickening polymer in the water flooding
compositions to obtain an
initial concentration value and a final concentration value.
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Results of the filtration tests for both filtering techniques are provided in
Figure 2.
Example 4 - Water Flooding Compositions - Coreflood Tests
Coreflood tests of a selected number of the water flooding compositions were
conducted to study the incremental oil recovery resulting from the use of the
water flooding
compositions over the oil recovery obtained from an initial water flood
procedure. In general, the
coreflood tests were performed by conducting a brine water flood first to
obtain a meaningful
water flood recovery value, conducting a water flood using one of the water
flood compositions,
and then conducting a second brine water flood as a chaser flood. The aqueous
medium for each
of the water flooding compositions was 1 % NaCl brine.
A schematic drawing of the apparatus which was used to conduct the coreflood
tests
is provided in Figure 7.
Referring to Figure 7, the coreflood test apparatus (50) comprises a sandpack
coreflood test core (52). The coreflood test core (52) has an upstream end
(54) and a downstream
end (56). The upstream end (54) of the coreflood test core (52) is in fluid
communication with a
pump (58) which is connected with a source of brine (60) and a source of water
flooding
composition (62).
The downstream end (56) of the coreflood test core (52) is in fluid
communication
with a backpressure regulator (64). The backpressure regulator (64) has a
liquid outlet (66).
An upstream pressure transducer (72) is connected with the upstream end (54)
of
the coreflood test core (52). A midstream pressure transducer (74) is
connected with the midpoint
of the length of the coreflood test core (52).
The properties of the coreflood test cores (52) for a number of tests are set
out in
Figure 3. For the coreflood tests, the length of the coreflood test cores (52)
was about 30
centimeters.
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In conducting the coreflood tests, the coreflood test core (52) was first
saturated
with a 1 % brine solution to obtain its brine permeability.
Oil was then injected into the coreflood test core (52) to displace mobile
water until
a constant pressure drop across the coreflood test core (52) was obtained and
water production
stopped. Properties of the oil are set out in Figure 4.
The oil permeability of the coreflood test core (52) was then measured. The
coreflood tests were carried out under net overburden pressure of 7000 kPa at
a constant core
temperature of 20 degrees Celsius.
The initial brine water flood was conducted by injecting a 1 % brine (NaCl)
solution into the upstream end (54) of the coreflood test core (52) at a
constant flow rate of 3.6
ml/hr. This flow rate equates to a linear velocity of 0.6 feet per day, which
is believed to be
representative of the flow rates which may be expected in typical reservoirs
far removed from a
wellbore. Effluent samples were collected at the downstream end (56) of the
coreflood test core
(52) in a series of pre-weighed tubes at a time interval of 100 minutes.
Pressure drops across the
coreflood test core (52) generated by the injected brine solution were
continuously monitored by
the upstream pressure transducer (72) and the midstream pressure transducer
(74). The initial
brine water flood was continued until about 1 pore volume (PV) had been
injected into the
coreflood test core (52).
A water flood composition was then injected continuously into the upstream end
(54) of the coreflood test core (52) until at least 2 pore volumes (PV) of the
water flood
composition had been injected into the coreflood test core (52).
Finally, a second brine water flood was injected into the upstream end (54) of
the
coreflood test core (52) as a chaser until about 1 pore volume (PV) had been
injected into the
coreflood test core (52).
The results of the coreflood tests are summarized in Figure 5.
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Analysis of Filtration Test Results and Coreflood Test Results
Referring to Figure 1, it is observed that the viscosity of the HPAM
(polyacrylamide) water flooding composition was very significantly higher when
the water
flooding composition was prepared using water as the aqueous medium than when
1 % brine
(NaCI) or hard brine was used as the aqueous medium. In contrast, the
viscosity of the HMHEC
water flooding compositions was generally higher when the water flooding
compositions were
prepared using 1 % brine (NaCI) or hard brine as the aqueous medium than when
water was used
as the aqueous medium. This phenomenon suggests that HMHEC water flooding
compositions
may exhibit superior durability for use in secondary oil recovery in brine
environments than
polyacrylamide water flooding compositions.
Also referring to Figure 1, it is observed that HMHEC water flooding
compositions
prepared using a relatively high molecular weight HEC backbone polymer (i.e.,
1,300,000) tend to
exhibit relatively high viscosity at relatively lower hydrophobe substitution
levels than do HMHEC
water flooding compositions which are prepared using a relatively low
molecular weight HEC
backbone polymer (i.e., 720,000). This phenomenon demonstrates that the use of
relatively higher
molecular weight HEC backbone polymers can be effective to achieve suitably
high viscosities at
relatively lower levels of substitution (and thus lower degrees of
association).
