Alkyl Sulfonates of Polyvinyl Alcohol as Viscosifiers in Saline Solutions

The present invention relates to alkyl sulfonate salts derived from polyvinyl alcohols which are viscosifiers, e.g., for enhanced oil recovery fluids and for use in drilling fluids.

In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplementary recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.

One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous displacing medium; that is, the injected displacing medium tends to channel through certain portions of the reservoir as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors such as differences in the mobilities of the injected displacing liquids and the displaced reservoir oil and permeability variations within the reservoir which encourage preferential flow through some portions of the reservoir at the expense of other potions.

Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing medium and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected displacing medium in order to increase the viscosity thereof and thus decrease its mobility to a value equal to or less than the mobility of the displaced reservoir oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Many polymeric thickening agents including both anionic and cationic polyelectrolytes have been proposed for use in such mobility control operations. Thus, U.S. Pat. No. 3,085,063 discloses waterflooding in which the water is thickened by the addition of polyvinyl aromatic sulfonates such as sulfonated polystyrene and copolymers of such vinyl aromatic sulfonates. Similarly, U.S. Pat. No. 3,984,333 discloses waterflooding involving the injection of an aqueous solution thickened by block copolymers in which the water-soluble blocks are sulfonated by polyvinylarenes and the relatively water-insoluble blocks are polymerized alpha olefins and/or hydrogenated dienes such as polyisoprene and polybutadiene. Synthetic anionic polymers such as those disclosed in the patents discussed above, as well as the more widely used partially hydrolyzed polyacrylamides, suffer a number of disadvantages in actual operations. Where the injected water or the reservoir water contains significant quantities of dissolved inorganic salts, their viscosity yield is decreased materially. Also U.S. Pat. No. 3,969,592 discloses water-soluble polymers manufactured by treating an aqueous suspension of protein with selected enzymes.

Also U.S. Pat. No. 4,110,232 discloses a waterflooding process for producing oil wherein an aqueous solution of a copolymer formed of hydrophobic olefinic segments and cationic aromatic segments is injected into oil-containing subterranean formation, and U.S. Pat. No. 4,222,881 discloses a waterflood oil recovery process involving the use of an amphoteric polyelectrolyte as a thickening agent for mobility control comprising a copolymer of a quaternary vinyl pyridinium sulfonate-styrene block copolymers.

During the drilling of an oil well, a usually aqueous fluid is injected into the well through the drill piper and recirculated to the surface in the annular area between the well-bore wall and the drill string. The functions of the drilling fluid include: lubrication of the drill bit, transportation of cuttings to the surface, overbalancing formation pressure to prevent an influx of oil, gas or water into the well, maintenance of hole stability until casings can be set, suspension of solids when the fluid is not being circulated, and minimizing fluid loss into and possible associated damage/instability to the formation through which drilling is taking place.

Proper overbalancing of formation pressure is obtained by establishing fluid density at the desired level usually via the addition of barite (greater than or equal to 95% barium sulfate). Transportation of cuttings and their suspension when the fluid is not circulating is related to the fluid viscosity and thixotropy which depend on solids content and/or use of a polymer. Filter loss control is obtained also by the use of clays and/or added polymers.

Fluid properties are constantly monitored during the drilling operations and tailored to accommodate the nature of the formation stratum being encountered at the time. When drilling reaches the producing formation special concern is exercised. Preferentially low solids content fluids are used to minimize possible productivity loss by solids plugging. Proper fluid density for overbalancing formation pressure may be obtained by using high salt concentration aqueous brines while viscosity and filter loss control may be obtained by polymer addition. Substantial future oil well drilling will be at depths between 15 and 30 thousand feet where temperatures encountered can be 350.degree. F. Temperatures such as these, coupled with the desire for low solids content and preferably no added solids, require brine tolerant and high temperature stable polymers for viscosity and filtration control. Conventionally employed polymers such as starch, carboxymethyl cellulose, and modified polyacrylates are not stable at the temperatures in question and some have severe brine tolerance limitations.

