Note: Descriptions are shown in the official language in which they were submitted.
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FILTER CAKE DEGRADATION COMPOSITIONS AND ASSOCIATED METHODS
BACKGROiTND
The present invention relates to at least the partial degradation of filter
cakes in
subterranean formations. More particularly the present invention provides
filter cake
degradation compositions and methods of degrading filter cakes.
In general, filter cakes are residues deposited on the walls of subterranean
well bores
as a result of various subterranean operations such as drilling, completion,
and work-over
operations. Such filter cakes are often tough, dense, substantially water
insoluble, and
usually capable of reducing the permeability of a surface on which they have
formed. In
general, filter cakes may prevent a fluid used in subterranean operations from
being lost into
the formation. Filter cakes also may prevent solids from entering the pores of
the formation,
thus preventing damage to the conductivity of the formation. Eventually, for a
subterranean
formation or portion of a subterranean formation to produce, the filter cake
is often removed
from the walls of the well bore.
Filter cakes are desirable, at least temporarily, in subterranean operations
for several
reasons. For instance, a filter cake may be used in a fluid-loss control
operation. In such an
operation, a filter cake may act to localize the flow of a servicing fluid and
minimize
undesirable fluid loss into the formation matrix. This is an important
function of a filter cake
because if too much fluid is lost the conductivity or permeability of the
formation may be
damaged. A filter cake also may add strength and stability to the formation
surfaces on
which the filter cake forms. For example, one type of drilling fluid, commonly
referred to as
a "drill-in fluid," may be used to drill a well bore while minimizing the
damage to the
permeability of the producing zone. Drill-in fluids may include a fluid-loss
additive (e.g.,
starch) and a bridging agent to block fluid entry into formation pores (e.g.,
calcium
carbonate). Typically, a drill-in fluid forms a filter cake on the walls of
the well bore that
prevents or reduces fluid loss during drilling, and upon completion of the
drilling operation,
stabilizes the well bore during subsequent completion operations. The filter
cake may be
beneficial to other well bore operations, for example, hydraulic fracturing,
and gravel
packing.
In general, filter cakes include bridging agents that block formation pores
and fluid-
loss additives that, inter alia, bind the bridging agents to the well bore and
further inhibit
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fluids from entering the formation. The fluid-loss additive component of a
filter cake
generally should form a coherent membrane so that the filter cake maintains
its integrity.
Although useful, the coherent membrane oftentimes can make it difficult to
remove the filter
cake from the face of the formation when it is desirable to do so. Typical
fluid-loss additives
include starches (e.g., xanthan, amylose, and/or amylopectin) and typical
bridging agents
include salts (e.g., calcium carbonate andlor sodium chloride). Starch is a
polysaccharide that
comprises monosaccharide units linked by glycosidic bonds, e.g., a-1,4
glucosidic bonds and
a-1,6 glucosidic bonds. In addition, filter cakes commonly include drilled
solids, weighting
agents, and viscosifying polymers that have been used to viscosify fluids used
in some
subterranean operations. Although some fluids used in well bore operations do
not form
filter cakes, these fluids may create conditions analogous to those found
within filter cakes,
e.g., by plugging formation pores. Therefore, the term "filter cake" when used
herein also
refers to these conditions.
Although desirable for a certain amount of time or during a certain operation,
to
produce the desirable fluids from the formation, at some point the filter cake
generally may
need to be removed. Accordingly, some subterranean fluids may comprise an
additional
component that is capable of degrading the fluid-loss additive of the filter
cake. Such
components include acids, enzymes, and oxidizers.
Although enzymes may be useful for degrading the fluid-loss additive component
of a
filter cake, enzymes may be unstable at certain elevated temperatures like
those frequently
encountered in some subterranean operations. At sufficiently high
temperatures, enzymes
can undergo irreversible denaturation (i. e., conformational alteration
entailing a loss of
biological activity). Enzymes also may be intolerant to the salt
concentrations commonly
found in well bores. In addition, the combination of salt concentration and
temperature may
cause enzymes to coagulate and precipitate as shown in Figure 1. Typical
enzymes often
produce this damaging precipitate at the enzyme concentrations, salinity, and
temperatures
needed to effectively remove the filter cake. This sort of precipitation is
particularly
problematic with filter cakes because the gelatinous precipitate may clog
formation pore
throats, which can decrease the permeability of the formation and ultimately
reduce
production from the formation.
