Method to produce a scale inhibitor

Abstract

A method for the preparation of a scale inhibitor and a method of inhibiting the formation of scale uses a water soluble polymeric gelling agent, in particular synthetic polymer, which has been degraded and reduced in its molecular weight.

Claims

1. A method for preparation of a scale inhibitor comprising: providing a viscosified treatment fluid containing at least water, a water soluble polymeric gelling agent, an oxidizing breaker compound, and no external scale inhibitor agent, wherein the water contains at least 1,000 ppm total dissolved solids (TDS); (ii) pumping the viscosified treatment fluid into a formation and (iii) allowing the oxidizing breaker compound to degrade the water soluble polymeric gelling agent being a synthetic polymer comprising: (I) at least structural units of formula (I) ##STR00006## wherein R1, R2 and R3 independently are hydrogen or C.sub.1-C.sub.6-alkyl, and (II) from 1 to 95% by weight structural units of formula (II) ##STR00007## wherein R4 is hydrogen or C.sub.1-C.sub.6-alkyl, R5 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine, A is a covalent C—S bond or a two-valent organic bridging group, after the breaker degrading the synthetic polymer into oligomeric or monomeric chain fragments; and the oligomeric or monomeric chain fragments preventing scale depositing in the formation.

2. The method of claim 1 wherein a viscosity of the viscosified treatment fluid after breaking in measure (iii) is less than 5 mPas.

3. The method of claim 1, wherein the water soluble synthetic polymer material is selected from the group consisting of polymers containing: (I) 10 to 90% by weight of structural units of formula I, and (II) 1 to 80% by weight of structural units of formula II, referred to a total mass of the polymer, with the proviso that the percentage of the structural units of formulae (I) to (II) refer to a total mass of the copolymer and a percentage of the structural units of formulae (I) to (II)-amounts to 100%.

4. The method of claim 1, wherein a quantity of the synthetic polymer ranges from 0.01 to 10% by weight, referred to a total mass of aqueous polymer solution.

5. The method of claim 1, wherein the breaker compound is selected from the group of inorganic peroxides, persulfates, percarbonates, and perborates.

6. The method of claim 1, wherein a quantity of the breaker compound ranges from 0.001 to 5% by weight, referred to a total mass of aqueous polymer solution.

7. The method of claim 1, wherein the viscosified treatment fluid containing at least a synthetic water soluble gelling agent contains multivalent metal ion.

8. The method of claim 1, wherein the viscosified treatment fluid is injected as hydraulic fracturing fluid.

Description

EXAMPLES

Example 1: Preparation of a Polymer Via Inverse Emulsion Polymerization

(1) 37 g sorbitan monooleate were dissolved in 160 g C.sub.11-C.sub.16 isoparaffin. 100 g water in a beaker were cooled to 5° C., then 50 g 2-acrylamido-2-methylpropane sulfonic acid and 10 g vinylphosphonic acid were added. The pH was adjusted to 7.1 with aqueous ammonia solution. Subsequently 268 g acryl amide solution (50 weight % in water) were added.

(2) Under vigorous stirring the aqueous monomer solution was added to the isoparaffinic mixture. The emulsion was then purged for 45 min with nitrogen.

(3) The polymerization was started by addition of 0.5 g azoisobutyronitrile in 12 g isoparaffin and heated to 50° C. To complete the reaction the temperature was increased to 80° C. and maintained at this temperature for 2 h. The polymer emulsion was cooled to room temperature. As product, a viscous fluid was obtained.

(4) The K-value of was determined to be 390 as 0.1 wt.-% polymer solution in deionized water containing 0.5 wt.-% of an ethoxylated C.sub.13 alcohol having a HLB of >10.

Example 2: Preparation of a Polymer Via Inverse Emulsion Polymerization

(5) A polymer emulsion was prepared according to example 1 but using 58 g 2-acrylamido-2-methylpropane sulfonic acid, 5.5 g acrylic acid and 238 g acryl amide solution (50 weight % in water). The K-value of this polymer determined as in Ex. 1 was 445.

Example 3: Preparation of a Polymer Via Gel Polymerization

(6) 400 ml deionized water and 19.3 ml 25 weight-% aqueous ammonia solution were placed in a reaction vessel. 20 g N,N-dimethylacryl amide, 30 g 2-acrylamido-2-methylpropane sulfonic acid, and 10 g acrylic acid were added under stirring. The solution was purged with nitrogen and heated to 50° C. The polymerization was started by addition of 5 ml of a 20% by weight aqueous solution of ammonium persulfate. To complete the reaction, the temperature was increased to 80° C. and maintained at this temperature for 2 h. After cooling to room temperature a highly viscous gel was obtained. The gel was dried at 90° C. in a vacuum drying oven and carefully chopped from time to time. The dried polymer was crushed to obtain small particles. The K-value of this polymer determined as in Ex. 1 was 392.

