SCALE INHIBITION BY SQUEEZE APPLICATION OF A PHOSPHONOMETHYLATED POLYAMINE INHIBITOR

Abstract: A method for long term inhibition of scale in a brine downhole in a well. An effective amount of an N-phosphonomethylated polymer of an epihalohydrin and an amine selected from the group consisting of alkylene diamines, di-alkylene triamines and tri-alkylene tetramines, in an epihalohydrin to amine molar ratio of from about 0.6:1 to about 2:1, the polymer having a molecular weight of from about 500 to about 40,000, is squeezed into the well.

Description: SCALE INHIBITION BY SQUEEZE APPLICATION OF A PHOSPHONOMETHYLATED POLYAMINE INHIBITOR Background of the Invention Field of the Invention The present invention relates to scale inhibition, and more particularly to improved methods of scale inhibition by squeeze techniques. 2. Description of the Prior Art Scale formation and deposition downhole in oil and gas wells is a common problem, especially in high volume wells. The types of scale that have been found to be troublesome include mineral scale such as calcite, gypsum, anhydrite, celestite and barite. Barite has proven especially difficult to treat. A variety of techniques have been employed for control of such scale formation and deposition. These techniques have involved applications of many different types of scale inhibitor compositions and mechanical means. Among the types of compositions that have been used for downhole scale inhibition may be noted organic phosphates or phosphonates, adducts of acrylic acid, phosphino polycarboxylic acids and the like and, more recently, the compositions as described in U.S. Patent No. 4,857,205, incorporated herein by reference. The techniques for attempting to control scale also have involved various methods for applying the inhibitor compositions to the wells. For example, pipes have been removed and scraped for reuse, pipelines and wells have been treated with chemical scale inhibitors by mere dosing of the pipeline or dosing of the well (such as at the top or bottom) with the inhibitor, sometimes with a slow-release mechanism. U.S. Patent Nos. 3,827,977 and 3,704,750 (both of which are incorporated herein by reference) identify several such techniques. Among the techniques that have been employed with certain chemical inhibitors is "squeezing." Squeeze treatments are well known processes that have been described, for instance, in Water-Formed Scale Deposits, by J.C. Cowan and D.J. Weintritt (Gulf Publishing Co., Houston, Texas, 1976), pp. 352-353, and in "Prevention of Downhole Scale Deposition in the Ninian Field", by P.J. Shuler and W.H. Jenkins, SPE Producti on Engineering, May 1991, pp. 221-226, both of which are incorporated herein by reference. According to such techniques, an amount of the inhibitor is injected into the well and "squeezed", or pushed under pressure, into the oil or gas reservoir formation accessed by the well. Generally, the inhibitor is squeezed into the formation with water, usually over-flushing the treated zone with the water. After the squeeze application, the well is placed back into service and the inhibitor, which has been adsorbed into the formation by the squeeze application, is desorbed into in the produced water, thereby protecting the wellbore and downhole equipment from scale. The efficacy of an inhibitor in a squeeze application depends on several variables. The ideal inhibitor provides long term control of scale by means of an inherently or automatically controlled slow, but sufficient rate of release of the quantity of inhibitor squeezed into the formation. In other words, a rate of release that is too high provides an inhibitor concentration that is wastefully high, thereby leading to large inhibitor requirements, frequent re-applications and an unduly short period of scale inhibition; that is, an unduly short squeeze life. As noted in U.S. Patent Nos. 3,704,750 and 3,827,977, when liquid inhibitors have been squeezed into well bores and into fracture planes in the formation, the release of the inhibitor into the fluids to inhibit scale formation relies on adsorption-desorption of the inhibitor from the sand grains, or in the alternative depends on the differential pressure along the fracture face to meter the inhibitor into the produced fluids, oil and water. With the inhibiting agents used heretofore in adsorption-desorption, there is generally a large amount of the agent that is not absorbed but is produced back, i.e., swept away with the produced fluids, in large wasteful concentrations . . . Squeezing liquid scale inhibitors into the well face or fracture face is also practiced. But here also are found disadvantages