CA1249426A - Stabilizing polymer thickened aqueous alkaline solutions with a mercaptobenzothiazole - Google Patents

Abstract

STABILIZING POLYMER THICKENED AQUEOUS ALKALINE
SOLUTIONS WITH A MERCAPTOBENZOTHIAZOLE

Abstract of the Disclosure A viscosity-stabilized viscous alkaline aqueous solution comprises an aqueous alkaline solution containing a water-thickening polymer and a 2-mercaptobenzothiazole polymer stabilizer.

Description

~ 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

-2- ~ 3292-2587 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 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 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 ~ 2 ~ t~

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 ) (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 -8- 3293-2~87 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 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~

~. .. ~,. , ~2~

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 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.

Claims (10)

WE CLAIM AS OUR INVENTION:

1. A viscosity stabilized solution comprising at least one water-soluble, water-thickening polymer dissolved in a substantially oxygen-free aqueous liquid which is sufficiently alkaline to form surface-active soaps of petroleum acids and contains an amount effective for polymer stabilization of a 2-mercaptobenzothiazole of the formula:

where R represents one or more hydrogen atoms or lower hydrocarbon radicals and X represents a hydrogen atom or other monovalent cation.

2. The solution of Claim 1 where 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.

3. The solution of Claim 1 containing at least about 2000 ppm of a readily oxidizable lower molecular weight alcohol or glycol.

4. The solution of Claim 3 in which said alcohol is methyl alcohol.

5. The solution of Claim 4 in which said thiazole is 2-mercaptobenzothiazole.

6. The solution of Claim 5 containing an alkali metal dithionite oxygen-scavenger.

7. The solution of Claim 1 in which said polymer is a polyacrylamide polymer.

8. The solution of Claim 1 in which said polymer is a hydroxyethyl cellulose polymer.

9. In an oil-recovery process in which an alkaline aqueous liquid is thichened with a water-soluble polymer and injected into a subterranean reservoir, the improvement comprising treating said aqueous liquid to remove substantially all dissolved oxygen;
adding to the deoxygenated liquid at least one water-soluble sulfur-containing antioxidant;
adding to the deoxygenated liquid a 2-mercaptobenzothiazole of the formula defined in Claim 1; and injecting the liquid into the reservoir.

10. The process of Claim 9 in which at least about 2000 ppm of a readily oxidizable alcohol or glycol is added to the liquid to be injected.