GB2205318A

GB2205318A - Polyvinyl saccharide/borate products

Info

Publication number: GB2205318A
Application number: GB08712960A
Authority: GB
Inventors: Teunis Graafland
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1987-06-03
Filing date: 1987-06-03
Publication date: 1988-12-07
Also published as: GB8712960D0

Abstract

Thickened aqueous systems containing a polyvinyl saccharide and a borate ion-yielding compound, in a ratio sufficient to provide a ratio of saccharide equivalents to borate ion equivalents in the range from 200 to 0.5. The systems are suitable for use in oil field operations.

Description

THICKENED AQUEOUS POLYSACCHARIDE SYSTEMS The present invention relates to thickened aqueous polysaccharide systems, to the preparation of said thickened aqueous polysaccharide systems and to their use.

Polysaccharides such as guar gum have a high water thickening capacity and for this reason are being used in a wide range of applications such as in water treatment, in oil fields operations and as additives in the paper making industry. Futhermore it is known that the water thickening capacity of these polysaccharides can be further increased by combining them with certain crosslinking agents such as borate ion-yielding compounds.

Although the valuable properties of these natural polymeric materials have long been recognized, many synthetic polymeric systems have been developed to replace said polysaccharides. Synthetic polymers have the advantage in that in general they can be tailor made to meet the specific requirements of a given application, and moreover do not suffer from the sometimes unreliable supply position of the natural products. An example of such a synthetic replacement for polysaccharides is polyacrylamide, which has successfully been applied as polymeric thickener for aqueous systems.

However the development of a synthetic replacement for the crosslinked polysaccharides described hereinbefore has not yet been very successful. Although polyvinyl alcohol, being a hydroxyl-rich polymer like polysaccharide, could have been expected to be a likely candidate to replace the polysaccharides in the crosslinked systems, its performance could not match that of the corresponding polysaccharide systems.

The problem underlying the present invention is finding a synthetic polymer-based crosslinked system which could act as a replacement for the crosslinked polysaccharide systems mentioned hereinbefore.

A class of hydroxyl-rich water-soluble polymers which are known to have water-thickening properties are the polyvinyl saccharides. Especially high molecular weight polyvinyl saccharide polymers prepared according to a process such as described in European Patent Application EP 0180262, were found to be good polymeric thickeners. Although these polymers also contain saccharide type moieties, there is a significant difference with the natural polysaccharides, in that they possess a carbon-carbon polymer backbone having pendant saccharide moieties, whereas with the natural polysaccharides the glucose moieties are linked to one another via ether linkages, thereby forming the polymer backbone.As a result of this difference in configuration it could be expected that there would also be a difference in performance when combined with a crosslinking agent such as a borate ion-yielding compound, i.e. a predominant formation of intramolecular crosslinks rather than intermolecular crosslinks. Such intramolecular crosslinked systems would be expected to have an inferior water thickening capacity.

Surprisingly it has now been found that combinations of high molecular weight polyvinyl saccharides and certain crosslinking agents have a very high water-thickened capacity.

Accordingly, the invention provides thickened aqueous polysaccharide systems comprising aqueous mixtures of a polyvinyl saccharide and a borate ion-yielding compound, wherein the polyvinyl saccharide and the borate ion-yielding compound are present in a weight ratio which is sufficient to provide a ratio of saccharide equivalents to borate ion equivalents in the range of from 200:1 to 1:2.

The polyvinyl saccharides which may be used in the present invention are homo- or copolymers derived from a water-soluble monovinyl saccharide monomer. When the polyvinyl saccharide is a copolymer, a water-soluble monomer bearing an acidic group is a preferred type of comonomer to be polymerized in combination with the monovinyl saccharide monomer. Preferably the polyvinyl saccharide copolymer is based on a monomer composition wherein the acidic group-bearing monomer makes up to 30 % mol of the total monomer composition.

The term "monovinyl saccharide monomer" herein refers to mono- or disaccharide compounds containing one polymerizable vinyl group per molecule.

The preferred water-soluble monovinyl saccharide monomers contain a vinylcarbonyloxy group, preferably a methacryloyl group as they result in polymers which generally are hydrolytically more stable than the corresponding acrylic acid based polymers. Particularly preferred is 3-0-methacryloyl D-glucose.