According to an aspect of the invention, there is provided a polymeric alkyl sulfonate salt of polyvinyl alcohol. More particularly, these sulfonate salts are of the formula

wherein R.sub.1 represents repeating polymeric units, said units consisting essentially of (i) units of the formula ##STR1## and (ii) units of the formula ##STR2## wherein: (a) R.sub.2 --O is ethyleneoxy, propyleneoxy or mixtures thereof;

(b) n is zero or greater (e.g., 0-6), provided that (R.sub.2 --O).sub.n has 18 or less carbon atoms;

(c) R.sub.3 is a hydrocarbyl group having at least 2 carbon atoms; and

(d) M is a cation (preferably a monovalent cation).

R.sub.2 may be CH.sub.2 CH.sub.2 or CH.sub.2 CH.sub.2 CH.sub.2.

When R.sub.2 --O represents mixtures of ethyleneoxy and propyleneoxy, R.sub.2 --O may be, e.g., CH.sub.2 CH.sub.2 O--CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 O--CH.sub.2 CH.sub.2 O or CH.sub.2 CH.sub.2 CH.sub.2 O--CH.sub.2 CH.sub.2 O--CH.sub.2 CH.sub.2 CH.sub.2 O. It is expressly noted that polyvinyl alcohol may be derivatized with a series of reactants in less than stoichiometric amounts to form, in a single polyvinyl alcohol backbone chain, a variety of units each conforming to formula III. Thus, formula I is intended to connote a single polymer having, e.g., one or more of the following units in combination: ##STR3##

According to another aspect of the invention, there is provided a method for preparing a polymeric alkyl sulfonate salt comprising the steps of:

(i) reacting polyvinyl alcohol or polyvinyl alcohol having pendant polyalkyleneoxy segments with ClCH.sub.2 CH.dbd.CH.sub.2 in the presence of Na and DMSO in an amount sufficient to convert from about 5% to about 40% of the OH groups of said polyvinyl alcohol into groups of the formula --OCH.sub.2 CH.dbd.CH.sub.2 ; and

(ii) reacting said --OCH.sub.2 CH.dbd.CH.sub.2 groups of step (i) with NaHSO.sub.3 in the presence of oxygen and water to convert said --OCH.sub.2 CH.dbd.CH.sub.2 groups into groups of the formula --OCH.sub.2 CH.sub.2 CH.sub.2 SO.sub.3.sup.- Na.sup.+.

According to another aspect of the invention, there is provided an aqueous fluid for use as either (a) a mobility control fluid in the secondary or tertiary recovery of oil or (b) a drilling fluid in the drilling of subterranean wells, said fluid having a salinity of at least 5 weight percent, said fluid comprising polymeric alkyl sulfonate salt according to formula (I) said polymeric alkyl sulfonate salt being present in an amount sufficient to thicken said fluid.

According to another aspect of the invention, there is provided an improved method for the recovery of oil from a subterranean oil reservoir penetrated by spaced injection and production systems in which an aqueous mobility control slug is introduced into said reservoir via said injection system to displace oil to said production system, the improvement comprising employing in said slug a thickening amount of one or more compounds according to formula (I).

According to another aspect of the invention, there is provided a method for maintaining the proper viscosity of an aqueous brine drill completion fluid used when drilling reaches a producing formation at a depth between 15 and 30 thousand feet, the temperature of said formation being more than about 325 degrees F, said method comprising incorporating in said fluid a viscosity increasing amount of one or more compounds according to formula (I).

The incorporation of from about 5 to 40 mer % of alkyl sulfonate groups, e.g., C.sub.2 -C.sub.6 alkyl sulfonate groups, particularly 3-propane sulfonate groups, onto the alcohol oxygens of polyvinyl alcohols imparts solubility to resulting polymers at high concentrations of salt and divalent ions.

