SILICATE SOLUTIONS

Abstract: An alkali metal silicate solution contains an agent effective to inhibit precipitation or gelation of the silicate in the solution in the presence of divalent metal ions. The agent is a water-soluble inorganic compound containing a univalent cation other than the cation of the alkali metal silicate and an anion effective to inhibit said precipitation or gelation.

Description: SILICATE SOLUTIONS The present invention relates to alkali metal silicate solutions and more particularly, but not exclusively, to such solutions for use in oil industry applications, e.g. for use in drilling fluids, drilling muds, and oil well working fluids, all referred to generically herein as drilling fluids. Alkali metal silicate solutions were extensively used in the oil industry for such applications over a period of many years. However more recently they have for the large part been superseded by more sophisticated fluids with improved properties. These fluids are however now giving rise to environmental concerns so that silicate solutions may come back into favour. Silicate solutions do however have the disadvantage that they gel or form precipitates on reaction with divalent metal ions (e.g. Ca 2 ' and Mg 2 + ) which are contained in groundwater and seawater. This is obviously an unwanted reaction which limits the usefulness of silicate solutions in oil industry applications. It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantage. According to a first aspect of the present invention there is provided an alkali metal silicate solution containing an agent effective to inhibit precipitation or gelation of said silicate in the solution in the presence of divalent metal ions wherein said agent is a water soluble inorganic compound containing a univalent cation other than the cation of the alkali metal silicate and an anion effective to inhibit said precipitation or gelation. The inorganic compound (also referred to herein as the 'inhibiting agent') may be one for which the anion prevents gelation or precipitation of the silicate either by (i) complex formation with the divalent metal, (ii) precipitation of an insoluble compound of the divalent metal, or (iii) reducing the degree of polymerisation of the silicate The anion is preferably hydroxide, carbonate or phosphate. Two or more inorganic compounds may be used as the inhibition agent to provide a combination of such anions. The term phosphate as used herein is used as a generic name for a group o: compounds sucn as (but not exclusively) phosphates, orthophospnates, metaphosphates, polyphosphates, pyrophosphates, etc, but for t rie purposes of the present specification excludes hydrogen phosphates and dihydrogen pnosphates. The inhibiting agent is preferaolv a pyrophosphate or polyphosphate. Preferably the univalent cation is an alkali metal. If the alkali metal silicate is sodium s-licate then the cation ( of the inhibiting agent) is other than sodium ar.α is preferably potassium. In accordance with the first aspect of the invention, the cation of the aikali metal silicate is different from that of the inhibiting agent for solubility reasons. The preferred cation for the alkali metal silicate :≤ sodium silicate wnereas that f or the inhibiting agent is potassiurr Inhibiting agents m the form of potassium salts have greater solubility in sodium silicate solutions than the equivalent sodium saits. We have also found that the use of an alkali metal hydroxide in conjunction with a phosphate or carbonate is particularly advantageous since the hydroxide provides not only enhanced inhibition of precipitation or gelation of the silicate but also lowers the viscosity of the solution without effecting other properties thereof. Thus the inclusion of an alkali metal hydroxide, in conjunction with a phosphate or carbonate, allows the viscosity of the solution to ne tailored to specific needs. As mentioned above, preferred inhibiting agents for use in the invention include carbonates and/or phosphates. This leads to a second aspect of the invention according to which there is provided an alkai; metal silicate solution containing an agent effective to inhibit precipitation or gelation of said silicate in the presence of divalent metal ions wherein said agent is a water soluble inorganic compound containing a univalent cation and an anion selected from carbonate and/or phosphate. The preferred phosphate for use in accordance with the second aspect of the