Note: Descriptions are shown in the official language in which they were submitted.
~ K 9037 CAN
STA~II.IZING POLYMEK T~IICKENED AQUEOUS ALKALINE
SOLUTIONS WITEI A MERCAPTOBENZOTHIAZOLE
Backgro~nd of the Invention
The present invention relates to an oil recovery process in which
an aqueous alkaline solution containing a water-soluble polymer for
reduclng the mobility of the solution is in~ected into a subterranean
reservoir to displace oil within the reservoir. More particularly the
inventlon relates to such a process in which the oil i8 displaced toward a
production well or production locatlon.
With respect to fluid drive oil recovery processes, it i~ known
to use water-soluble anionic polymers as mobility reducing agents. For
15 example, U.S. Patent No. 3,208,518 suggests using such a solution with an
initial pH which reduces the solution viscosity during injection and then
rises to increase the solution viscosity within the reservoir. U.S. Patent
No. 3,343,601 suggests that polymer thickened flood water be deoxygenated
by adding a water-soluble hydrosulfite before or after adding the polymer.
20 U.S. Patent No. 3,581,824 suggests that~ within a reservoir, a polymer
solution be contacted with an aqueous solution containing divalent cations
to agglomerate the polymer~ for selectively plugging portions of the
reservoir. U.S. Patent No. 3,676,494 describes sulfur-containing aromatic
carboxylic acid amides and indicates that they can stabilize
oxygen-sensitive organic materials against oxidation. U.S. Patent No.
3,801,502 suggests that the viscosity increasing capability uf xanthan gum
polymers be increased by adding various materials inclusive of
water-soluble alcohols. U.S. Patent No. 4,317,759 discloses that a
combination of a phenolic antioxidant and a mercaptobenzimidazole can
stabili~e an aqueous polyacrylamide polymer solut~on.
Summary of the Invention
The present invention relates to a solution of a water-soluble
polymer ln a subst~ntially oxygen-free aqueous liquid which ls sufficiently
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alkaline to form soaps of petroleum acids and is stabilized by the presence
of an effective amount of a 2-mercaptobenzothiazole of the formula:
I l . sx
~S~
where R represents one or more hydrogen a-toms or lower hydrocarbon radicals
and X represents a hydrogen atom or other monovalent cation. Preferably,
X is hydrogen, and at least one R is alkyl and the molecular configuration
of the thiazole provides properties of water-solubility and oxygen reactivity
at least substantially equaling those of 2-mercaptobenzothiazole.
A particularly preferred entodiment of the present invention
comprises an improver~nt of an oil-recovery process in which a substantially
oxygen-free alkaline aqueous fluid containing a water-thickening polymer is
mi æ d with an amount effective for polymer stabilization of a 2-mercaptobenzothi-
azole of the above formNla and the mixture is injected into an oil-containing
subterranean reservoir.
escription of the Drawing
Figure 1 is a plot of variations with increasing am~unt of additives
in the viscosity of an aqueous alkal.ine polymer solution. Fig~re 2 is a sche-
rnatic illustration of an improved apparatus for measuring variations with time
in the viscosity of a liq~Lid. Figures 3 and 5 are plots of variations with time
of viscosities OI various aqueous alkaline polymer solutions. Figure 4 is a
plot of variations with time of the viscosities of aqueous alkaline polymer
solutions at different t~mperatures.
2a~ 2 ~ 9 ~ 2 6 3293-2587
Description of the Invention
Ihe chenLical composition of water-soluble polymers which are
effective as water-thickening agents, such as polyacrylamide polymers or copo-
lymers or xanthan gums, is such that the polymers are suscep-tible to chemical
deyradation or depolymerization. Such a degradation (which increases with
increasing temperature) reduces the viscosity of a solution containing the
polymers. Two paths by which such a degradation can occur comprise hydrolysis
and free-radical reactions. me hydrolysis involves
~,,
~2~
the reaction of molecules of water with amide or ether-type linkages ln the
structure of the polymer. The free-radical reactions are usually those
initiated when the polymer solution is mi~ed with air or oxygen. Such an
oxygen-containing mixture tends to form hydroperoxides and the
decomposition of the hydroperoxides produces reaction-initiating
free-radicals that propagate polymer-degrading radical reactions.
Numerous types of materinls and techniques for treating squeou~
solutions to remove ~issolved oxygen are known to those skilled in the art.
