U.S. patent application number 11/190431 was filed with the patent office on 2005-11-24 for well treatment fluid and methods with oxidized polysaccharide-based polymers.
This patent application is currently assigned to Halliburton Energy Services, Inc.. Invention is credited to Dalrymple, Eldon Dwyann, Eoff, Larry S., Reddy, B. Raghava.
Application Number | 20050261139 11/190431 |
Document ID | / |
Family ID | 32988384 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050261139 |
Kind Code |
A1 |
Reddy, B. Raghava ; et
al. |
November 24, 2005 |
Well treatment fluid and methods with oxidized polysaccharide-based
polymers
Abstract
The present invention provides a well treatment fluid containing
water, a amine-based polymer, and an oxidized polysaccharide-based
polymer. The oxidized polysaccharide-based polymer is able to
crosslink with the amine-based polymer and produce a gel having a
viscosity of greater than about 20 cp, measured at a pH of about 4
to about 7 and at STP. The present invention also provides a method
of treating a subterranean formation penetrated by a wellbore
comprising the steps of: (a) forming the well treatment fluid, and
(b) contacting the subterranean formation with the fluid.
Inventors: |
Reddy, B. Raghava; (Duncan,
OK) ; Eoff, Larry S.; (Duncan, OK) ;
Dalrymple, Eldon Dwyann; (Duncan, OK) |
Correspondence
Address: |
Robert A. Kent
2600 S. 2nd Street
Duncan
OK
73536-0440
US
|
Assignee: |
Halliburton Energy Services,
Inc.
|
Family ID: |
32988384 |
Appl. No.: |
11/190431 |
Filed: |
July 27, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11190431 |
Jul 27, 2005 |
|
|
|
10394461 |
Mar 21, 2003 |
|
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Current U.S.
Class: |
507/209 |
Current CPC
Class: |
C09K 8/685 20130101;
C09K 8/887 20130101; C09K 8/512 20130101; C09K 8/12 20130101 |
Class at
Publication: |
507/209 |
International
Class: |
C09K 003/00 |
Claims
What is claimed is:
1. A well treatment fluid for use in a well, the well treatment
fluid comprising: (a) water; (b) an amine-based polymer; and (c) an
oxidized polysaccharide-based polymer, which is able to crosslink
with the amine-based polymer in water to produce a gel having a
viscosity of greater than or equal to about 20 cp.
2. The well treatment fluid of claim 1, wherein the amine-based
polymer comprises at least one member selected from the group of
chitosan, oxidized chitosan, polyvinylalcoholamine and
polyethyleneimine.
3. The well treatment fluid of claim 2 wherein the oxidized
chitosan is prepared by oxidizing chitosan-based polymers selected
from the group consisting of chitosan, chitosan salts, and any
combinations in any proportion thereof.
4. The well treatment fluid of claim 1 wherein the amine-based
polymer is prepared by oxidizing a chitosan-based polymer with an
oxidizer selected from the group consisting of sodium hypochlorite,
sodium periodate, hydrogen peroxide, peracetic acid, and any
mixture in any proportion thereof.
5. The well treatment fluid of claim 1 wherein the oxidized
polysaccharide-based polymer is selected from the group consisting
of dialdehyde starch, dialdehyde cellulose, oxidized starch,
oxidized cellulose, oxidized agarose, oxidized partially-acetylated
cellulose, oxidized HEC, and combinations thereof.
6. The well treatment fluid of claim 5 wherein the oxidized starch,
oxidized cellulose, oxidized agarose, oxidized HEC, or oxidized
partially-acetylated cellulose is prepared by oxidizing about 1% to
about 25% of the glucose units of the polysaccharide-based polymer
with an oxidizer selected from the group consisting of alkali,
alkaline earth and transition metal salts of periodate,
hypochlorite, perbromate, chlorite, chlorate, hydrogen peroxide and
combinations thereof.