Examples of other polymerizable water-soluble monovinyl saccharide monomers bearing a vinylcarbonyloxy group are 3-O-acryloyl-Dglucose, 6-O-acryloyl-D-galactose, 6-O-methacryloyl-D-galactose, l-O-acryloyl-1-sorbose, l-O-methacryloyl-L-sorbose, 1-0-acryloyl-mannose and l-0-methacryloyl-mannose.

A method for the preparation of the polymerizable water-soluble vinylcarbonyloxy group-containing monosaccharides has been disclosed by Black et al, Makromol.

Chem. 117 (1968), 210.

Other suitable water-soluble monovinyl saccharide monomers contain a vinyloxy group, for example 3-0-vinyl-Dglucose, 6-O-vinyl-D-galactose and 1-0-vinyl-L-sorbose.

Methods for the preparation of the polymerizable water-soluble vinyloxy group-containing monosaccharides have been disclosed by e.g. Reppe et al, in Annalen, 1956, 601, 81 and Watanabe and Colon in J. Amer. Chem. Soc. 1957, 79, 2828.

The water-soluble monomer carrying an acidic group may suitably be a water-soluble a,ss-olefinically unsaturated monomer bearing a carboxyl or a sulfonic(-SO2OH) group.

Suitable such a,ss-olefinically unsaturated monomers include acrylic acid, methacrylic acid, itaconic acid and 3-0-methacryloyl-Dgluconic acid. Methacrylic acid is a preferred cr,B-olefinically unsaturated monocarboxylic acid.

The polyvinyl saccharide can be prepared by polymerizing a monovinyl saccharide monomer, optionally together with a comonomer, in an aqueous medium in the presence of a free-radical initiator at a temperature in the range of from 0 to 95 "C.

The free radicals to be used in the process of the present invention may originate from compounds which are capable of generating free radicals via a reduction-oxidation reaction, so called redox initiators, as well as from compounds which are capable of generating free radicals by decomposition, e.g. upon heating thereof. Redox initiator systems have been described e.g; in Prog. Polym. Vol. 8, pp 61-131(1982). Preferred redox initiator systems are selected from the group comprising a peroxide, a persulfate , a peroxydiphosphate or a permanganate type of oxidant, and a reducing agent. Preferred reducing agents are based on a reducing acid of sulfur.

Compounds which are capable of generating free radicals via a decomposition reaction, have been described e.g. in J. Macro. Sci. Rev. Macrol. Chem., C 20(1), 149-205(1981) and include azo and diazo compounds, organic peroxides, hydroperoxide, peroxydicarbonate and persulfate type of free-radical initiators, and hydrogen peroxide.

In the process for the preparation of the cross linked polymers of the present invention 2,2'-azobisisobutyronitrile (AIBN) is a preferred free-radical initiator.

Although the amount of free radical or redox initiator to be used may vary over wide ranges there is a preference to employ said initiator in a molar ratio of free radical initiator or oxidant compound of the redox initiator to total monomer of not higher than 1:100 and more preferably not higher than 1:500.

When employing a free-radical initiator of low water-solubility it is advantageous to add said initiator as a solution in a water-miscible solvent such as acetone.

The polymers may be isolated by precipitation in an excess of a suitable organic liquid e.g. an alcohol such as ethanol and 2-propanol. The precipitate may subsequently be washed employing an alcohol as mentioned hereinbefore and finally dried.

Mixtures of water-soluble monovinyl saccharide monomers and/or mixtures of water-soluble monomers carrying an acidic group may also be used.

The nature of the borate ion-yielding compound is not critical provided said compound is a water-soluble borate ion-yielding compound, and includes alkali metal and alkaline earth metal borates, as such or in the hydrated form, as well as boric acid. Suitable borates include sodium metaborate, potassium metaborate, calcium metaborate, sodium metaborate tetrahydrate, sodium tetraborate, sodium tetraborate tetrahydrate and sodium tetraborate decahydrate. Sodium tetraborate decahydrate, commercially available as borax, is a preferred borate.

Preferably the weight ratio of the polyvinyl saccharide and the borate in the aqueous mixture is sufficient to provide a ratio of saccharide equivalents to borate ion equivalents in the range of from 100:1 to 1:1.