In general, such treatments are effected by (or completed by) dissolving a
strong reducing agent ~or oxygen scavenger) in the solution. In a solution
in which a radical degradable polymer is present, a combination of excess
oxygen and oxygen scavenger may create an oxidation reduction couple or
redox system. Therefore, although oxygen has been effectively scavenged,
the solution may lose viscosity very quickly if any minute oxygen
introduction occurs. ~nd thus, preferably, a free-radical terminating
chemical is also added to the solution. This will provide protection
against oxygen leakage into the solution and also provide protection
against free-radicals in the solution whose source was not atmospheric
oxygen (such as unreacted monomer).
The aqueous liquid used in the present process is an alkaline
solut~on, havlng a p~l above about 10. Such a solution preferably has a
total dissolved salt content of not more than about 100,000 ppm and a
hardness compatible with the alkalinity of the solution. When deoxygenated
for use in the present process, such a water is preferably substantially
completely free of dissolved oxygen and its total dissolved salt content
preferably includes from about 100 to 500 parts per million S03
group-containin~ oxygen scavenger (in terms of S03 group equivalent).
Water-soluble inorganic compounds that contain or form ions
having an S03 group are particularly suitable oxygen-scavengers for use in
the present oil recovery process. Such compounds include water-soluble
alkali IQetal sulfites, bisulfites, dithionites, etc. As known to those
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skilled in the art, such an oxygen scavenger is preferably used in a slight
stoichiometric excess (relative to the amount needed to remove
substantially all of the dlssolved oxygen in the solution being treated).
Such an excess is preferably from about 10 to 500% more than
stoic!liometric. In the alkaline solution of the present invention, the
oxygen-scavenger ~s preferably an alkali metal dithionite.
A sulfur-containing antioxidant used in the present process
preferably comprises substantially any such water-soluble antioxidant
composition (transfer agent, terminator, peroxide decomposer) which is
effective with respect to decomposing peroxides in aqueous solutions, and
is capable of protecting an alkaline-soluble polymer sOlueiOn from drastic
loss of viscosity due to being stored overnight at 80C with 36 ppm
hydrogen peroxide at atmospheric pressure. Examples of such compounds
include relativ&ly water-soluble thia~oles, thioethers, mercaptans, and the
like. Particularly suitable examples are 2-mercaptobenzothiazole,
thiodiacetic acid (thiodiglycolic acid, 3,3'-thiodiacetic acid,
3,3'-thiodipropionic acid (dithiodiglycolic acid) and their water-soluble
homologues.
Readily oxidizable alcohols or glycols which are suitable for use
in the present process include substantially any water-soluble primary and
secondary alcohols or glycols that are easily oxidized, and are capable of
protecting a water-soluble anionic polymer solution from drastic 1088 of
viscosity due to being stored overnight at 80C with 36 pp~ hydrogen
peroxide at atmospheric pressure. Examples of such compounds include
methanol, ethanol, allyl alcohol, isopropyl alcohol, isobutyl alcohol,
ethylene glycol, and the like.
Water-soluble anionic polymers suitable for use ln the present
invention include the essentially linear hlgh molecular weight
polyacrylamide polymers and copolymers, which may have some or all of the
amide groups hydrolyzed to carboxyl groups and polysaccahride polymers such
as xanthan gums or alkyl or hydroxyalkyl ethers of cellulose. Examples of
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particularly suitab:Le polymers inclu~e the polyacrylamide polymers such as
* *
Pusher-500 or Pusher-700 available from Dow Chemical Compa1ly, or Cyanatrol
950 available from ~merican Cyanamicl Company.
As known to those skilled in tlle art, in an oil recoverJ process
in which fluids are displaced witllin a subterranean reservoir by injecting
a viscosity enhanced aqueous solution, the effective viscoslty
should be at least substantially equal to and preferablY ~reater
than that of the fluid to be disr,laced. In the present process,
~he concentration of e.g.~ an anionic polyacrylamide, in such
lO a solution should be in the order of about 500 to 3,000 parts by
weigllt of hydroli~ed polyacrylamide per million parts by weight of aqueous
liquid. Such concentration generally provides viscosities in the order of
from about 4 to 50 centipoises at room temperature, in a water containing
about 25,000 parts per million total dissolved solids.