7. The well treatment fluid of claim 1 wherein the oxidized
polysaccharide-based polymer is selected from the group consisting
of dialdehyde starch, dialdehyde cellulose, oxidized starch and
combinations thereof.
8. The well treatment fluid of claim 1 wherein the oxidized
polysaccharide based polymer comprises oxidized starch, oxidized
cellulose, oxidized agarose, oxidized hydroxyethylcellulose, or
oxidized partially-acetylated cellulose prepared by oxidizing about
1% to about 25% of the glucose units of the polysaccharide-based
polymer with an oxidizer selected from the group consisting of
alkali, alkaline earth and transition metal salts of periodate,
hypochlorite, perbromate, chlorite, chlorate, hydrogen peroxide and
combinations thereof.
9. The well treatment fluid of claim 8 wherein the oxidized
polysaccharide is oxidized starch and said oxidized starch is
present in an amount of up to about 10 wt %.
10. The well treatment fluid of claim 9 wherein the ratio of the
oxidized starch to the amine-based polymer is from about 20:1 to
about 1:20.
11. The well treatment fluid of claim 1 wherein the amine-based
polymer is oxidized chitosan which is present in an amount of up to
about 10 wt %.
12. The well treatment fluid of claim 1 wherein the ratio of the
oxidized chitosan to oxidized polysaccharide is from about 50:1 to
about 1:50.
13. The well treatment fluid of claim 1 wherein the water for the
well treatment fluid is selected from the group consisting of fresh
water, seawater, natural brine, formulated brine, 2% KCl solution,
and any mixtures in any proportion thereof.
Description
[0001] This application is a divisional of application Ser. No.
10/394,461 filed on Mar. 21, 2003, now pending.
FIELD OF THE INVENTION
[0002] The invention relates to aqueous well fluids containing
amine containing or amine-based polymers and oxidized
polysaccharide-based polymers and to methods of drilling and
servicing wells using such fluids.
BACKGROUND OF THE INVENTION
[0003] As hydrocarbon-producing wells mature, water production
becomes a serious problem. Remediation techniques for controlling
water production are generally referred to as conformance control.
Remediation techniques are selected on the basis of the water
source and the method of entry into the wellbore. Conformance
control treatments include sealant treatments and relative
permeability modifiers (also referred to as disproportionate
permeability modifiers).
[0004] In previous years, water-soluble chromium (+3) crosslinked
polyacrylamide gels have been used in conformance control
treatments. The gel time was usually controlled by the addition of
materials that chelate with chromium in competition with the
polymer-bound carboxylate groups. The crosslinking reactions in
these gel systems take place by the complexation of Cr (+3) ions
with carboxylate groups on the polymer chains. Because of the
nature of the chemical bond between Cr (+3) and the pendant
carboxylate groups, formation of insoluble chromium species can
occur at high pH values. Other problems with these systems include
thermal instability, unpredictable gel time, and gel instability in
the presence of chemical species that are potential ligands.
[0005] Another water-based gel system for conformance control is
based on phenol/formaldehyde crosslinker system for homo-, co-, and
ter-polymer systems containing acrylamide. The crosslinking
mechanism involves hydroxymethylation of the amide nitrogen, with
the subsequent propagation of crosslinking by multiple alkylation
on the phenolic ring. Because of the nature of this chemical bond,
the gel time is controllable over a wide temperature range.
Although these gels work well, phenol and formaldehyde are highly
toxic.
[0006] U.S. Pat. No. 5,836,392 discloses a system based on a
polyethyleneimine (PEI) crosslinker and a copolymer of acrylamide
and t-butyl acrylate (PA-t-BA). PEI is such a low-toxicity material
that the Food and Drug Administration has approved it in the United
States for food contact. Although non-toxic, PEI can bio-accumulate
or persist in the environment for long periods.
[0007] It has been shown that chitosan can be used in place of
polyethyleneimine to crosslink acrylamide based polymers.