It has been observed that for a given saccharide/borate ion equivalent ratio the viscosity of the thickened aqueous mixtures of the present invention may vary with the pH of said mixture. The pH of the aqueous mixture will not only be determined by the composition of the polyvinyl saccharide polymer, i.e. acid-free or acid-containing, but also by the nature of the borate ion yielding source. Whereas boric acid tends to lower the pH of the mixture the use of a borate such as borax tends to raise the pH. It has furthermore been observed that the high viscosity to gel-like consistency of said mixtures may be maintained over a wide range of saccharide/borate ion equivalent ratio's when the pH of the system is at least 7 and preferably in the range of from 7 to 10.In said range of pH values the high viscosity and gel-like aqueous mixtures may already be obtained at relatively high saccharide/borate ion equivalent ratios, i.e.

at relatively low borate ion concentrations. Hence mixtures having a pH in the range of from 7 to 10 are preferred, and for this reason it may be beneficial to include a buffer in the composition of the thickened aqueous mixtures of the present invention.

Although the high viscosity and gel-like thickened aqueous mixtures may already be obtained at very low polyvinyl saccharide concentrations, there appears to be a minimum requirement regarding said polyvinyl saccharide concentration in order to obtain the gel-like mixtures, for which a polyvinyl saccharide concentration of more than 0.75 8 w appears to be required. The maximum polyvinyl saccharide concentration will generally be determined by the handlability of the viscous aqueous polyvinyl saccharide solutions. A too high viscosity may jeopardize the homogenation of the polyvinyl saccharide/borate ion-yielding compound mixtures.

The viscosity of an 8-10 8 w aqueous polyvinyl saccharide solution at 20 "C is still sufficiently low to allow easy handling and mixing. Should thickened aqueous polyvinyl saccharide/borate ion-yielding compound mixtures be required wherein the polyvinyl saccharide content is considerably higher or alternatively the water content much lower, it is always possible to remove the excess water from said mixtures, e.g. by evaporation.

The preparation of the thickened aqueous polyvinyl saccharide/borate ion-yielding compound mixtures may conveniently be conducted by mixing an aqueous solution of a borate ion-yielding compound with an aqueous polyvinyl saccharide solution in a ratio which is sufficient to provide a ratio of saccharide equivalents to borate ion equivalents in the range of from 200:1 to 1:2.

Preferably the polyvinyl saccharide content of the mixture is in the range of from 0.75 to 3 * w. Should it be required for the composition of the thickened aqueous mixtures of the present invention to include a pH buffer, said buffer may conveniently be introduced by dissolving the polyvinyl saccharide in a suitable pH buffer solution instead of employing e.g. demineralized water.

The actual quality of the water used in the preparation does not appear to be critical as it has been demonstrated, when the polyvinyl saccharide solutions were prepared by dissolving the polyvinyl saccharide in aqueous NaCl solutions having a molarity of up to 4.0 M, that the presence of NaC1 did not appear to be harmful for the viscosity of the ultimate mixtures.

The thickened aqueous polyvinyl saccharide/borate ion-yielding compound mixtures of the present invention are especially suited to be used in oil field operations e.g. for fracturing subterranean formations.

The invention will be further illustrated by the following Examples for which the following information is provided.

Preparation of 3-O-methacryloyl-D-glucose (MG).

This monomer was prepared according to the method described in European Patent Application EP 0180262.

Preparation of polyvinyl saccharide homopolymer based on MG and polyvinyl saccharide copolymer based on MG/methacrylic acid(MA) in a 75/25 molar ratio.

The polymers were prepared according to the method described in European Patent Application EP 0180262 at a temperature of 40 ec and in a monomer to AIBN molar ratio of 2000:1.

Preparation of the aqueous polyvinyl saccharide solutions.

The polymer solutions were prepared by the addition of 1 g of polymer to a jar containing 100 ml of demineralized water and subsequently placing the jar on a roller table for approximately 24 hours at 23 "C, which period of time was sufficient to dissolve the polymer. For the Examples wherein a buffered or NaCl containing polymer solution was required the demineralized water was replaced by a buffer solution or a NaCl solution respectively.

Preparation of the 0.1 M aqueous borax solution.

38.14 g of borax was dissolved in demineralized water and subsequently the volume was made up to 1 1. When borax solutions of lower molarity were required e.g. 0.01 M and 0.001 M, these were obtained by diluting the 0.1 M borax solution with demineralized water.

Buffer solutions.

Commercially available buffer solutions ex Merck were employed having a pH of 5, 7 and 9.6 respectively.

Example I.

Thickened aqueous poly MG/borax solutions.