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SUMMARY
The present invention relates to at least the partial degradation of filter
cakes in
subterranean formations. More particularly the present invention provides
filter cake
degradation compositions and methods of degrading filter cakes.
In one embodiment, the present invention provides a method of degrading a
fluid-loss
additive component in a portion of a filter cake in a subterranean formation
comprising:
contacting the fluid-loss additive component with a filter cake degradation
composition that
comprises a precipitation resistant enzyme, wherein the precipitation
resistant enzyme is
capable of degrading the fluid-loss additive component; and allowing the
filter cake
degradation composition to at least partially degrade the fluid-loss additive
component in a
portion of the filter cake.
In one embodiment, the present invention provides a filter cake degradation
composition comprising a precipitation resistant enzyme component that will at
least partially
degrade a portion of a filter cake.
The features and advantages of the present invention will be readily apparent
to those
skilled in the art upon a reading of the description of the embodiments that
follows.
BRIEF DESCRIPTION OF THE FIGURES
A more complete understanding of the present disclosure and advantages thereof
may
be acquired by referring to the following description taken in conjunction
with the
accompanying drawings. The patent or application file contains at least one
figure executed
in color. Copies of this patent or patent application publication with color
figures) will be
provided by the Office upon request and payment of the necessary fee.
FIGURE 1 illustrates an embodiment of a precipitated enzyme.
FIGURE 2 illustrates a graph of the change in permeability possible if using
certain
methods of the present invention.
FIGURE 3 illustrates a graph of the change in permeability with a comparative
test
sample.
While the present invention is susceptible to various modifications and
alternative
forms, specific exemplary embodiments thereof have been shown in the figures
and are
herein described. It should be understood, however, that the description
herein of specific
embodiments is not intended to limit the invention to the particular forms
disclosed, but on
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the contrary, the intention is to cover all modifications, equivalents, and
alternatives falling
within the spirit and scope of the invention as defined by the appended
claims.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
The present invention relates to at least the partial degradation of filter
cakes in
subterranean formations. More particularly the present invention provides
filter cake
degradation compositions and methods of degrading filter cakes.
In general, the present invention provides filter cake degradation
compositions and
methods of degrading the fluid-loss additive components of filter cakes. In
certain
embodiments, the methods of the present invention degrade at least a portion
of the fluid-loss
additive component of a filter cake in a subterranean formation. The term
"degrade," as used
herein, refers to at least a partial degradation of the fluid-loss additive
component of the filter
cake, e.g., by hydrolysis. In certain embodiments, the methods of the present
invention also
may comprise degradation of bridging agents from a filter cake in a
subterranean formation.
In certain exemplary embodiments, the methods of the present invention
compromise the
integrity of the filter cake to a degree at least sufficient to allow any
pressure differential
between formation fluids and the well bore to induce flow from the formation.
The filter cake degradation compositions of the present invention comprise
precipitation resistant enzymes. Suitable precipitation resistant enzymes
should be capable of
hydrolyzing starch and should be resistant to precipitation under conditions
sometimes found
in subterranean well bores, e.g., elevated temperatures. Precipitation
resistant enzymes
suitable for use in the methods of the present invention generally catalyze
the hydrolysis of
the fluid-loss additive component of a filter cake, e.g., by chemically
removing any of the
linkages between the monomers of a starch molecule. In certain embodiments,
the
precipitation resistant enzymes include hydrolase enzymes of enzyme
classification (E.C.)
number 3.2, according to the Recommendations of the Nomenclature Committee of
the
International Union of Biochemistry on the Nomenclature and Classification of
Enzymes. In
certain embodiments of the present invention, glycosidase enzymes (E.C. 3.2.1)
may be used.
In certain exemplary embodiments, the precipitation resistant enzymes include
a-amylase
enzymes (E.C. 3.2.1.1), J3-amylase enzymes (E.C. 3.2.1.2), glucan 1,4-a-
glucosidase
enzymes (E.C. 3.2.1.3), or combinations thereof. Examples of suitable
precipitation resistant
enzymes that are commercially available, include, but are not limited to,
Liquezyme~ X
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(Novozymes A/S of Bagsaerd, Denmark) and Optisize HT (Genencor International,
Palo
Alto, California). The precipitation resistant enzymes of the present
invention should resist
precipitation in temperatures ranging from about 10°C (50°F) to
about 150°C (327°F) and
pHs ranging from about 2 to about 11. In addition, the precipitation resistant
enzymes should
resist precipitation at salt concentrations of up to at least about 2.5 molar;
and may resist
precipitation at salt concentrations up to at least 5 molar. The term "salt"
refers to salts of
monovalent cations and anions. A person of ordinary skill in the art, with the
benefit of this
disclosure, will recognize how the valency of the salt will affect molarity
and ionic strength.