as a large amount of the inhibiting agent is recovered almost immediately, thus reducing the life of the treatment. Frequent re-applications are therefore required. Rates of release that are too high frequently occur immediately after treatment and tend to be especially wasteful because the rate of release usually is extremely high for a short period, wasting large amounts of inhibitor quickly, and then drops precipitously to a level too low to afford satisfactory inhibition. Frequent re-application is then required. On the other hand, a very low rate of release does not provide a high enough concentration of inhibitor to be effective. In fact, as further noted in U.S. Patent Nos. 3,704,750 and 3,827,977, in some "cases the inhibiting agent is irreversibly adsorbed in the formation and no inhibiting agent is desorbed by the fluids to prevent scale formation. " Thus, it is not enough that the inhibitor effectively and efficiently inhibit scale formation. An ideal inhibitor would exhibit a high degree of absorptivity with respect to the formation, but then will release the inhibitor at a relatively constant, slow rate that produces a generally consistent and constant concentration of inhibitor in the produced water that is low, but sufficient to inhibit scale effectively. As a result, the efficacy and desirability of the inhibitor depend not only on the efficacy of the inhibitor against scale, but also on the degree of absorptivity of the inhibitor with respect to the formation and then the rate of release profile (initial rate of release, as well as long term release rate) . Therefore, predicting the desirability or efficacy of an inhibitor in a squeeze process is difficult. Accordingly, while the article Using Statistical Experimental Design to Optimize the Performance and Secondary Properties of Scale Inhibi tors for Downhole Application, in Recent Advances in Oilfield Chemistry, April 1994, notes that the compositions of U.S. Patent No. 4,857,205 have been used to inhibit scale formation under low pH conditions in downhole application and discusses the use of squeeze treatments, it nowhere teaches or suggests squeezing the compositions of U.S. Patent No. 4,857,205 to achieve long term scale inhibition. Thus, a large number of compositions and methods have been tried and even practiced commercially. Nevertheless, none of the techniques has proved entirely satisfactory in treating scale, especially with respect to mineral scale which has been found to be particularly resistant to inhibition. Summary of the Invention The present invention, therefore, is directed to a novel method for long term inhibition of scale in a brine downhole in a well. The method comprises squeezing into the well an effective amount of an N-phosphonomethylated polymer of an epihalohydrin and an amine selected from the group consisting of alkylene diamines, di-alkylene triamines and tri-alkylene tetramines, in an epihalohydrin to amine molar ratio of from about 0.6:1 to about 2:1, the polymer having a molecular weight of from about 500 to about 40,000. Among the several advantages of this invention, may be noted the provision of a method that achieves long term scale inhibition; and the provision of such method that reduces inhibitor waste. Brief Description of the Drawing Figure The lone drawing figure, figure 1, is a graph showing the relative desorption return profiles resulting from tests of three scale inhibitors, with open circles representing a conventional polymeric scale inhibitor, squares representing a conventional phosphonate scale inhibitor and black circles representing a polymer of the method of the present invention. Detailed Description of the Preferred Embodiments In accordance with the present invention, it has been discovered that long term inhibition of scale, especially mineral scale, even barite scale, in a brine downhole in a well can be achieved by injecting into the well and then squeezing an effective amount of the N-phosphonomethylated polymer of an epihalohydrin and an amine, which may be an alkylene diamine, a dialkylene triamine, or a tri-alkylene tetramine in a molar ratio of from about 1:1 to about 2:1, the polymer having a molecular weight of from about 500 to about 40,000. It has been found that squeezing this product affords surprisingly long term scale inhibition — in fact, much longer term than provided by prior art squeeze inhibitors and much longer term than provided by application of the inhibitor by other techniques. The products utilized in the present squeeze method include those described and used in the methods of U.S. Patent No. 4,857,205. However, whereas the products illustrated