invention is pyrophosphate or polyphosphate. For compositions in accordance with the second aspect of the invention, it is preferred (but not essential) that the cation of the inhibition agent is other than the cation of the alkali metal silicate. Compositions in accordance with the second aspect of tne invention preferably also include the hydroxides of a univalent cation as a viscosity modifier. The invention (both first and second aspects) is applicable mcst particularly to sodium silicate solutions. For use in the inventicn, the silicate may have a wide range of Si0 2 :M 2 0 weight ratios, typically 1.6:1 to 3.6:1. As mentioned above, the preferred inhibiting agents are hydroxides, phosphates and carbonates. We have established that t .e latter two agents (phosphate and carbonate) are useful over a wide range of Si0 2 :Na 2 0 weight ratios. However if hydroxide is to be used as the sole inhibiting agent then it is most effective at Si0 2 :N ' a-C ratios of 2.5:1 to 3.6:1. Generally the amount of alkali metal silicate solids will comprise 30 to 99% (preferably 50 to 99%) by weight of the total weight of tr.e alkali metal silicate solids and inhibition agent which will correspondingly be 1 to 70% (preferably 1 to 50%) by weight on the same basis. Within this broad range we have identified more preferred ranges which are dependent on the inhibition agent(s) used and, for the more preferred ranges, on the Si0 2 :Na 2 0 ratio of the silicate employed. If the inhibition agent is phosphate and/or carbonate then it is preferred that the solution comprises 15 to 55% by weight of tnc inhibition agent based on the total weight of solids (silicate plus inhibition agent). To attain maximum inhibition, the amount of carbonate or phosphate is determined by reference to the Si0 2 :Na,C weight ratio (assuming the silicate is sodium silicate) in accordance with the following Table 1. Table 1 SiO-.:Na.,0 Ratio Amount of Inhibition Agent 1.6 to 2.1 15-30% 2.1 to 2.6 20-45% 2.6 to 3.6 35-55% In the above table the amount of inhibition agents is expressed as a percentage of the total amount of silicate plus inhibition agent. If the inhibition agent comprises a hydroxide plus at least one otner inhibition agent ( e.g. carbonate and/or phosphate) then it is preferred that the hydroxide comprises up to 10% by weight of total solids ( inhibition agent plus silicate) and the other inhibition agent comprises 15% to 60% on the same basis. To attain maximum inhibition, the amount of the 'other ' inhibition agent is determined by reference to tnε 5i0 2 :N ' a-,0 weight ratio (assuming the silicate is sodium silicate) in accordance with the following table. Table 2 SiQ. : Na.,Q Ratio Amount of Hydroxide Amount of 'other' Inhibition Agent 1.6 to 2.1 i to 10% 15-35% 2.1 to 2.6 1 to 10% 20-40% 2.6 to 3.6 1 to 10% 40-60% In the above table, the amounts of hydroxide and 'other' inhibition agent are expressed as percentages of the total amount of silicate, hydroxide and 'other' inhibition agent. The use of a hydroxide as the sole inhibition agent is limited by its ability to reduce the degree of polymerisation within the solution. The preferred amount of the hvdroxide is 1 to 20% by weight (based on the total weigh t of the hyαroxide (e.g. alkali metal hydroxide ) and silicate), more preferably 2 to 12% on the same basis. Solutions in accordance with the invention may be produced in a wide range of concentrations. It will however generally be most convenient initially to formulate more concentrated solutions for transport to the site at which the solution is to be used and then to dilute the "concentrate" down to the required use strength. If the inhibiting agent is a carbonate or hydroxide then such a concentrate may conveniently be prepared by mixing a commercial grade of silicate solution (which would generally contain 35% to 55% by weight solids) with a solution of appropriate strength of the carbonate or hydroxide. If however the inhibiting agent is a phosphate, it may be more convenient to dilute a commercial silicate solution to give a solids content of 15-40% (eg 20-25%) by weight before admixture with a solution of the phosphate since it may be difficult to obtain the solution of the phosphate at higher silicate concentrations. It is also possible to provide a two component system comprising (i) a concentrated silicate solution, and (ii) an inhibiting agent solution, the two solutions being mixed on site prior to use. Alkali metal