Chitosan's usefulness as a crosslinker has been limited, however,
by its relative poor solubility in aqueous solutions. For example,
commercial sources of chitosan are only sparingly soluble in water;
about 1-2% active solutions are the highest concentrations that can
be made while maintaining usable viscosity. While this is a step
forward in the effort to provide more environmentally acceptable
systems, the major component, or base polymer, of such a gel system
is still a non-biodegradable polymer. The major component of such
gel system is generally a homopolymer or copolymer of acrylate-type
monomers, such as acrylic acid, acrylamide, vinylpyrrolidone etc.
The backbone of such polymers contains continuous carbon-carbon
single bonds, which are of poor biodegradability. Since the
chitosan crosslinker is only a minor component of the gel
composition, the total system is still predominantly
non-biodegradable due to poor biodegradability of the synthetic
base polymer.
SUMMARY OF THE INVENTION
[0008] The present invention provides a well treatment fluid
containing water, an amine-based polymer, and an oxidized
polysaccharide-based polymer. The oxidized polysaccharide-based
polymer is able to crosslink with the amine-based polymer and
produce a gel having a viscosity of greater than about 20 cp. The
present invention also provides a method of treating a subterranean
formation penetrated by a wellbore comprising the steps of: (a)
forming the well treatment fluid, and (b) contacting the
subterranean formation with the fluid.
[0009] These and other embodiments of the present invention will be
apparent to one skilled in the art upon reading the following
detailed description. While the invention is susceptible to various
modifications and alternative forms, specific embodiments thereof
will be described in detail and shown by way of example. It should
be understood, however, that it is not intended to limit the
invention to the particular forms disclosed, but, on the contrary,
the invention is to cover all modifications and alternatives
falling within the spirit and scope of the invention as expressed
in the appended claims. The compositions can comprise, consist
essentially of, or consist of the stated materials. The method can
comprise, consist essentially of, or consist of the stated steps
with the stated materials.
DETAILED DESCRIPTION
[0010] In general, the present invention provides a well treatment
fluid containing water, an amine-based polymer, and an oxidized
polysaccharide-based polymer. The oxidized polysaccharide-based
polymer is able to crosslink with the amine-based polymer and
produce a gel having a viscosity of greater than about 20 cp. The
viscosity of the gel is preferably measured at a pH of about 4 to
about 7 and at STP. The well treatment fluid of the present
invention is useful as a well drilling and servicing fluid in
various operations such as drilling, fracturing, sand control, lost
circulation control, completion, conformance control, work over,
and the like.
[0011] The present invention relates to aqueous conformance control
fluids and treatments. The water used for the well treatment fluid
can be of any convenient source, including fresh water, seawater,
natural brine, formulated brine, 2% KCl solution, and any mixtures
in any proportion thereof. Formulated brine is manufactured by
dissolving one or more soluble salts in water, natural brine or
seawater. Representative soluble salts are the chloride, bromide,
acetate and formate salts of potassium, sodium, calcium, magnesium
and zinc. The preferred water for the well treatment fluid is 2%
KCl solution or seawater.