structurally in that patent are formed from reacting the epihalohydrin and amine in a molar ratio of about 1:1, it is now understood that products prepared from the reaction of the epihalohydrin and the amine in other molar ratios, such as from about 1:1 to about 2:1, may be used effectively in the present method. The polymeric product of the subject invention may be produced in the following manner. An aqueous amine solution, specifically an aqueous alkylene diamine solution, an aqueous di-alkylene triamine solution or an aqueous tetramine solution, may be mixed with an epihalohydrin (such as epichlorohydrin) and the resulting mixture stirred rapidly to effect a polymerization reaction. The amine may correspond to the formula H 2 NR(NHR) X NH 2 , wherein R is an alkylene group (preferably unsubstituted) of from about 2 to about 12 carbons, preferably about 2 to about 6 carbons, such as ethylene or propylene, or (less preferably) a cycloalkylene group of from about 3 to about 12 carbons, and x is 0, 1 or 2, preferably 0 or 1. If R is a substituted alkylene group, at least one hydrogen (preferably one hydrogen) of the alkylene is substituted with a radical selected from among alkyls of from 1 to about 4 carbons, methyl ammonium, ethyl ammonium and ammonium groups, and the other hydrogens of the alkylene are unsubstituted. Nevertheless, it is preferred that R be a straight chain, unsubstituted alkylene group, most desirably ethylene. Particularly preferred amines include ethylenediamine, diethylenetriamine, triethylene tetramine, hexamethylenediamine, pentylenediamine and 2-methylpentyldiamine. Preferably, the epihalohydrin and amine are reacted in a molar ratio of about 1:2 to about 1:1, most preferably about 1:1. Reaction at this ratio tends to form a generally linear polymer of the idealized formula H 2 NR(NHR) X NH[CH 2 CH(OH)CH 2 NHR(NHR) X NH] n H wherein R and x are defined as above and n is an integer from 1 to about 100 or more, typically from about 1 to about 50, such as from about 5 to about 50, especially about 5 to about 25. However, the epihalohydrin and the amine may be reacted in higher epihalohydrin to amine molar ratios, such as up to about 2:1 or more. In such cases, the polymer tends to include branching, thereby departing from the noted idealized linear structure. Regardless of the epihalohydrin to amine ratio, the resulting polymer is then mixed with phosphorous acid and enough of a strong acid such as hydrochloric acid or sulfuric acid to produce a reaction mixture of pH less than about 1 and heated with formaldehyde to effect a reaction. Because the product may include the anion from the strong acid and certain anions, such as the sulfate anion, may tend to combine with ambient cations in the brine treated with the polymer to form insoluble scale such as barium sulfate, it is preferred that the strong acid not include such cations. Therefore, hydrochloric acid is particularly suitable as the strong acid. Moreover, the amine/formaldehyde/phosphorous acid reaction seems to proceed best in hydrochloric acid as well. The phosphorous acid should be added to the polymerization reaction product in a phosphorous acid to amine NH equivalent molar ratio of from about 0.6:1 to about 1:1, preferably from about 0.7:1 to about 1:1, especially about 0.8:1 to about 1:1. Upon heating the mixture to reflux, formaldehyde (e.g., formalin) is added slowly in at least a 1:1 formaldehyde to phosphorous acid molar ratio. Whereas U.S. Patent No. 4,857,205 notes advantageous results in the phosphorous acid to amine NH equivalent molar ratio range of from about 0.8:1 to about 0.9:1, complete phosphonomethylation (i.e., a molar ratio of about 1:1) is believed most desirable here. The resulting polymer, if linear, therefore may be represented by the idealized formula R' 2 NR[N(R' )R] X NR' {CH 2 CH(OH)CH 2 NR'R[N(R' )R] X NR' } n H wherein R, n and x are defined as above and each R' is independently selected from among hydrogen and the phosphonomethyl radical corresponding to the formula -CH 2 PO(OH) 2 , provided that on average about 6 to 10, preferably about 7 to 10, more preferably about 8 to 10, most preferably about 10, of every 10 R 1 groups correspond to the phosphonomethyl radical. As noted, although this structure is the idealized representation of the product, it may also include to some degree replacement with the anion from the strong acid. As also noted, use of epihalohydrin to amine molar ratios of greater than 1:1 tends to produce a related branched product. The molecular weight of the composition may be, for example, from about 500 to about 40,000, typically from about 500 to about 20,000, such