silicate solutions in accordance with the invention are particularly suitable for use in oil industry applications. As such, the solutions may be formulated into drilling fluids (including drilling muds clear drilling fluids, workover fluids, and oil well working fluids). Such fluids may comprise up to 25% by weight, more usually 5-10% by weight, of alkali metal silicate. The fluids may, for example, be used in shale stabilisation processes. The invention will be illustrated by reference to the following non-limited Examples. Example 1 A silicate solution (designated Sample 1) in accordance with the invention was prepared by mixing 0.2 g 50% w/w potassium hydroxide solution and 1 g tetrapotassium pyrophosphate in 88.8 g deionised water, adding 10 g of Q79 sodium silicate solution (available ex Brunner Mond) and stirring for 2 minutes. Q79 has a Si0 2 :Na 2 0 weight ratio of 3.3 and a silicate solids content of 38.1% by weight. The tolerance of the solution to precipitation and/or gelation of the silicate was determined by titrating the solution with synthetic seawater whilst measuring the turbidity of the solution by light transmission. The end point was determined by the point at which light dispersion causes a drop in transmitted light due to the intervention of particulate matter. The synthetic seawater used in this procedure had the composition shown in the Handbook of Chemistry and Physics, 58th Edition, F-203 save that components having a concentration less than 100 ppm were excluded. The result is shown in Table 3. A further sample (Sample 2) in accordance with the invention was prepared by the same procedure but using 0.2 g 50% w/w potassium hydroxide solution, 2 g tetrapotassium pyrophosphate, 87.8 g deionised water and 10 g of Q79 sodium silicate solution. The tolerance to precipitation and/or gelation was measured as previously. A further sample (Sample 3) containing potassium hydroxide (in the amount shown in Table 3 ; but no tetrapotassium pyrophosphate was also prepared and the tolerance measured as previously. By way of comparison, Sample 4 which contained neither potassiurr, hydroxide nor tetrapotassium pyrophosphate was prepared and its tolerance also measured. The results are shown in Table 3. Table 3 Sample Silicate Weight Potassium Hydroxide Tetrapotassium Volume seawater to Pyrophosphate precipitation/gelation g/lOOg g/lOOg g/100g cm3 1 10 0.1 1 73 2 10 0.1 2 91 3 10 0.34 0 47 4(comparative) 10 0 0 39 It can be seen from Table 3 that all of Samples 1 to 3 sh superior tolerance to precipitation/gelation than Sample 4 which did n contain an inhibition agent. The result was best for Sample 2. Example 2 A silicate solution was prepared by taking 1 g 50% w/w potassiu hydroxide and 10 g 50% tetrasodium pyrophosphate with 30 g deionis water, adding 50 g Q79 sodium silicate and stirring for 2 minutes. A 20 g aliquot of the solution was made up to 100 g with deionis water and tested for its tolerance to sea water as described in Examp 1 above. The result was substantially the same as for sample 1 (Examp 1) above. Example 3 Using a sodium silicate solution having a solids content of 53. and a Si0 2 :Na 2 0 weight ratio of 2, Samples 4 and 5 (comparative) havi the composition shown in Table 4 were prepared. The samples we tested for their tolerance to seawater as described in Example 1 abov The results are shown in Table 4 and once again the superior resu obtained by the presence of potassium hydroxide and tetrapotassiu pyrophosphate (as compared to no inhibiting agent at all) are clear demonstrated. Table 4 Sample Silicate Weight Potassium Hydroxide Tetrapotassium Volume seawater to Pyrophosphate precipitation/gelation g/lOOg g/lOOg g/100g cm3 5 10 0 . 25 1 90 6 10 0 0 71

Claims: CLAIMS 1. An alkali metal silicate solution containing an agent effective to inhibit precipitation or gelation of said silicate in the solution in the presence of divalent metal ions wherein said agent is a water soluble inorganic compound containing a univalent cation other than the cation of the alkali metal silicate and an anion effective to inhibit said precipitation or gelation. 2. A solution as claimed in claim 1 wherein the anion is hydroxide, carbonate or phosphate. 3. A solution as claimed in claim 2 wherein the anion is pyrophosphate or polyphosphate. 4. An alkali metal silicate solution containing an agent effective to inhibit precipitation or gelation of said silicate in the presence of divalent metal ions wherein said agent is a water soluble inorganic compound containing a univalent cation and an anion selected from carbonate or phosphate. 