[0012] The amine-based polymers preferably have pendant amino
groups on the polymer which can react with the carbonyl group of
the oxidized polysaccharides. The pendant amino group may be a
primary or secondary amine. Examples of amine-based polymers which
are useful in the present invention include, but are not limited
to, polyvinylamine, polyvinylalcoholamine, chitosan, polylysine and
the like. Preferred amine-based polymers include chitosan,
polyvinylalcoholamine and polylysine. Occasionally, a pendant amine
may be a part of a branched structure of a polymer. Examples of
such polymers which are useful in the present invention include
polyethyleneimine. Poly(vinylalcohol vinylamine) polymers, also
referred to as vinylalcohol/vinylamine copolymers are available,
for example, from ERKOL Corp. of Tarragona Spain. The mole %
vinylamine present in the polymer may range from about 1 to about
12% and the molecular weight of the polymer may be in the range of
from about 10,000 to about 150,000. Although chitosan has been used
in aqueous conformance control, its usefulness has been severely
limited by its relative poor solubility in water. Chitosan is a
beta-(1.fwdarw.4)-polysaccharide of D-glucosamine, and is
structurally similar to cellulose, except that the C-2 hydroxyl
group in cellulose is substituted with a primary amine group in
chitosan. Chitosan occurs in nature in small amounts and is
biodegradable. Chitosan degrading enzymes, namely chitonases,
chiosanases, and lysozymes that degrade chitin-derived materials
occur in bacteria, fungi, algae, mammals, birds, fish, etc. The
biodegradability of chitosan is comparable to hydroxyethylcellulose
(HEC). In-house Biochemical-Oxygen-Demand (BOD) and
Chemical-Oxygen-Demand (COD) tests show a BOC/COD of 54% compared
to 32% for HEC (according to HACH Method 8000 that is described in
the HACH Water Analysis Handbook, 3rd ed., Hach Company
(1997)).
[0013] Chitosan is a partially or fully deacetylated form of
chitin. Chitin is a naturally occurring polysaccharide, which is
the second most abundant natural product on earth preceded only by
cellulose. Structurally, chitin is a polysaccharide consisting of
beta-(1.fwdarw.)2-acetamido-2-deoxy-D-glucose units, some of which
are deacetylated. Chitin is not one polymer with a fixed
stoichiometry, but a class of polymers of N-acetylglucosamine with
different crystal structures and degrees of deacetylation and with
fairly large variability from species to species. Typically,
chitosan has a degree of deacetylation that is between 50% and
100%. The degree of deacetylation in the commercially available
chitosan is usually in the 70% to 78% range.
[0014] The large number of free amine groups (pKa=6.3) makes
chitosan a polymeric weak base. However, because chitosan is a
polysaccharide containing many primary amine groups, it forms
water-soluble salts with many organic and inorganic acids. For
example, chitosan is somewhat more soluble in dilute aqueous acids,
usually carboxylic acids, as the chitosonium salt. Nevertheless,
the solubility of chitosan in acidified water, for example in
acetic or hydrochloric acid, is still only in the 1 to 2% range. If
the pH of the solution is increased above 6.0, polymer
precipitation occurs, thus limiting its solubility. The viscosity
of the aqueous chitosan solution depends on the molecular weight of
the polymer.
[0015] In an advantageous embodiment, the present invention employs
an oxidized chitosan-based polymer to overcome the relative poor
solubility of chitosan. Thus, the oxidized chitosan-based polymer
can serve both as a crosslinker and base polymer to the oxidized
polysaccharide-based polymer. Some examples of chitosan-based
polymers suitable for oxidation in the present invention include
chitosan, chitosan salts with mineral and organic acids, and any
combination in any proportion thereof. Some commercial examples of
chitosan include Chitosan Lactate available from Vanson HaloSource
and Hydagen HCMF available from Cognis.
[0016] To oxidize the chitosan-based polymer, a wide variety of
oxidizers can be used. Examples of oxidizers include sodium
hypochlorite, sodium periodate, hydrogen peroxide, peracetic acid,
and any mixture in any proportion thereof. The selection of the
oxidizer and the concentration of oxidizer should be sufficient to
oxidize or degrade the chitosan-based polymer to a desired
solubility. Oxidizing the chitosan-based polymer increases its
solubility. Without being limited by the theoretical explanation,
the oxidation of the chitosan-based polymer divides the polymer
into shorter chain segments, thus increasing its solubility.
Increased solubility of the chitosan-based polymer may also be
explained by the introduction of carboxyl groups. By increasing
solubility of the chitosan-based polymer, it can be used in higher
concentration in fluids, thereby utilizing it as the base polymer
in the fluids, rather than merely as a crosslinker.