as from about 3,000 to about 20,000, especially about 3,000 to about 10,000, depending on R, x and the degree of polymerization (n) . The resulting mixture, containing the resulting N-phosphonomethylated amino-2-hydroxypropylene polymer in acid form, may be used directly as a scale inhibitor, but it is highly acidic. Therefore, it may be desirable to neutralize the polymer, completely or partially, by the addition of a base, preferably, ammonium hydroxide, sodium hydroxide or potassium hydroxide, but also possibly an alkylamine such as diethylamine or triethylamine, or an alkanolamine such as ethanolamine, diethanolamine or triethanolamine, to produce a pH of, for example, about 2 to about 10. Thus, water-soluble salts or partial salts of the polymer may be formed by replacement of some or all of the hydroxyl hydrogens of the phosphonomethyl groups, so that the phosphonomethyl groups may correspond to the formula -CH 2 PO(OM) 2 , wherein each M is independently selected from among hydrogen and a cation such as the ammonium ion, the sodium ion or the potassium ion. The amine salts are also possible. The reaction scheme discussed above is described in more detail in U.S. Patent No. 4,857,205 with respect to the relatively lower epihalohydrin to amine ratios. The inhibitor composition may include other ingredients as well. For example, a freezing point depressant such as methanol, ethylene glycol or isopropanol may be included. Typically, the scale inhibiting composition is diluted prior to use so that the expanded solution comprises about 20% to about 30% by weight of the phosphonic acid or its salt. In practice, the inhibitor composition is squeezed into the well, and thus into the formation producing the fluids withdrawn by the well. The fluids typically include brine, which contains (among other things) mineral salts such as the barium species that, without the subject treatment, results in scale deposition. Often, the brine is acidic (e.g., a pH of from about 3 to about 6) and may be relatively hot (e.g., about 50°C to about 150°C) . After the inhibitor charge is injected into the well, tests show that it adsorbs very quickly and that its release soon reaches what appears, at least for thousands of eluted pore volumes thereafter, to be a steady state. Thus, according to the tests, the inhibitor release reaches an effective scale inhibition level of about 3 to about 25 ppm (mg inhibitor per liter of brine) in less than the equivalent of 200 eluted pore volumes of flushes and maintains a release in that range for at least about 3,000 eluted pore volumes, preferably at least about 4,000 eluted pore volumes. In fact, the level of inhibitor release has been noted in the test to remain between about 3 and about 10 ppm, generally about 5 ppm, from about 500 eluted pore volumes to about 4,500 eluted pore volumes, when the test was terminated. This level is exceptional not only because it provides a concentration for effective scale inhibition, but also because it is a very low effective level, thereby avoiding waste and allowing prolonged release and efficacy. U.S. Patent No. 4,857,205 teaches simply that such compositions are employed in oil and gas fields by mere injection or other delivery of the composition to the reservoir to provide a concentration of phosphonic acid or salt thereof in the conduit being treated of from about 1 ppm to about 100 ppm by weight, thereby to inhibit scale formation. However, it has now been discovered that if the composition is applied by squeeze techniques, extremely prolonged inhibitor release results. The release profile has been found to indicate not only that the immediate high level release, which tends to waste a large portion of the inhibitor, is far less than takes place when conventional inhibitors are squeezed, but also that the subsequent release occurs at a very consistent low level that is just sufficient to afford long-lasting scale inhibition without wasteful overdose. This combination has been found to result in dramatically longer lasting treatment not only than that afforded by other application techniques, but even than that afforded by conventional inhibitors applied by squeeze techniques. Accordingly, less inhibitor and fewer applications are needed. The following examples describe preferred embodiments of the invention. Other embodiments within the scope of the claims herein will be apparent to one skilled in the art from consideration of the specification or practice of the invention as disclosed herein. It is intended that the specification, together with the examples, be considered exemplary only, with the scope and spirit of the invention being