Example 4: Preparation of a Polymer Via Gel Polymerization

(7) A polymer was prepared according to example 3 but using 12.7 ml 25 weight-% aqueous ammonia solution, 10 g 2-acrylamido-2-methylpropane sulfonic acid, 15 g vinylsulfonic acid, and 60 g acryl amide solution (50 weight % in water). The K-value of this polymer determined as in Ex. 1 was 355.

Example 5 to 12

(8) A polymer solution was prepared by mixing 200 ml tap water, 0.7 g of an ethoxylated C.sub.13 alcohol having a HLB of >10 and polymer emulsion or polymer powder. Mixing was continued until the emulsion was completely inverted or the polymer was completely dissolved.

(9) The viscosity of the linear gel was determined using a Fann rheometer at 100 rpm. Then ammonium persulfate was added under mixing. The mixture was transferred into a borosilicate glass bottle, the bottle was closed and put into a water bath of 95° C. for 4 h to break the polymer. After cooling, the viscosity was determined using Ubbelohde capillary.

(10) 30 g of a 1% by weight CaCl.sub.2) solution were prepared and solution of the broken polymer was added. After thorough mixing, 30 g of a 1% Na2SO4 solution were added under mixing. Then the solution was checked visually.

(11) Details on the used polymer, the concentrations and the results are summarized in table

(12) TABLE-US-00001 Preparation of linear gel Broken Scaling test Emulsion, Powder, Viscosity, polymer Broken Result Ex. Polymer g g mPas Viscosity, mPas polym., g after 1 h 5 — crystals 6 Ex. 1 2.5 30 0.78 0.3 clear solution 7 Ex. 1 2.5 30 0.78 10 clear solution 8 Ex. 2 2.66 46.5 0.81 10 turbid 9 Ex. 2 2.66 46.5 0.81 0.3 crystals 10 Ex. 3 0.78 24 0.8 10 crystals 11 Ex. 4 0.78 23 0.85 10 clear solution 12 CMHPG 1.0 78 0.76 10 crystals

(13) The test results clearly show that broken polymer containing monomers bearing phosphonic and/or sulfonic groups are effective to prevent the precipitation of CaSO.sub.4. On the other hand, broken polymers without a phosphonic group but containing carboxylic group cannot inhibit the formation of CaSO.sub.4-crystals. Also, the broken solution of natural based CMHPG does not inhibit scaling.

Examples 13 to 19

(14) Broken polymer solutions from different polymers were prepared according to the procedure described in examples 5 to 12.

(15) A synthetic brine containing 36 500 ppm Na, 500 ppm K, 7500 ppm Ca, 3000 ppm Mg, 1000 ppm sulfate and 75 000 ppm chloride was prepared.

(16) To 200 ml of the synthetic brine broken polymer solution was added, the quantities given in table 2. The solution was transferred into teflon lined autoclaves, sealed and put in an electric oven at 175° C. for 20 h. After cooling down the autoclaves using cold water, they were opened and CaSO.sub.4 crystals grown at the wall due to the reduced solubility at high temperature were carefully removed and stirred in the water. The solution or slurry was filtered using a 20 to 40 μm filter paper with 1.5 cm in diameter. The filter paper was then dunked into a 10 ml of cold distilled water and held there for 20 h at a temperature between 7 to 10° C. At the low temperature CaSO.sub.4 from the filter paper can dissolve again. The Ca content in the water was determined using ICP EOS. From the Ca content the quantity of CaSO.sub.4 that precipitated during the scaling experiment was calculated.

(17) TABLE-US-00002 Broken polymer solution Scaling test Quantity Visual Ca content CaSO.sub.4 Viscosity added inspection after precipitated of broken to 200 ml after 20 h at dissolution in the fluid, Made from polymer synth. 177° C. before in 10 ml calculated, Ex. polymer of solution, mPas brine, g filtration water, ppm mg/kg 13 — fine crystals 386 131 14 CMHPG 0.76 1.0 crystals 298 101 15 Ex. 1 0.78 0.5 very few tiny 27 9 crystals 16 Ex. 1 0.78 2.0 very few tiny 48 16 crystals 17 Ex. 2 0.81 1.0 crystals 289 98 18 Ex. 4 0.85 0.5 few tiny 64 22 crystals 19 Commercial 1.0 turbid 49 17 phosphonate (of 1% scale inhibitor solution)

(18) It can be clearly seen that the broken polymer solutions from polymers prepared according to Ex. 1 and Ex. 4 prevent CaSO.sub.4 from precipitation. Similar results are obtained using a commercially available phosphonate scale inhibitor.

(19) In contrast, solution of broken polymer of Ex. 2 showed comparable results as the reference example without addition of scale inhibitor. Also solution of broken CMHPG could not prevent precipitation of CaSO.sub.4.