[0017] In the present invention, the oxidized polysaccharide-based
polymer is able to crosslink with the chitosan-based polymer in
water and produce a gel having a viscosity of greater than or equal
to about 20 cp. The viscosity is preferably measured at a pH of
about 4 to about 7 and at STP. Gels ranging from stiff and ringing
type to "lipping" gels can be obtained.
[0018] Without being limited by any theoretical explanation, it is
believed that oxidation of the polysaccharide-based polymer
introduces carbonyl groups. Thus, the oxidized polysaccharide-based
polymer can include a number of carbonyl-based functional groups
such as aldehydes, anhydrides, carboxyl groups, ketones and esters.
The amine-based polymer includes an amino nucleophile bearing an
electron pair, thus enabling it to react or crosslink with the
carbonyl groups of the oxidized polysaccharide-based polymer. The
terms nucleophile and nucleophilic refer to a negative ion or
neutral molecule, such as a primary or secondary amine group on the
amine-based polymer, which brings an electron pair into a chemical
reaction with another electron accepting reactive group or positive
ion, called an electrophile. An electrophile, such as the oxidized
polysaccharide-based polymer having or modified to have carbonyl
groups is capable of accepting the electron pair, thereafter
forming a covalent bond.
[0019] Some non-limiting examples of water-soluble or
water-swellable oxidized polysaccharide-based polymers include
dialdehyde starch (DAS), dialdehyde cellulose, oxidized starch,
oxidized cellulose, oxidized agarose, oxidized partially-acetylated
cellulose, and combinations thereof. It should be noted that DAS or
polymeric dialdehyde is also referred to as oxidized starch. DAS is
preferred as the oxidized polysaccharide-based polymer, which has
as low as 12% oxidation. DAS can be obtained from any of a number
of chemical suppliers, such as Sigma Chemical Company (Catalog N.
P9265) or a manufacturer, Monomer-Polymer & Dajac Laboratories,
Inc. Further examples of oxidized polysaccharide-based polymers
include aldehyde agarose, oxidized gums such as guar, locust bean
gum, gum Arabic, tragacanth, gutta percha or xanthan and alginate
salts.
[0020] Oxidized polysaccharide-based polymers can also contain
ketones, anhydrides and active esters. Ketones useful in the
practice of the present invention can include alpha-dicarbonyl
compounds, beta-dicarbonyl compounds, gamma-dicarbonyl compounds,
and alpha, beta-unsaturated ketones.
[0021] DAS, or oxidized starch is the preferred
polysaccharide-based polymer used to crosslink the chitosan-based
polymer and form rigid gels at useful temperatures. Starch
(C.sub.6H.sub.10O.sub.5).sub.n, is a mixture of linear (amylose)
and branched (amylopectin) polymers of .alpha.-D-glucopyranosyl
units. It is a principal reserve polysaccharide in plants, and
constitutes a substantial portion of the human diet; thus it is a
biodegradable option for well treatment fluids. Starch suitable for
oxidation as in the present invention can include a number of
starch-based polymers. In one embodiment, the starch is selected
from the group consisting of corn starch, potato starch, waxy
maize, dextrinized starch and any mixtures in any proportion
thereof. Additionally, the starches may be modified chemically
prior to oxidation. Examples of such starches include cationic
starches, hydroxyethylated starches, hydroxypropylated starches,
carboxylated starches and the like.
[0022] The extent of oxidation of the polysaccharide-based polymer
can be controlled by, for example, the amount of oxidizer added,
the duration of the oxidation process and the temperature of
reaction. For example, oxidation time for starch can be attained in
24 hours. Oxidized starch is prepared by oxidizing about 1% to
about 25% of the glucose units of the starch. A variety of
oxidizers can be used to oxidize the starch, including oxidizers
selected from the group consisting of alkali, alkaline earth and
transition metal salts of, for example, periodate, hypochlorite,
perbromate, chlorite, chlorate, hydrogen peroxide, peracetic acid
and combinations thereof. Without being limited by theoretical
explanation, it is believed that oxidation of the hydroxyl groups
of C-2, C-3 and C-6 carbons of the glucose units in the starch
gives electrophilic functional groups such as aldehydes, ketones,
or carboxylic acids, which react with nucleophiles such as
chitosan-based polymers.