5. A solution as claimed in claim 4 wherein the cation is a pyrophosphate or a polyphosphate. 6. A solution as claimed in claim 4 or 5 wherein the solution additionally comprises a hydroxide. , 7. A solution as claimed in any one of claims 1 to 6 wherein the univalent cation is an alkali metal. 8. A solution as claimed in any one of claims 1 to 7 wherein the alkali metal silicate is sodium silicate having a SiO 2 :Na O weight ratio of 1.6: 1 to 3.6: 1. 9. A solution as claimed in any one of claims 1 to 8 wherein the alkali metal silicate is sodium silicate and the cation of said agent is potassium. 10. A solution as claimed in any one of claims 1 to 9 wherein the amount of silicate solids in the solution comprise 30 to 99% by weight of the total weight of the silicate solids and agent. 1 1. A solution as claimed in claim 4 wherein the carbonate or phosphate is present in an amount of 15 to 55% by weight based on the total weight of the silicate and inhibition agent. 12. A solution as claimed in claim 1 wherein the inhibition agent comprises a carbonate or phosphate and the inhibition agent is present in an amount of 15 to 55% by weight based on the total weight of the silicate and inhibition agent. 13. A solution as claimed in claim 11 or 12 wherein the silicate is sodium silicate having a SiO : :Na 2 O ratio of 1.6 to 2.1 and the inhibiting agent is present in an amount of 15 to 30% by weight based on the total weight of the silicate and inhibition agent. 14. A solution as claimed in claim 1 1 or 12 wherein the silicate is sodium silicate having a SiO 2 :Na 2 O ratio of 2.1 to 2.6 and the inhibiting agent is present in an amount of 20 to 45% by weight based on the total weight of the silicate and inhibition agent. 15. A solution as claimed in claim 11 or 12 wherein the silicate is sodium silicate having a SiO 2 :Na 2 O ratio of 2.6 to 3.6 and the inhibiting agent is present in an amount of 35 to 55% by weight based on the total weight of the silicate and inhibition agent. 16. A composition as claimed in claim 4 wherein the inhibition agent comprises a hydroxide in addition to the phosphate or carbonate, the amount of said hydroxide being up to 10% by weight of the total weight of the inhibition agent and silicate and the amount of the carbonate or phosphate being 15% to 60% on the same basis. 17. A composition as claimed in claim 1 wherein the inhibition agent comprises a hydroxide together with a phosphate or carbonate, the amount of said hydroxide being up to 10% by weight of the total weight of the inhibition agent and silicate and the amount of the carbonate or phosphate being 15% to 60% on the same basis. 18. A solution as claimed in claim 16 or 17 wherein the silicate is sodium silicate having a SiO 2 :Na2O weight ratio of 1.6 to 2.1 and the solution contains 1 to 10% by weight of the hydroxide (based on the total weight of the silicate and inhibition agent) and 15 to 35% by weight of the carbonate or phosphate on the same basis. 19. A solution as claimed in claim 16 or 17 wherein the silicate is sodium silicate having a SiO 2 :Na2O weight ratio of 2.1 to 2.6 and the solution contains 1 to 10% by weight of the hydroxide (based on the total weight of the silicate and inhibition agent) and 20 to 40% by weight of the carbonate or phosphate on the same basis. 20. A solution as claimed in claim 16 or 17 wherein the silicate is sodium silicate having a SiO 2 :Na2O weight ratio of 2.6 to 3.6 and the solution contains 1 to 10% by weight of the hydroxide (based on the total weight of the silicate and inhibition agent) and 40 to 60% by weight of the carbonate or phosphate on the same basis. 21. A solution as claimed in claim 1 wherein a hydroxide is present as the sole inhibition agent, the amount of the hydroxide being 1 to 20% by weight of the total weight of the silicate and hydroxide. 22. A solution as claimed in claim 21 wherein the hydroxide is present in an amount of 2 to 12% by weight based on the total weight of the silicate and hydroxide.

Inventors: HUGHES PHILIP GEORGE, THORPE MALCOLM JOHN, HUGHES, PHILIP, GEORGE, THORPE, MALCOLM, JOHN
Applicants: BRUNNER MOND & CO LTD, HUGHES PHILIP GEORGE, THORPE MALCOLM JOHN, BRUNNER MOND & COMPANY LIMITED, HUGHES, PHILIP, GEORGE, THORPE, MALCOLM, JOHN
Application year: 1996
Main classification: C01B33
IPC codes: C01B33, C09K8