In certain exemplary embodiments, the precipitation resistant enzymes of the
filter cake
degradation compositions of the present invention are capable of degrading
starch without
precipitation in saturated brines (e.g., sodium chloride) at a temperature up
to about at least
90°C.
The precipitation resistant enzymes may be present in the compositions of the
present
invention in an amount sufficient to degrade at least a desired portion of a
filter cake. In
some exemplary embodiments, the precipitation resistant enzymes may be present
in an
amount in the range of from about 10 kilo novo units (~ to about 150 KNU. One
KNU
is defined as the quantity of enzyme which degrades 4.87 grams of starch
(Merck, soluble
amylum, Erg. B6, Batch No.: 6380528), at pH 5.6, and at a temperature of
37°C.
The filter cake degradation compositions of the present invention may be used
in any
form including a solid, a liquid, an emulsion, or a combination thereof. The
precipitation
resistant enzymes in the compositions of the present invention also may be
used as, or with,
encapsulated particles, particles that are impregnated on a carrier, solids,
liquids, emulsions,
or mixtures thereof. The filter cake degradation compositions may be designed
to have a
delayed effect on a portion of a filter cake, for instance, when the process
will involve a long
pump time and consequently it is necessary to delay the enzymatic action of
the precipitation
resistant enzymes. Examples of delayed forms include encapsulated embodiments
and solid
embodiments. If immediate enzymatic action is desired, a liquid form may be
preferable,
e.g., in an aqueous solution. In certain embodiments of the present invention,
the
precipitation resistant enzymes in the filter cake degradation compositions
may be spray-
dried, freeze-dried, or the like. In certain embodiments, cells capable of
producing the
precipitation resistant enzymes that have been lyophilized may provide the
precipitation
resistant enzymes. In certain embodiments, the precipitation resistant enzymes
of the present
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invention may be provided, inter alia, in a purified form, in a partially
purified form, as
whole cells, as whole cell lysates, or any combination thereof. One of
ordinary skill in the art
with the benefit of this disclosure will be able to determine the appropriate
form for a given
application.
In certain embodiments of the present invention, the filter cake degradation
compositions of the present invention may comprise other additives, including,
but not
limited to, glycerol, bactericides, microbiocides, surfactants, chelating
agents, foaming
agents, and the like. With the benefit of this disclosure, one of ordinary
skill in the art will
recognize when such additives may be useful in a given application.
In certain embodiments, the filter cake degradation compositions of the
present
invention may comprise agents designed to remove or dissolve bridging agents
in a filter
cake. Examples of such agents include, but are not limited to, complexing
agents (e.g., salts
of ethylenediaminetetraacetic acid, a salt thereof, or other chelating
agents), organic acids, or
acid precursors (e.g., diethylene glycol diformate, glycerol diacetate, and
glycerol triacetate).
Some organic acids of this type may react with the bridging agents (e.g., acid-
soluble
bridging agents like calcium carbonate) and, in the presence of a conjugate
base, may form a
buffered system with a pH of about 4 or greater. Similarly, in the case of the
acid precursors,
which can produce organic acids in situ, since the acid is produced very
slowly, the pH may
stay in a range where precipitation resistant enzymes are active (e.g., in the
range of from
about 4 to about S.5).
In certain embodiments, the filter cake degradation compositions of the
present
invention may be used in conjunction with agents designed at least to
partially remove a
bridging agent component of the filter cake. For example, a strong acid, such
as hydrochloric
acid or hydrofluoric acid, may be used in a two-stage, sequential process.
Such a process
may involve treatment of the filter cake with a filter cake degradation
composition of the
present invention and then treatment of the filter cake with the strong acid.
Thus, inactivation
of the precipitation resistant enzyme at the resultant low pHs created by a
strong acid may be
avoided. In embodiments where the bridging agent is water soluble, e.g., a
salt, the bridging
agent may be removed with fresh water or water undersaturated with respect to
the water-
soluble bridging agent.
The filter cake degradation compositions of the present invention may be
contacted
with a filter cake to degrade at least a portion of the filter cake using any
method. For
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instance, the filter cake degradation compositions may be incorporated in a
clean-up fluid.