[0023] The concentration of the polysaccharide-based polymer and
amine-based polymer in the fluids is selected to be sufficient to
impart to the fluids the rheological characteristics desired. The
weight ratio of oxidized amine-based polymer to oxidized starch is
from about 50:1 to about 1:50. In one advantageous embodiment that
uses oxidized starch as the polysaccharide-based polymer, the
chitosan-based polymer includes an oxidized chitosan-based polymer
that is present in the amount of up to about 10 wt % of the water.
At such high concentration in the well treatment fluid, the
oxidized chitosan-based polymer serves as the base polymer. The
oxidized starch, in turn, serves as a crosslinker to the oxidized
chitosan-based polymer. In one embodiment, the oxidized starch is
present in the amount of up to about 10 wt % of the water. In such
embodiments, the chitosan-based polymer serves as a crosslinker to
the oxidized starch. In one aspect, the weight ratio of the
oxidized starch to chitosan-based polymer is from about 20:1 to
about 1:20. Thus, the oxidized chitosan-based polymer can serve
both as a crosslinker and a base polymer to the oxidized
polysaccharide-based polymer. The oxidized chitosan-based
polymer/oxidized polysaccharide-based polymer combination provides
an environmentally acceptable system since both the crosslinker and
the major component are natural, non-toxic polymers.
[0024] Since chitosan-based polymers are typically more expensive
than starch, the economic aspects of biodegradability can be
improved by using oxidized starch as the base polymer and either a
substantially biodegradable polyvinylalcoholamine or a
biodegradable chitosan-based polymer as the crosslinker. It is
understood by those skilled in the art that both the amine-based
polymer and the oxidized polysaccharide-based polymer can serve as
a crosslinker. The polymer in lower concentration serves as a
crosslinker to the polymer in higher concentration. However in the
case of chitosan, because the non-oxidized chitosan-based polymer
is of low solubility, the chitosan-based polymer must be oxidized
to serve as the base polymer, or polymer of higher
concentration.
[0025] To further illustrate the present invention, and not by way
of limitation, the following examples are provided.
EXAMPLE 1
[0026] Table 1 provides the results obtained with oxidized starch
and chitosan. The gel times are controlled by the degree of
oxidation and the base polymer to cross-linker ratio, as well as
the pH and temperature.
1TABLE 1 Chitosan concentration Chitosan to Temp- Gel in 1% acetic
% starch oxidized starch erature Time solution oxidation ratio
(wt%) pH (.degree. F.) (Hr:min) 1% 19 2:1 5 175 1:30 1% 12.5 2:1 5
175 8
EXAMPLE 2
[0027] To improve the relative overall biodegradability of the gel
compositions, starch was oxidized to different degrees to introduce
carbonyl groups, and the resulting products were reacted with
chitosan and oxidized chitosan. Initial experiments were directed
to using oxidized chitosan as the base polymer and the oxidized
starch as the crosslinker. The compositions yielded gels and the
results are presented in Table 3. The degree of starch oxidation
also effected the gel time. For example, when the degree of starch
oxidation is high (higher level of carbonyl formation), the gel
time is shorter than when the degree of oxidation is low.
2TABLE 2 Starch/Oxidizer.sup.1 % Chitosan/ Temp. Gel Weight Ratio %
Starch (.degree. F.) pH Time(HRS) 50/1 5/2 160 4.9 1 50/1 5/2.5 160
4.9 8 50/1 5/1.5 160 4.9 None in 65 hrs 36/1 5/2 160 4.9 4.8 36/1
5/1.5 160 4.9 23 36/1 5/1 160 4.9 None in 63 hrs 50/1 5/2.5 160 4.9
8 50/1 5/2 190 4.9 5 50/1 5/1.6 190 4.9 11 50/1 5/1.5 160 4.9 None
in 65 hrs 50/1 5/1.5 190 4.9 22 50/1 5/1.3 190 4.9 24 .sup.1sodium
periodate was used as the oxidizer.