In the present process, the concentration of antioxidant can be
relatively low, in the order of about 50 parts per million (weight per
weight of aqueous liquid) and preferably ~rom about 200 to 800 parts per
million. The readily oxidizable alcohol or glycol concentration can be
from about 50 to 5,000 yarts per million, and preferably from about 500 to
2~ 2,000 parts per million. The concentration of chemicals in the present
process depen~s upon the total amount of oxygen in the make-up brine, and
the amount of oxygen wl1ich contacts the solution during mixing and
injection into the reservoir.
It appears likely that hydroxyl radicals are formed from oxygen
at a rate which increases with increasing temperature and those radicals
extracting hydrogen atoms from polymers can weaken the polymer. It is
known that a rigorous exclusion of oxygen from a polymer solution can
stabilize the solutlons at relatively high temperatures.
Polymer-stabilizing materials usually comprise additives which function as
oxygen scavengers or antioxidants ~ut most of those materials which are
* Trade Mark
~KAC8429903
~L~ek~
useful in neutral solutions, for example, formaldehyde or thiourea are
chemically unstable in alkaline solutions.
Since the transformation of dissolved oxygen into reactive
radicals in aqueous solution does not occur instantaneously and is
dependent on temperature as well as whether transition metal ions are
present in the solution, a relatively quick, but effective, screening
procedure has been developed. This comprises adding to polymer-thickened
solutions, such as polyacrylamide polymer so1ution, chemicals which are
capable of immediately forming reactive radicals similar to those which
would subsequently be formed from dissolved oxygen. Such chemicals can
include ammonium peroxysulfate which reacts with water producing peroxide
radicals and is used in oil field production operations as a viscosity
breaker for polymer-thickened fracturing fluids or compounds such as
hydrogen peroxide or substituted peroxides which are radlcal formers. The
concentrations of such radical-initiator chemicals which are used in the
screening procedures are relatively small. The stoichiometric proportions
- of the inhibitor chemicals such as the present benzomercaptothiazoles are
significantly higher, so ~hat a fair test of their inhibitor effectiveness
is provided. It was found that meanin~ful indications of inhiblting
capabilities are obtained by determining a ratio comprising the viscosity
of the polymer solution nfter 24 hours divided by the viscoslty of the
freshly prepared solutlon.
In general, the pH of an aqueous alkaline solution in~ected into
a subterranean reservoir in order to form surface active soaps of petroleum
acids contained in the reservoir oil and enhance the recovery of oil i9
relatively high, such as a pH of at least about 10 to 13 or more. ~he
alkalin~ty can be established by incorporaeing in the aqueous solution
alkaline materials such as the alkali metal or ammonium hydroxidesJ
carbonates or s1icates, or the like. Where ehe dissolving of silica i8
apt to be a problem in a siliceous reservoir, the alkaline solution can
advantageously contain an amount of soluble silicate partIcularly suited
BK~C8429903
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for the reservoir temperature, for example, as described in U.S. Patent
4,458,755 by J. G. Southwick and R. C. Nelson.
A11 of the aqueous alkaline polymer solutions used in the tests
described hereill had a pH of about 13 and contained amounts of polymer
providing an initial solution viscosity of about 30 centipoise.
Typical 24-hour screening tests and the compositions of the
tested fluids are presented in Table I.
-
TABLE I
24-HOUR "SCREENING TESTS" OF CHEMICALS TO
PROVIDE VISCOUS STABILITY OF POLYMER SOLUTIONS
All samples contained: 1% NaOH
1.5 NaCl
2500 ppm polyacrylamide
120 ppm (0.0005M) ammonium peroxysulfate
lOO ppm sodium dithonite
Molar Vi~cosity Ratios
Tests Chemical Additives Concentration (Before/After 24 Hrs )
50C 70C 80C
(1) 800 ppm TEPA (Tetra- .0042 .50 .44 .28
ethylenepentamine
500 ppm Allyl Alcohol .0086
(2) Like (1) without
ammonium peroxysulfate .96
(3) 800 ppm TEPA .0042 .58 .54 .49
2000 ppm Allyl ~lcohol .0344
(4) 800 ppm TEPA .on42 .84 .77 .63
1000 ppm 3-3' .0056
Thiodipropionic ~cid
(5) 800 ppm TEPA .0042 .98 .96 .91
1000 ppm 2-MBI .006
(2-Mercaptobenzimidazole)
1600 ppm Methyl
Alcohol .S
(6) 1500 ppm 2~MBT .009 1.16 1.22 1.22
(2-Mercaptobenzothiazole)
40,000 pp~ Methyl Alcohol
In each of the tests, the so]utions were prepared by using an
o~ygen scavenger along with an antioxidant or free-radical neutrallzer and
BKAC8429903
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a sacrificial agent such as an alcohol or glycol. As indicated by tests 1
and 2, the tetraethylene pentamine and alkyl alcohol system w~s only
marginally effective in the alkaline solution containing ammonium
peroxysulfate, although that pentamine ls commonly used as a stabilizer for
water-soluble polymers and is available from Union Carbide under the trade
mark "UCAR". The activity of ammonium peroxysulfate in generating free
radicals is shown by the fact that whell it was absent, in Test 2, the
pentamlne-containing solutlon showed good stability at 70C.