The term "clean-up fluid" refers to any fluid introduced into a subterranean
formation for the
purposes of facilitating the degradation of a filter cake. In certain
embodiments, the filter
cake degradation compositions of the present invention are internally
incorporated in a
servicing fluid, externally applied to a servicing fluid, or any combination
thereof. The term
"servicing fluid" refers to any fluid suitable for use in subterranean
operations. Examples of
servicing fluids, include, but are not limited to, drill-in fluids, fracturing
fluids, and gravel
packing fluids. For applications such as, e.g., fracturing and gravel packing,
the precipitation
resistant enzyme may be incorporated internally in the fluid or onto a
particulate used in the
process. In one embodiment, the filter cake degradation compositions of the
present
invention may be pumped to the location of the treatment zone at a rate
sufficient to introduce
sufficient precipitation resistant enzymes to at least partially degrade the
fluid-loss additive
component in a portion of a filter cake. To achieve certain beneficial effects
of the present
invention, the filter cake degradation compositions of the present invention
may be shut in
the formation for a time sufficient to at least partially degrade the fluid-
loss additive
component of a filter cake. This shut-in-time may be affected by the activity
and/or
concentration of the precipitation resistant enzyme and/or by the
environmental conditions of
the well bore, such as temperature, pH, and the like. If necessary, the pH of
the treatment
fluid may be adjusted through the use of acids, bases, or buffers. One of
ordinary skill in the
art with the benefit of this disclosure will recognize the conditions that
might affect the
requisite shut-in time needed to achieve a desired result.
An example of a method of the present invention is a method of degrading a
fluid-loss
additive component in a portion of a filter cake in a subterranean formation
comprising:
contacting the fluid-loss additive component with a filter cake degradation
composition that
comprises a precipitation resistant enzyme, wherein the precipitation
resistant enzyme is
capable of degrading the fluid-loss additive component; and allowing the
filter cake
degradation composition to at least partially degrade the fluid-loss additive
component in a
portion of the filter cake.
An example of a composition of the present invention is a filter cake
degradation
composition comprising a precipitation resistant enzyme component that will at
least partially
degrade a portion of a filter cake.
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To facilitate a better understanding of the present invention, the following
examples
of specific embodiments are given. In no way should the following examples be
read to
limit, or to define, the scope of the invention.
EXAMPLES
Methods and Procedures
In general, the test method for assessing enzyme activity generally involved
addition
of a known amount of an enzyme concentrate to a solution containing a standard
amount of a
standard starch in solution. The course of the resulting reaction is then
followed by testing
for the presence of starch with an iodine test solution. The starch was judged
to have been
consumed when coloration due to a complex formed between starch and iodine was
no longer
observed as compared to a colored glass standard. Reaction conditions were
controlled to a
pH of 5, a temperature of 37°C, and included a trace concentration of
calcium (0.0003
molar).
The enzyme assay was performed as follows. 5 milliliters iodine solution B was
pipetted into at least 5 test tubes per sample, which were placed in a water
bath at 40°C. 20
milliliters starch solution was pipetted into a large test tube. The pH was
checked to ensure a
pH of 5. 5 milliliters of calcium chloride solution was then added to the test
tube. The test
tube was warmed to 40°C before adding an enzyme solution. An amount of
enzyme solution
was gradually added to the mixture and mixed. The reaction was allowed to
proceed at 40°C.
At suitable intervals, 1 milliliter of the reaction mixture was removed and
added to the test
tubes containing 5 milliliters iodine solution B. Each tube was shaken briefly
and color
checked to determine the presence of starch.
Iodine solution A was made as follows. 22 grams of potassium iodine were
dissolved
in approximately 60 milliliters of demineralised water in a 500 milliliter
volumetric flask. 11
grams of iodine were dissolved in the flask, which was then filled to the mark
with
demineralised water.
Iodine solution B was made as follows. 80 grams of potassium iodine was
weighed
out and added to a 2,000 milliliter volumetric flask. Then 8 milliliters of
iodine solution A
was added and the flask was filled to the mark with demineralised water.
A stock salt solution was made as follows. 9.36 grams NaCI, 69 grams KH2P04
and
4.8 grams Na2HPOa. were weighed out and poured into a 1,000 milliliter
volumetric flask,
which was then filled to the mark with demineralized water. The pH of the
solution was
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checked, and when necessary, adjusted using HCl or NaOH as appropriate to
reach a pH of
5.2.