indicated by the claims which follow the examples. In the examples, all percentages are given on a weight basis unless otherwise indicated. Example 1 Water (130 gm.) was added to diethylenetriamine (361 gm.) in a 1-L glass flask fitted with a reflux condenser, a mechanical stirrer, a thermometer and an addition funnel, and the resulting solution was stirred mechanically. The stirred solution was heated in the flask to 90°C and epichlorohydrin (328.4 gm.) was slowly to the flask with vigorous stirring. External cooling was applied to the flask to control the exothermic reaction so that the temperature of the reaction mixture was maintained in the range of about 90°C to about 95°C. After addition of the epichlorohydrin was completed, the reaction mixture was stirred for a further four hours at about 90°C to about 95°C. At the end of this reaction period, external cooling was applied to the flask again, while concentrated (ca. 34%) hydrochloric acid (181 g .) was added slowly at a rate such that the temperature did not exceed about 95°C. The resulting aqueous mixture containing about 55% low molecular weight polymer was cooled and stored. Aqueous (ca. 70%) phosphorous acid (221 gm.) was added to concentrated (ca. 34%) hydrochloric acid (201 gm.) in a 1-L flask fitted with a reflux condenser, a mechanical stirrer, a thermometer and an addition funnel, and the resulting solution was stirred mechanically while a sample (172.4 gm.) of the low molecular weight polymer that had been cooled and stored was added at a rate such that the temperature of the additive mixture did not exceed about 80°C. Upon completion of the addition, the mixture was heated to a temperature in the range of about 98°C to about 102°C. Then, aqueous (ca. 37%) formaldehyde (154.2 gm. ) was added over a period of about 1.5 hours, while heating and cooling as necessary to maintain the mixture at a temperature in the range of about 98°C to about 102°C. When the addition was complete, the reaction mixture was stirred at about 98°C to about 102°C for three more hours. The mixture was then cooled and aqueous (ca. 50%) sodium hydroxide solution (251.4 gm.) added slowly at a rate such that the temperature did not exceed about 90°C. Example 2 An industry recognized core flood test was carried out to compare the squeezing of the composition of Example 1, above, with the squeezing of a polymer and the squeezing of a phosphonate, both of which are standard, conventional squeeze inhibitors. A core flood test rig was set up with brine from a reservoir pumped by a Pharmacia High Precision Pump through a 3-way valve to resin coated core in a core holder with two differential pressure tapping points (one upstream and one downstream) . The valve and core holder were located in a 70°C oven. The effluent from the core was directed to a sample receiver. The three cores (one each for the composition of Example 1, above, the polymer and the phosphonate) were initially conditioned with synthetic seawater at room temperature. A lithium tracer study was then performed to determine the accurate pore volume (PV) of the cores by passing a 5 ppm lithium trace in synthetic seawater through the cores collecting the effluents and measuring the lithium concentration. The permeability of the cores was then determined by measuring the differential pressure across the cores at various brine rates, constructing a p/Q plot (where p=Differential Pressure and Q=flow rate) and measuring the slope of the line. The cores were then saturated with inhibitor solutions at room temperature. For this test, all inhibitor strengths were standardized at 5% active inhibitor concentration in synthetic seawater. The saturation of the cores was stopped after about 7 pore volumes, when full inhibitor absorbance had been achieved and the influent and effluent levels of inhibitor were equal. The cores were then "shut in" at 70°C for 16 hours. Next, inhibitor desorption was carried out by flushing the cores with synthetic seawater, collecting the effluents via a fraction collector and measuring the inhibitor concentration by plasma emission spectroscopy. The inhibitor concentration was plotted against eluted pore volumes to show the comparative desorption return profiles of the three scale inhibitors. Figure 1 is a graph of these return profiles, with open circles representing the conventional polymer, squares representing the conventional phosphonate and black circles representing the polymer produced by the method of Example 1, above. It is accepted that a 5 ppm (5 mg/L) dose of product is sufficient to afford desirable scale control in the majority of squeeze treatments. In view of the above, it