EXAMPLE 3
[0028] The results of using oxidized starch as the base polymer and
the oxidized and non-oxidized, chitosan-based polymers as
crosslinkers are shown in Table 4. The results indicate that the
ratio of the base polymer to crosslinker as well as the
concentration of the two components may be used to optimise the gel
time.
3TABLE 3 Oxidized Starch as the Base Polymer and
Oxidized/Nonoxidized Chitosan as the Crosslinker % Starch/ Temp.
Gel Time Sample Crosslinker % Chitosan (F.) (Hrs) 1 Nonoxidized
8/0.2 180 None in 3 day Chitosan 2 Nonoxidized 2/0.8 180 < than
45 min Chitosan 3 Oxidized 7/2 180 No gel in 4 Chitosan .sup.1 days
4 Oxidized 4.9/4.9 180 <30 minutes Chitosan 5 Oxidized 5/2 180
<30 minutes Chitosan 6 Oxidized 5/1 180 No gel in 4 Chitosan
days .sup.1 Chitosan was oxidized with hydrogen peroxide in a
suspension at elevated temperature followed by acidification with
acetic acid
[0029] The degree of oxidation of the polysaccharide-based polymer
can be used to control gel time. In the case where chitosan is used
as the crosslinker or base polymer, the degree of oxidation of
chitosan can also be used to control the gel time. Besides the
degree of oxidation, a number of other variables can be used to
control gel time, or impact the gelling of the oxidized or
non-oxidized, chitosan-based polymer with the oxidized
polysaccharide-based polymers, irrespective of which serves as the
crosslinker. Such variables include type of polysaccharide-based
polymer, crosslinker concentration, pH of the gel system, mix
water, application temperature and chitosan modification.
[0030] In comparing the ability of the oxidized
polysaccharide-based polymers to crosslink or react with the
oxidized/non-oxidized, chitosan-based polymer, the order of ease of
crosslinking reactivity may be effected by steric resistance to the
approach of the amino group of the oxidized/non-oxidized
chitosan.
[0031] Yet another variable that can be used to control gel time is
the pH of the gel system. The crosslinking reaction proceeds with
decreasing pH of the gel system. This observation is in accordance
with expectations for amine-type crosslinkers, such as
oxidized/non-oxidized, chitosan-based polymers. Without being
limited by theory, it is believed that the lone pair of electrons
on the amine nitrogen groups of the amine-based polymer is expected
to be protonated in acidic media, thus making them unavailable to
initiate a nucleophilic attack on the oxidized polysaccharide-based
polymer. Therefore, varying the pH of the gel system can by used to
control gel time.
[0032] Still another variable that can be used to control gel time
is the mix water, which is believed to effect the crosslinking
reaction. The crosslinking reaction can proceed significantly
faster in fresh water compared to seawater. Similar results occur
in the crosslinking reactions of the prior art, such as
crosslinking reactions of (PA-t-BA) with (PEI).
[0033] Therefore, gel time can be controlled by variables that
include the degree of oxidation of the polysaccharide-based
polymer, the pH of the solution, and base polymer/crosslinker ratio
and the corresponding solution concentrations.
[0034] In high temperature applications, it can be more
advantageous to use the oxidized chitosan-based polymer as a
crosslinker because non-oxidized, chitosan-based polymers have very
short gel times at high temperatures. It is difficult to achieve
practical gel times with non-oxidized, chitosan-based polymers at
such temperatures. Surprisingly, the oxidation of the
chitosan-based polymer made the gelling compositions essentially
salt-insensitive. It should be noted that gel time with the
oxidized chitosan-based polymer appears to be more strongly
influenced by temperature than other polymer/crosslinker ratios, or
the corresponding concentrations especially at temperatures higher
than 200.degree. F., which may be indicative of the sterically
hindered environment of the amino group in the oxidized
chitosan-based polymer. The base polymers have different
reactivities with crosslinkers, which allow the selection of
suitable water-soluble polymer for a wide range of formation
temperatures in conformance applications. The chitosan-based
polymer/polysaccharide gel system has thermal stability in the
temperature range applicable to many conformance-related
applications, making it commercially useful.