A starch solution was made as follows. An equivalent amount of 6.95 grams of
dry
matter content starch was added to demineralized water to a volume of 1,000
milliliters. The
percentage dry matter content (DM %) of the starch was analyzed at
105°C (water
determination at 105°C). The starch was suspended in 100 milliliters of
demineralised water.
The starch solution was then transferred quantitatively while stirring to a
beaker containing
200 milliliters boiling demineralised water. The solution was boiled for
approximately 30
seconds. The solution was transferred quantitatively to a 1,000 milliliter
volumetric flask and
cooled to room temperature. The pH was adjusted to 5 with HCl or NaOH as
appropriate.
The solution was then made up to the mark with demineralised water.
A calcium chloride solution was prepared as follows. 0.82 grams of CaCl2 in
100
milliliter solution of demineralised water. The pH was then adjusted to 5 with
HCl or NaOH
as appropriate.
An enzyme solution was prepared as followed. Enzyme concentrates were
dissolved
in 100 milliliters deionised water and diluted to the degree necessary to
yield a measurable
rate (e.g., 5 to 20 minutes) in the test procedures described. The pH was then
adjusted to 5
with HCl or NaOH as appropriate.
A test method for precipitation of enzymes based on salinity was conducted as
follows. Three test brine solutions were made up by combining fresh water,
saturated sodium
chloride brine (density 1.2 kilograms per liter) and sodium chloride brine at
SO% saturation
(density 1.2 kilograms per liter). An enzyme sample at a moderately high
concentration and
at a low concentration were added to the brine and then heated to 90°C.
The appearance of
the solutions was monitored for formation of a precipitate.
Simulation of the tendency of a precipitated enzyme to block porous media was
tested
by noting the rate of flow through fine filter paper and core flow studies
using sandstone of
low permeability.
A pore blockage test using fllterpaper was conducted as follows. A steel cell
of
diameter 5 centimeters and length 150 centimeters was fitted with filter paper
having a pore
dimension of 2.7 microns. The cell was then filled with 100 milliliters of
water, sealed and
pressurized to 100 pounds per square inch. The rate of water discharge through
the
filterpaper was timed in seconds to measure the initial inj ectivity of water
through the
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filterpaper. Enzyme solutions of known concentrations were prepared in brine
solutions and
either heated and allowed to cool before being tested or tested without prior
heating. The cell
was then filled with 100 milliliters of the enzyme solutions, sealed, and
pressurized to 100
pounds per square inch and the rate of discharge was timed. Next, water was
injected
through the filter paper as described above to check whether any precipitated
enzyme creates
a lasting permeability reduction in the filterpaper.
A pore blockage test using a test core was conducted as follows. An enzyme
solution
was injected into a test core at ambient temperature. The temperature was then
raised to
93°C to induce precipitation. The direction of flow into the core was
reversed to assess
whether any precipitation occurring inside the core affected the permeability
of the core.
Specifically, Berea sandstone core plugs were cut, dried, and vacuum saturated
in 1.2 specific
gravity NaCI brine. A core plug was then mounted in the permeameter and sealed
with 500
pounds per square confining pressure. The temperature was increased to
200°F while
maintaining the confining pressure. Soltrol~ 170, an isoparaffin solvent
commercially
available from Chevron Phillips Chemical Company, The Woodlands, Texas, was
flowed
through the core in the production direction until a stable permeability was
measured (Ki).
Ten pore volumes of the 0.5% vlv enzyme in a NaCI brine solution having a
specific gravity
of 1.2 was flowed through the core in the injection direction. The core was
shut in and held
for 24 hours at 200°F. Flow of Soltrol~ 170 was resumed in the
production direction and
continued until the permeability reached a stable value (Kf).
One example of a precipitation resistant enzyme suitable for use in the
methods of the
present invention, Liquizyme X (commercially available from Novozymes A/S,
Bagsaerd,
Denmark), was compared to comparative test samples of other enzymes. The
comparative
test samples were: Termamyl~ 120L (commercially available from Novozymes A/S,
Bagsaerd, Denmark); Ban~ (commercially available from Novozymes A/S, Bagsaerd,
Denmark); and Nervanase~ BT2 (commercially available from Rhodia Food Ltd,
Cheshire,
United Kingdom).
The activities per gram of the various enzyme samples tested as quoted by
suppliers
and estimated according to the method outlined above are summarized in Table
1.