will be seen that the several advantages of the invention are achieved and other advantageous results attained. As various changes could be made in the above methods and compositions without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Claims: What is claimed is: 1. A method for long term inhibition of scale in a brine downhole in a well, the method comprising squeezing into the well an effective amount of an N-phosphonomethylated polymer of an epihalohydrin and an amine selected from the group consisting of alkylene diamines, di-alkylene triamines and tri-alkylene tetramines, in an epihalohydrin to amine molar ratio of from about 0.6:1 to about 2:1, the polymer having a molecular weight of from about 500 to about 40,000. 2. A method as set forth in claim 1 wherein the N-phosphonomethylated polymer is a reaction product of phosphorous acid and a polymer of the epihalohydrin and the amine. 3. A method as set forth in claim 1 wherein the N-phosphonomethylated polymer is a salt of a reaction product of phosphorous acid and a polymer of the epihalohydrin and the amine. 4. A method as set forth in claim 1 wherein the N-phosphonomethylated polymer has a molecular weight of from about 500 to about 20,000. 5. A method as set forth in claim 4 wherein the N-phosphonomethylated polymer is a reaction product of phosphorous acid and a polymer of the epihalohydrin and the amine. 6. A method as set forth in claim 4 wherein the N-phosphonomethylated polymer is a salt of a reaction product of phosphorous acid and a polymer of the epihalohydrin and the amine. 7. A method as set forth in claim 1 wherein the N-phosphonomethylated polymer corresponds to the idealized formula R' 2 NR[N(R' )R] X NR' {CH 2 CH(OH)CH 2 NR'R[N(R' )R] X NR' } n H wherein R is an alkylene group of from about 2 to about 12 carbons or a cycloalkylene group of from about 3 to about 12 carbons, x is 0, 1 or 2, n is an integer from 1 to about 100 and each R' is independently selected from among hydrogen and the phosphonomethyl radical corresponding to the formula -CH 2 PO(OH) 2 , provided that on average about 6 to 10 of every 10 R' groups correspond to the phosphonomethyl radical. 8. A method as set forth in claim 7 wherein R is an alkylene of from 2 to about 6 carbons. 9. A method as set forth in claim 8 wherein n is an integer from 1 to about 50. 10. A method as set forth in claim 1 wherein the N-phosphonomethylated polymer is a salt of the polymer corresponding to the idealized formula R' 2 NR[N(R' )R] X NR' {CH 2 CH(OH)CH 2 NR'R[N(R » )R] X NR' } n H wherein R is an alkylene group of from about 2 to about 12 carbons or a cycloalkylene group of from about 3 to about 12 carbons, x is 0, 1 or 2, n is an integer from 1 to about 100 and each R' is independently selected from among hydrogen and the phosphonomethyl radical corresponding to the formula -CH 2 PO(OH) 2 , provided that on average about 6 to 10 of every 10 R' groups correspond to the phosphonomethyl radical. 11. A method as set forth in claim 10 wherein R is an alkylene of from 2 to about 6 carbons. 12. A method as set forth in claim 11 wherein n is an integer from 1 to about 50. 13. A method as set forth in claim 1 wherein the brine is acidic. 14. A method as set forth in claim 1 wherein, after the squeezing, the N-phosphonomethylated polymer is released into the brine at a level of from about 3 to about 25 mg/L after at least about 3,000 eluted pore volumes. 15. A method as set forth in claim 14 wherein, after the squeezing, the N-phosphonomethylated polymer is released into the brine at a level of from about 3 to about 25 mg/L after at least about 4,000 eluted pore volumes. 16. A method as set forth in claim 1 wherein, after the squeezing, the N-phosphonomethylated polymer is released into the brine at a level sufficient to provide effective scale inhibition after at least about 3,000 eluted pore volumes. 17. A method as set forth in claim 16 wherein, after the squeezing, the N-phosphonomethylated polymer is released into the brine at a level sufficient to provide effective scale inhibition after at least about 4,000 eluted pore volumes. 18. A method as set forth in claim 1 wherein the amine includes at least one alkylene group of from 2 to about 6 carbon atoms. 19. A method as set forth in claim 1 that effects mineral scale inhibition. 20. A method as set forth in claim 19 that effects barite scale inhibition.

Inventors: JACKSON GEORGE E, MCLAUGHLIN KEVIN, POYNTON NEIL, PRZYBYLINSKI JOHN L, JACKSON, GEORGE, E., MCLAUGHLIN, KEVIN, POYNTON, NEIL, PRZYBYLINSKI, JOHN, L.
Applicants: PETROLITE CORP, PETROLITE CORPORATION
Application year: 1996
Main classification: E21B43
IPC codes: C09K8