As with all anionic polymers, the viscosity of aqueous solutions of these polymers falls with increasing salt content but, unlike other anionic polymers, at a salt concentration equivalent to about 10% w/v NaCl the viscosity begins to increase so that by about 20-25% salt the viscosity is as high or higher than in deionized water. The effect of rising salt is independent of the cation used with makes these polymers eminently suitable for enhanced oil recovery in reservoirs of higher brine content or for use in drilling fluids where high concentrations of calcium or other divalent ions may be present. The polymers are extremely stable both thermally and hydrolytically which also lends to their utility in the above-named processes.

The alkyl sulfonate salts according to the present invention may be prepared by a particular combination of process steps which may be carried out according to conventional technology. For example, a 3-propane sulfonate salt may be prepared according to the following reaction sequence: ##STR4##

In the above sequence, it is expressly noted that no stereochemical regularity is implied by the structures given. It is also noted that the number calculated by the formula, m/n.times.100, corresponds to the value for mer %.

Since polyvinyl alcohol is customarily made from the hydrolysis of polyvinyl acetate, it is likely that the polymeric alkyl sulfonate salt of formula (I) may contain units of the formula ##STR5##

The polymeric alkyl sulfonate salts of formula (I) may have a molecular weight of at least 25,000, e.g. from about 30,000 to about 500,000. It is contemplated that the higher molecular weight compounds made from super-high viscosity polyvinyl alcohols will have the most desirable thickening properties for mobility control fluids and drilling fluids.

In view of the compatibility of compositions used in the present invention with divalent metal ions, a preferred application of the invention is in reservoirs in which the connate water contains significant divalent ion concentrations and in situations where the available water for flooding or drilling contains divalent metal ions inconsistent with the use of conventional non-brine tolerant thickeners. Thus, a preferred application of the present invention is in those situations in which the reservoir waters and/or the waters employed in formulating the flooding medium or drilling fluid exhibit a divalent metal ion concentration of at least 0.1 weight percent. The invention is particularly useful where the reservoir, injection, and/or drilling waters have divalent metal ion concentrations of 0.5 weight percent or more. A similar consideration applies with regard to those situations in which the waters exhibit moderate to relatively high salinities even though the divalent metal ion concentration may be relatively low. Thus, another application of the invention is those situations in which the reservoir, injection and/or drilling waters have salinities of at least 5.0 weight percent, especially at least 10 weight percent, whether provided by monovalent salts such as sodium chloride or monovalent salts and divalent salts such as calcium or magnesium chloride.

The amount of formula (I) compound which may be used in accordance with aspects of the invention should be sufficient to achieve the desired thickening effect. For example, this compound may be added to water and injected into an oil-containing formation in a sufficient amount and concentration to effectively thicken the water so as to provide improved efficiency in driving the oil through the formation to the production well. This thickening agent may be added in concentrations so as to provide a graded viscosity at the trailing edge of the mobility control slug as disclosed in a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, or graded viscosities at both the leading and trailing edges of the mobility control slug as disclosed in U.S. Pat. No. 4,018,281 to Chang. Alternatively, the thickening agent concentration may be relatively constant throughout. Normally, the viscosity of at least a portion of the mobility control slug should be at least as great as that of the reservoir oil and typically it will be within the range of about 1 to 4 times the viscosity of the reservoir oil.

Secondary oil recovery may be carried out utilizing injection and production systems as defined by any suitable arrangement of wells. One well arrangement commonly used in waterflooding operations and suitable for use in carrying out the present invention is an integrated five-spot pattern of the type illustrated in U.S. Pat. No. 3,927,716 to Burdyn et al. Other well arrangements may be used in carrying out aspects of the present invention, examples of which are set forth in the Burdyn et al patent. By the term "pore volume" as used herein is meant that volume of the portion of the formation underlying the well pattern employed, as described in greater detail in the Burdyn et al patent.

The mobility control slug may be injected in amounts ranging, e.g., from about 0.1 to about 1.0 pore volumes, more particularly, from about 0.25 to about 0.5 pore volumes.