[0035] The well treatment fluid of this invention generally will
contain materials to provide various characteristics of properties
to the fluid. Thus, the well treatment fluid can contain one or
more viscosifiers or suspending agents in addition to the
chitosan-based polymer, weighting agents, corrosion inhibitors,
soluble salts, biocides, fungicides, seepage loss control
additives, bridging agents, deflocculants, lubricity additives,
shale control additives, pH control additives, and other additives
as desired. The well treatment fluid can also contain one or more
materials that function as encapsulating or fluid loss control
additives to restrict the entry of liquid from the fluid to the
contacted shale. Representative materials include partially
solubilized starch, gelatinized starch, starch derivatives,
cellulose derivatives, humic acid salts (lignite salts),
lignosulfonates, gums, biopolymers, synthetic water soluble
polymers, and mixtures thereof. If desired, water-soluble potassium
salts can be incorporated into the fluids of this invention to
increase the potassium ion content thereof. Other materials that
may be added to fluids to enhance the shale stabilizing
characteristics of the fluids are potassium chloride, potassium
formate, and potassium acetate.
[0036] Mixtures of oxidized polysaccharide-based polymers and
amine-based polymers can be prepared for addition to the well
treatment fluid of this invention for maintenance of the properties
thereof, or indeed, for preparing the initially prepared oil and
gas well drilling and servicing fluids before adding the fluids of
the present invention thereof. By oxidizing the
polysaccharide-based polymer and crosslinking such polymer with a
chitosan-based polymer, a treatment fluid made entirely of
biodegradable material can be achieved. Furthermore, by oxidizing
the chitosan-based polymer, the solubility of the chitosan-based
polymer is increased so as to enable the chitosan-based polymer to
be used in higher concentration in the treatment fluid, while still
retaining the reactivity of the chitosan-based polymer.
[0037] An improved method of this invention for treating a
subterranean formation penetrated by a wellbore is comprised of the
following steps: (a) forming a well treatment fluid comprising
water, the amine-based polymer, and a oxidized polysaccharide-based
polymer; and (b) contacting the subterranean formation with the
well treatment fluid. The oxidized polysaccharide-based polymer is
able to crosslink with the amine-based polymer in water and produce
a gel having a viscosity of greater than or equal to about 20 cp.
In one embodiment, forming the well treatment fluid further
includes the step of crosslinking the chitosan-based polymer with
the polysaccharide-based polymer. Preferably, the viscosity of the
gel of greater than or equal to about 20 cp is measured at a pH of
about 4 to about 7 and at STP. In one embodiment, the
chitosan-based polymer includes an oxidized chitosan-based
polymer.
[0038] In one embodiment, contacting the subterranean formation
with the well treatment fluid further includes introducing the well
treatment fluid into the wellbore penetrating the subterranean
formation. As indicated above, the treatment fluid in the present
invention is useful in drilling a well wherein there is circulated
in a wellbore (borehole) a drilling fluid during the drilling
thereof. The well treatment fluid of this invention is circulated
or spotted within a borehole during well drilling or servicing
operations. The well treatment fluid can be formulated to provide
viscous gels to overcome lost circulation problems in a
wellbore.
[0039] After careful consideration of the specific and exemplary
embodiments of the present invention described herein, a person of
ordinary skill in the art will appreciate that certain
modifications, substitutions and other changes can be made without
substantially deviating from the principles of the present
invention. The detailed description is illustrative, the spirit and
scope of the invention being limited only by the appended
Claims.
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