The additive combination of Test 5, including TEPA and 2
mercaptoben~imidazole is currently prohibitively expensive for use as an
Inhibitor in an oil recovery process. The current price is about $17 per
pound for that imidazole.
Test 6 showed the very significant stabilization provided by the
2-mercaptobenzothiazole (2-MBT) stabilizer of the present invention. The
mercaptobenzothiazole is a compound wiclely used in commercial operations,
such as rubber vulcanization, and is currently available at $1.50 dollars
per pound. Test 6 used an unnecessarily large proportion of methyl alcohol
but resulted in a solution in which the viscosity increased about the
initial value. Such an increase in viscosity is comnton for polyacrylamide
2~ solutions wllich have been stabilized. The increase in viscosity is not
caused by the additive but is a result of alkaline hydrolysis of some of
the amide groups of the polymer.
Figure 1 shows a plot of viscosity degradation in 24-hours at
80C in aqueous alkaline solutions containing 2500 ppm polyacrylamide, 100
ppm soditml dithionite~ 1% sodium hydroxlde and 1.5% sodlum chloride with
increasing "fracticns of a package" of stabilizer consisting tin total) of
1000 parts per million of 2-mercaptobenzothiazole and 1.67~ methyl alcohol.
As indicated, tllere was no loss in the effectiveness occurring when the
full package amount was reduced to half, but after that, si~nlficant
furthèr reductions in the stabilizer resulted in losses of performance. In
these tests the lowest effective concentration was about 500 ppm 2-MBT,
BK~C8429903
2,~
8300 ppm methanol and about 100 ppm sodium dithionate. Similar tests of
the effect of alcohol concentration have indicated a nearly equal
performance for methanol concentrations between about 2100 to 8300 ppm,
with 500 ppm 2-MBT.
The total amount of oxygen which can contact an aqueous polymer
solution is a critical factor regarding the rate and extent of the
degradatlon of viscosity. Viscosity debllitat:Lon tests are co~monly
performed by heat-sealing polymer solutions within glass containars, 8uch
as 20cc Wheaton glass ampules. When such ampules are filled with liquid,
with minimum space being allowed for the heat-sealing operation, about a
5cc air space remains. In view of this, a new technique and apparatus has
been developed for ensuring that less air is allowed to contact the
polymer.
~igure 2 is a schematic illustration of the present tubular
viscosity monitoring apparatus. The liquid to be tested is placed in tube
1 which is an elongated container having a relatively small internal
diameter and walls composed of materials which are rela~lvely stable to the
polymer solution and are capable of being sealed while the interior of the
tube is evacuated. In a preferred embodiment, the tube is vacuum sealed;
is 12-inches long and has an inner diameter of about 6 mm.
Calculations have indicated that when such a tube is heat-sealed
with the air space under a vacuum of 2l-22 inches of mercur~, the
difference in the air space volume relative to that in a non-vacuum-sealed
glass ampule is such that about 20 times fewer oxygen molecules are present
in the space within the vacuum-sealed tube. The calculations predict that
1.13 X 1018 molecules of oxygen remain in the air space.
When a particular kind and amount of inhibitor causes a
particular polymer to remain stable through a given period, it is apparent
that the inhibitor package is sufficient for controlling degradation for
that amount of oxygen. However, that inhibitor package may provide
BKAC8429~93
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insufficient control for levels 20 times greater, sucll as the levels ln
non-vacuum-sealed ampules.