Poly MG/borax solutions were prepared, having a poly MG concentration of 10gel and a MG to borax molar ratio in the range of from 400:1 to 1:1, by adding to a glass jar containing 2.5 ml of an aqueous poly MG solution (20 g poly MG/1) A ml of a X M aqueous borax solution (both prepared as described hereinbefore) and C ml H20 and homogenizing the mixture for 45 minutes at 23 "C on a roller table. Subsequently the viscosity of the poly MG/borax solution was measured employing a Low Shear 30 Contraves viscometer at 25 "C and a shearrate z - 7.46 sec'l The viscosity of each of the systems was compared with that of a borax-free poly MG solution containing 10 g poly MG/1.

The values which correspond with the underlined capital letters as used hereinbefore are given in Table 1 hereinafter, which Table also gives the viscosity data of the different solutions.

Example II.

Buffered thickened aqueous poly MG/borax solutions.

Following the procedure of Example I buffered thickened aqueous poly MG/borax solutions were prepared using buffered poly MG solutions having a pH of 5.0, 7.0 and 9.6 respectively instead of a poly MG solution in demineralized water. The viscosity date of the different system is given in Table 2 hereinafter.

Example III.

NaCl-containing thickened aqueous poly MG/borax solutions.

Thickened aqueous poly MG/borax solutions were prepared following the procedure as described in Example I, but employing NaCl-containing aqueous poly MG solutions, i.e. a 0.4 and 4.0 M NaCl solution in water. The poly MG content of the ultimate solutions was 10 g/l while the MG to borax molar ratio was in the range of from 200:1 to 4:1.

The compositions of the different solutions are given in Table 3 together with the corresponding viscosity data.

Example IV.

Buffered thickened aqueous poly MG/boric acid solutions.

Following the procedure described in Example II buffered thickened aqueous poly MG solutions were prepared but employing a boric acid solution, in stead of a borax solution, in combination with a buffered aqueous poly MG solution having a pH-7. The poly MG content of the ultimate solutions was 10 g/l while the MG/boric acid molar ratio was in the range of from 27:1 to 1:0.81. The molarity of the aqueous boric acid solutions employed was 0.5 M and 0.1 M respectively and had been prepared in the same manner as the borax solutions.

The compositions of the thickened poly MG/boric acid solutions are given in Table 4 hereinafter together with the corresponding viscosity data.

Example V.

Thickened aqueous poly MG/MA (75/25)/ borax solutions.

Thickened aqueous poly MG/MA (75/25)/ borax solutions were prepared following the method as described in Example I but using an aqueous poly MG/MA (75/25) solution instead of a poly MG/MA solution. The poly MG content of the ultimate solutions was 10 g/l while the MG/borax molar ratio was in the range of from 360:1 to 1:1.1. The compositions of the thickened solutions are given in Table 5 hereinafter together with the corresponding viscosity data.

Table 1.

I Borax Water MG/borax 1 M ml ml molar m.Pa.S X A C ratio (f 7.46s-1) 0.001 0.5 2.0 4001) 72.5 2.8 0.001 1.0 1.5 200 207.2 8.5 0.01 0.2 2.3 100 > 450 > 17 0.01 0.5 2.0 40 207.2 8.0 0.01 1.0 1.5 20 214.6 8.29 0.1 0.2 2.3 10 296.0 11.43 0.1 0.5 2.0 4 314.5 12.14 0.1 1.0 1.5 2 303.4 11.71 0.1 2.0 0.5 1 247.9 9.57 2.5 1) 25.9 #: viscosity of poly MG/borax solution (PMG - poly MG) #: viscosity of poly MG solution 1) Comparative experiment, not according to the invention. Table 2

pH MG 5.0 7.0 9.6 MG/borax # # pH # # # pH # # pH # # # molar M.Pa.s. # pMG/borax # pMG/borax M.Pa.s # pMG/borax ratio (γ=7.46s-1) sol. (γ=7.46s-1) sol. (=7.46s-1) sol.

4001) 25.60 0.99 5 88.80 3.2 7 83.99 1.5 9-10 200 25.23 0.98 425 15.5 7 159 2.9 9-10 100 27.16 1.05 5 > 450 > 15.5 7 444 8.1 9-10 40 25.97 1.01 5 > 450(5) > 15.5 7 > 450 > 8 9-10 20 26.64 1.03 5 > 450(5) > 15.5 7 > 450 > 8 9-10 10 92.50 3.6 5 (3) (4) (5) - 7 > 450 > 8 9-10 4 > 400 > 15 5 (3) (4) (5) - 7-8 > 450 > 8 9-10 2 > 450 > 15 5-6 (4) (5) - 8 259 4.7 9-10 1 (3) - 8 370 14 9 229 4.2 10 ~1) 25.75 - 5 27.38 - 7 55.13 - 9-10 # : viscosity of p MG/borax solution (3) : precipitation observed (5) : opaque # : viscosity of p MG solution (4) : jelly-like consistency 1) : Comparative experiment, not according to the invention.