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Table 1
Enzyme Activity
(KNU/g enzyme
concentrate
Li uiz a X 200 a rox
Termam 1 120L 120 a rox
Ban 240L 240 a rox
Nervanase BT2 120 a rox
The exemplary precipitation resistant enzyme, Liquizyme X, and comparative
test
samples were tested using the method to determine enzyme precipitation
described above.
The precipitation tendencies of the comparative enzyme samples were determined
at 20°C
and 95°C and in different concentrations of sodium chloride in the
brine carrier. Table 2
shows the thermal precipitation potential of the Liquizyme~ X and comparative
test samples.
Table 2
Enzyme Activity Precipitation Precipitation
KNU/100 mL observed observed
at at 95C
20C
NaCI Molari 5.27M 2.63M 5.27M 2.63M
Li ui a X 40 no no no no
Li uiz a X 100 no no no no
Termam 1 120L 24 no no es no
Termam 1 120L 60 no no es es
Ban 240L 48 no no es es
Ban 240L 120 no no es es
Nervanase BT2 24 no no es es
Nervanase BT2 65 no no es es
The data in Table 2 exemplify the stability of precipitation resistant enzymes
in
sodium chloride brine. The comparative samples all produced precipitates.
The exemplary precipitation resistant enzyme, Liquizyme~ X, and comparative
test
samples were tested using the methods to determine pore blockage using the
filter paper
method as described above. Table 3 shows the effect of temperature on enzyme
precipitation
based on injection through filterpaper.
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Table 3
Test mixture Observation Injectivity
sec./100
mL
At 23C At 90C Initial Final
Liquizyme'~ X No effect No effect 8 8
100 KNU/100 mL in water
Liquizyme~' X No effect No effect 8 8
100 KNU/100 mL NaCI brine 2.63
Liquizyme~' X No effect No effect 8 9
100 KNU/100 mL NaCI brine (5.27
Nervanase~' BT 2 No effect Cloudy 8 30
60 KNU/100 mL in water
Nervanase'e' BT 2 No effect Precipitate9 90
60 I~NU/100 mL in NaCI brine formed
2.63M
Nervanase~' BT 2 No effect Precipitate8 236
60 KNU/100 mL in NaCI brine formed
5.27M
Termamyl~' 120L No effect cloudy 10 50
60 KNU/100 mL in water
Termamyl"5' 120L No effect Very cloudy8 74
60 KNU/100 mL in NaCI brine
2.63M
The results in the Table 3 demonstrate that the comparative test samples tend
to
precipitate when heated. In the case of the Termamyl~ 120L and the Nervanase~
BT2 there
is evidence that a precipitate capable of reducing the permeability of
filterpaper is produced
even when the solvent is fresh water. Additionally, Table 3 demonstrates that
the
concentration of sodium chloride in the carrier brine has a marked impact on
the precipitation
tendency. In the case of the Nervanase~ BT2, the precipitation in saturated
sodium chloride
was severe. Table 3 also shows that the exemplary precipitation resistant
enzyme,
Liquizyme~ X, is resistant to precipitation over the entire sodium chloride
concentration
range tested.
The effect of precipitation on return permeability was determined using the
pore
blockage method using a test core as described above. The Termamyl~ 120L (60
KNU/100
mL) comparative test sample was compared to the exemplary precipitation
resistant enzyme
Liquizyme~ X (100 KNU/100 mL) in 5.27M sodium chloride brine. Upon heating to
90°C
the Termamyl~ 120L solution developed an obvious precipitate whereas the
solution of
Liquizyme~ X did not. Termamyl~ 120L demonstrated the potential to be more
damaging to
permeability than a precipitation resistant enzyme. Figure 2 illustrate the
return of
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13
permeability when Liquizyme~ ~ is used in an exemplary composition and method
of the
present invention. Following the injection of 10 pore volumes of the Liquizyme
X sample
and a static aging period of 24 hours at 90°C only a mean volume of 20
pore volumes of oil
produced through the core was required to restore permeability to 100% of the
original when
flow was recommenced in the production direction. In the case of the Termamyl~
120L
comparative test sample, over the same time span return permeability was only
33% of the
original after a flow of more than 400 pore volumes through the core as shown
in Figure 3.
Thus, the tests carned out with the sandstone core demonstrate that the
precipitation
resistant enzymes are less damaging to the core's permeability.
[0001] Therefore, the present invention is well adapted to attain the ends and
advantages mentioned as well as those that are inherent therein. While
numerous changes
may be made by those skilled in the art, such changes are encompassed within
the spirit of
this invention as defined by the appended claims.