In the present viscosity tester the tube 1 is provided with a
small chemically inert ball, such as a 3 mm dlameter Teflon ball 2.
means, such as a pair of timing marks 3 and 4 are provided for indicatin~
when the ball has traveled a given distance within the liquid. A guide for
malntaining the liquid-contaillirlg tube in a given vertical alignment is
provided by inclined plane 5 which supports the tube nt a sultable angle,
such as 30 from horizontal.
Experiments have indicated that the time required for the ball to
roll 6 inches down the tube while the tube is inclined 30 from horizontal
and contains an aqueous polyacrylamide solution of 30 cp viscosity
corresponds to a shear rate of about 20.0 reciprocal seconds, wbich is in
the neighborhood oE the shear rate in a typical waterflood process. In a
preferred procedure a fall-time (or roll-down time) measured for a freshly
prepared polymer solution is compared with that for the same solution after
storage. While not identical, such fall time ratios are roughly equivalent
- to solution viscosity ratios. Where greater accuracy is desired,
calibration curves can be made from polymer solutions of differing
viscosities, with such a curve preferably being measured for each
particular type of polymer, such as xanthan gums, hydroxyethyl celluloses,
polyacrylamides, etc. -- since the falling ball measurements are not
performed at constant shear rates.
Figure 3 shows variations of polymer solution viscosity with time
for the specified series of solutions. Each of the specified solutions
also contained 2500 ppm polyacrylamides, 1% sodium hydroxide, and 1.5%
sodium chloride. The solutions ~ere maintained at 74C and the viscosities
were measured by fall time ratios.
The data from these tests differs from those of the screening
test. In these long-term tests, the inhibitor package with TEPA, methanol
and sodium dithionite provided more protection than was indicated in the
* Trade Mar~
BKAC8429903
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--11--
screening tests. Also, in the long-term tests, little difference was
apparent for the ~-NBT system of the present invention with and without
methanol.
Figure 4 shows a plot of variations of solution viscosity with
time for the aqueous alkaline polymer solutions stored at 165F and 225F.
In each case, the solutions contained 2500 ppm of the polyacrylamide
flvailable Erom Amerlcan Cyanamid Company under the trnde name "Cyanatrol
950", 1% sodium hydroxide, 1.5% sodium chloride, 500 ppm of
2-mercaptoben~othiazole, 2100 ppm methyl alcohol and 100 ppm sodium
dithionite. The viscous stabillty was less at the higher temperature, but
the results provided by the present mercaptobenzothiazole stabilizer were
excellent. The viscosity for the solution contalning that stabilizer
remained higher than the original solution after nearly one year of storage
at 225~F.
Figure 5 shows plots of viscosities with time for, respectlvely,
a xanthan gum polymer, a hydroxyethyl cellulose, and a polyacrylamide
polymer in solutions stabili~ed in accordance with the present invention.
In addition to the indicated kinds and amounts of polymers, each of the
solutions contained 1% sodium hydroxide, 1.5% sodium chloride~ 500 ppm
20 2-mercap~obenzothiazole, 2100 ppm methyl alcohol and 100 ppm sodium
dithionite. Each of the solutions had a pH of about 13 and was maintained
for the indicated time at 165F.
The viscosity of a xanthan gum polymer decreases in an alkaline
solution and the rate of decrease incr~ases with incraases in the pH of the
solution. It seems likely that, in the above tests, although the chemieal
inhibitor system effectively halted the oxidative degradation, the xanthan
polymer lost viscosity due to the pH 13 of the solutions. Such a 1068 was
not encountered in polyacrylamide solution. This migllt be due to the
xanthan polymer containing an ordered helical structure which is less
stable in alkaline solution. It is also possible that some linkages
betweell saccharide re~idues in the xanthan gum polymer are alka1ine lablle
BKAC8429903
and hydroly~e at elevated temperatures. Whatever the reason, the xanthan
gum polymers may simply be unsuitable for use in alkaline solutions of
significantly high pH.
The hydroxyethyl cellulose solutions exhibited stability in the
high pH solutions at 165F but, in other tests, were found to be degraded
rapidly at 225. Hydroxyethyl cellulose is moderately stable in alkaline
solutionR and may fln~ application in oil-recovery situations in which, due
to hlgh salinities and/or the possib;llity of mechanical degr~dation oE
polyacrylamide polymers, the conditions are unsuited for the xanthan gum or
acrylamide polymers.
BKAC8429903