Table 3

Molarity of aqueous NaCl solution 0.4 4.0 MG/borax # # # # molar m.Pa.s. # m.Pa.s. # ratio (γ=0.87s-1) (γ=8.7s-1) 200 - - #1000 gel 100 #1000 gel #1000 gel 70 - - #1000 gel 40 - - (4) 4 (3) - (3) 1) 27.53 ~ 34.48 # : viscosity of p MG/borax solution # : viscosity of pMG solution (3) : precipitation observed (4) : flocculation observed 1) : comparative experiment, not according to the invention.

Table 4

MG/boric acid 1 171 pH molar ratio m.Pa.s '72 pMG/boric acid -7.46s-1 solution 27 400 15 7 8 > 450 > 16 7 4 450 16 7 2.5 450 l6 7 1.15 (3)(4) - 6-7 0.81 (4) - 6-7 wl) 27.38 - 7 # : viscosity of p MG/boric acid solution # : viscosity of pMG solution (3) : jelly-like consistency (4) : flocculation observed 1) : comparative experiment, not according to the invention.

Table 5

MG/borax '71 '71 p11 molar ratio m.Pa.s 2 pMG/MA-borax (-3-7. 46 s-1) solution 360 118.4 1.2 4 180 133.9 1.4 4 90 192.4 2.0 4-5 36 325.6 3.4 5 18 525 5.5 5 9 > 600 > 6 6 3.6 > 600 > 6 7-8 1.8 600 6 9 0.9 600 6 9-10 1) 96.57 - 4 1 : viscosity of p MG/MA-borax solution # : viscosity of p MG/MA solution 1) : comparative experiment, not according to the invention.

Claims

1. Thickened aqueous polysaccharide systems comprising aqueous mixtures of a polyvinyl saccharide and a borate ion-yielding compound, wherein the polyvinyl saccharide and the borate ion-yielding compound are present in a weight ratio which is sufficient to provide a ratio of saccharide equivalents to borate ion equivalents in the range of from 200:1 to 1:2.

2. Thickened polysaccharide systems as claimed in claim 1, wherein the polyvinyl saccharide is a homopolymer.

3. Thickened polysaccharide systems as claimed in claim 1, wherein the polyvinyl saccharide is a copolymer of a water-soluble vinylsaccharide monomer and a water-soluble acidic group-bearing monomer, wherein the acidic group-bearing monomer comprises up to 30 % mol of the monomer composition.

4. Thickened polysaccharide systems as claimed in any of the preceding claims, wherein the polyvinyl saccharide is based on a water-soluble monovinylsaccharide monomer containing a vinylcarbonyloxy group.

5. Thickened polysaccharide systems as claimed in claim 4, wherein the vinylcarbonyloxy group is a methacryloyloxy group.

6. Thickened polysaccharide systems as claimed in claim 5, wherein the monomer containing a methacryloyloxy group is 3-0-methacryloyl-Dglucose.

7. Thickened polysaccharide systems as claimed in any of the preceding claims, wherein the borate ion-yielding compound is sodium tetraborate decahydrate.

8. Thickened polysaccharide systems as claimed in any of the preceding claims, wherein the ratio of saccharide equivalents to borate-ion equivalents is in the range of from 100:1 to 1:1.

9. Thickened polysaccharide systems as claimed in any of the preceding claims, wherein the polyvinyl saccharide content is more than 0.75 % by weight.

10. Thickened polysaccharide systems as claimed in any of the preceding claims, wherein a pH buffer is present.

11. Thickened polysaccharide systems as claimed in any of the preceding claims, wherein the pH is in the range of from 7 to 10.

12. Thickened aqueous polysaccharide systems as claimed in claim 1, substantially as described hereinbefore, with reference to the Examples.

13. A process for the preparation of thickened polysaccharide systems as claimed in any of the preceding claims, which process comprises mixing an aqueous solution of a borate ion-yielding compound with an aqueous polyvinyl saccharide solution in a ratio which is sufficient to provide a ratio of saccharide equivalents to borate ion equivalents in the range of from 200:1 to 1:2.

14. Thickened aqueous polysaccharide systems as claimed in any of claims 1 to 12 whenever used in oil field operations.