Also referring to Figure 1, it is observed that qualitatively, the best
overall results
with respect to injectability and oil recovery were achieved using either the
HPAM water flooding
composition or the HMHEC 0603 water flooding composition, suggesting that
relatively higher
molecular weight HEC backbone polymers with a modest level of hydrophobe
substitution can be
used to overcome injectability problems which may result from the use of
relatively lower
molecular weight HEC backbone polymers with a higher level of hydrophobe
substitution.
Referring to Figure 2, it is observed in the sandpack filtration tests that
the HMHEC
0603 water flooding compositions exhibited a very stable and consistent
viscosity pre-injection and
following injection of two pore volumes, indicating that HMHEC 0603 water
flooding
compositions can be considered to satisfy the requirement of injectability.
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Referring to Figure 5, it is observed in the coreflood tests that the HMHEC
0603
water flooding composition exhibited the highest oil recovery (slightly higher
than the HPAM
water flooding composition) while the HMHEC 1206 water flooding composition
exhibited a
much lower oil recovery. Furthermore, both the HPAM water flooding composition
and the
HMHEC 0603 maintained a stable and consistent viscosity pre-injection and post-
injection, while
the HMHEC 1206 water flooding composition exhibited a dramatic decrease in
viscosity from pre-
injection to post-injection. This phenomenon suggests that an HMHEC water
flooding
composition containing a relatively high molecular weight backbone polymer,
such as HMHEC
0603 can provide secondary oil recovery results which are comparable to a
polyacrylamide
(HPAM) water flooding composition.
Referring to Figure 8, it is observed that a water flooding composition
containing a
relatively low molecular weight backbone polymer, such as HMHEC 1206 may
exhibit a
continuous increase in required injection pressure during a water flooding
procedure, while a
polyacrylamide (HPAM) water flooding composition may exhibit a relatively
stable and consistent
required injection pressure during a water flooding procedure.
Referring to Figure 9, it is observed that a water flooding composition
containing a
relatively high molecular weight backbone polymer, such as HMHEC 0603 may
exhibit a required
injection pressure during a water flooding procedure which is comparable to
that exhibited by a
polyacrylamide (HPAM) water flooding composition.
Referring to Figure 10, it is observed that a water flooding composition
containing a
relatively high molecular weight backbone polymer, such as HMHEC 0603 may
exhibit an oil
recovery during a water flooding procedure which is comparable to that
exhibited by a
polyacrylamide (HPAM) water flooding composition, while a water flooding
composition
containing a relatively low molecular weight backbone polymer, such as HMHEC
1206 may
exhibit a significantly lower oil recovery during a water flooding procedure.
Referring to Figure 11, it is observed that for water flooding compositions in
which
the aqueous medium was comprised of 1 % NaCl, a polyacrylamide (HPAM) water
flooding
composition exhibited increased effective viscosity in the sandpack filtration
tests as the flow rate
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CA 02684230 2009-10-30
increased. An explanation for this result is that the transportation of
polymer solutions in porous
media is affected by both shear and elongational viscosities. In the case of
the HPAM water
flooding composition, its elongational viscosity increased greatly at high
flow rates, leading to
higher effective viscosities. Referring also to Figure 10, it is observed that
for water flooding
compositions in which the aqueous medium was comprised of 1 % NaCl, HMHEC
water flooding
compositions tended to exhibit a relatively stable and consistent effective
viscosity in the sandpack
filtration tests within the range of flow rates that was studied.
Referring to Figure 12, it is observed that water flooding compositions
containing a
moderately high to high molecular weight backbone HEC polymer (HMHEC 0318 and
HMHEC
0603) wherein the aqueous medium was comprised of a hard brine solution
exhibited higher
effective viscosities in the sandpack filtration tests at relatively low
concentrations of the
thickening polymer (i.e., less than 2000 ppm) than did water flooding
compositions comprising
HMHEC 0318 or HMHEC 0603 at relatively higher concentrations of the thickening
polymer (i.e.,
2000 ppm) wherein the aqueous medium was comprised of 1 % NaCl.
In summary, an HMHEC water flooding composition containing a relatively high
molecular weight backbone polymer (i.e., at least about 1,000,000) and a
moderate level of
substitution of the hydrophobic modifier may be capable of providing
performance in water
flooding applications which is comparable to the performance of an HPAM
(polyacrylamide) water
flooding composition with respect to injectability and oil recovery, and which
may be superior to
the performance of an HPAM (polyacrylamide) water flooding composition with
respect to
durability in the presence of a brine environment.
In this document, the word "comprising" is used in its non-limiting sense to
mean
that items following the word are included, but items not specifically
mentioned are not excluded.
A reference to an element by the indefinite article "a" does not exclude the
possibility that more
than one of the elements is present, unless the context clearly requires that
there be one and only
one of the elements.
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