GB2089362A - Additives for use in aqueous solutions in the recovery stage of an oil or gas well fracturing process - Google Patents

Abstract

The invention comprises a water soluble additive for a solution to be utilized in the recovery stage of an oil or gas well fracturing process, said additive comprising an ethylcyano derivative of a polygalactomannan. The invention also includes a process for the preparation of such additives comprising reacting a polygalactomannan with aerylonitrile, in the presence of an alkaline catalyst.

Description

SPECIFICATION Additives for use in aqueous solutions in the recovery stage of an oil or gas well fracturing process The present invention relates to improvements in the formulation and synthesis of modified polygalactomannans which are to be employed as additives to aqueous solutions used in the recovery stage of an oil or gas well fracturing process.

The technical and scientific literature has already made known the typical characteristics of water soluble polymers derived from cellulose or polygalactomannans and treated in a special way in order to improve their solubility. Generally known as well are their chemical derivatives which are produced by substitution of the methylether, ethylether, hydroxyalkyether, particularly hydroxypropylether groups with carboxymethyl groups and/or their alkali salts on the hydroxyl group. In the same way, the characteristics of such derivatives are known, such derivatives being synthetized according to more or less complicated technical procedures.

The main characteristic of both the water soluble polymers derived from cellulose or polygalactomannans, as well as their derivatives is the improved productivity of oil or gas wells during the recovery stage of the fracturing treatment. In facer such polymers allow a larger fracturing treatment through lower friction pressure. Moreover, these solutions are generally employed together with solids of selected size (generally granulated quartz sands, small glass spheres or other similar material) known as "propping agents". The best products of this line allow a better yield, also thanks to the improved permeabitity of the formation which helps the fluids to come to the surface.

For a more accurate description of the advantages offered by the present invention, we will firstly summarize the various stages of oil or gas recovery through fracturing: An aqueous solution of viscous polymer is pumped at a high rate into a formation. The fracturing process occurs as a consequence of hydrostatic pressure action. The above mentioned aqueous solutions must previously have con-ectly added thereto propping agents and breaking agents made of specific enzymes or oxidizers, usuaily persalts.

When the pumping of the aqueous solution is completed, the hydrolitic effect produced by the breaking agents takes place after some hours or some days, according to the added quantities. The reduction in viscosity during the aqueous stage causes the oil or gas to flow towards the collecting base of the well, such flow being assisted by the propping agents. After the hydrolysis step, the suspension properties of the hydrolyzed thickener in respect of eventual insoluble, pseudo-colloidal substances or inorganic microsolids have to be taken into acccunt. Such properties are appreciative, especially when considering that at this stage the viscosity reaches its lowest value.

Whilst the viscosity of the solution decreases considerably as a consequence of the enzymatic hydrolysis, a build-up of insoluble products takes place. Such insoluble products tend to settle on the propping agents in the fracture and themfore reduce the porosity or permeability thereof, thus causing a very negative effect. Usually the presence of the insoluble products causing such damage is quite visible, since they tend to build up visible sediments in the hydrolyzed solution. When these sediments are present there is e corresponding tendency by the propping agents, to reduce porosity.

An aqueous solution which does not give rise to visible sediments, is certainly to be considered as superior from this point of view since it will pass through the propping agents without obstructing them.

Such a better behaviour might be due to various causes, such as a iower build-up of insoluble products during hydro!ysis, smaller sizes of insoluble particles, or an improved suspending property of the hydrolyzed solution.

It is not easy to find out which is the effective causa or how much the various possible causes can help the better behaviour of the solutions in which sediments are not produced or in which they are produced in a smaller amount. In any event, this is not important since experience shows that the reduction or elimination of sediments depends on the better behaviour or improved properties of the solutions for fracturing obtained with the water soluble polymeric additives of the present invention. The tendency of thickeners to reduce the porosity or permeability of the propping agents, can be quantitatively evaluated as well, by filtering through common filters. The solutions obtained from the improved products of the present invention prove to be considerably more filterable than the traditional ones.Nevertheless, as previously mentioned, this advantageous property of the products according to the present invention, can be evaluated as well by checking the presence or absence of sediments.

According to the present invention there is provided water soluble polymeric additives for solutions to b utilised in the recovery stage of an oil or gas well fracturing process, said additives comprising ethylcyano derivatives of polygalactomannans.

The additives of the invention may have a degree of substitution of from 0.3 to 3.0, more preferably from 0.5 to 1.5.

The ethylcyano radical may be substituted by other radicals, such as alkyl, hydroxyalkyl, carbamil, carboyyethyl, and alkaline carboxyethyl, up to a maximum substitution of 70%.

Preferably, the postgalactomannans are derived from guar or locust seeds.

The additives of the invention are obtained by the reaction of acrylonitrile with a polygalactomannan in the presence of an alkaline catalyst. The degree of substitution can vary from 0.3 to 3.0; in any event a degree of substitution of from 0.5 to 1.5 is preferable. This reaction can be accompanied by other substitutions, such as etherification with alkyl or hydroxyalkyl groups. Whilst the reaction is being carried out, a partial degradation of the cyanoethyl groups into carbamoilic as well as a saponification to alkaline ethylcarboxyl are permitted. In any case such substitutions should not exceed 70%. The reaction can be carried out either under dry conditions, or in a slurry of a water-soluble solvent of the polygalactomannan with acrylonitrile in the presence of an alkaline catalyst.The preferred alkaline catalyst is sodium hydroxide, but potassium hydroxide, a tertiary amine or an organic quaternary ammonium base can also be employed.

The basic reaction, with the glucosidic radical being indicated by R, can be shown as follows: alkali

The physical conditions preferably needed for this reaction are temperatures ranging from 200C to 800C and pressures ranging from 0 to 8 atm, depending on the formulation and on the desired final product. Among the various solvents which are usually employed as suspending means, the ones providing a slurry are preferable but, in any case, they should not react with the other integers of the reaction. The ethers derived from ethyleneglycol, such as dimethyl, diethyl, dipropyl, dibutyl ethers, are suitable.When the reaction is completely carried out under the right conditions and the desired characteristics are achieved, the final product must be neutralized with an inorganic or organic acid, in such a way as to completely destroy any eventual ether group of the cellulosic molecule with alkali, for example, sodium cellulose. By observing the above mentioned conditions, products can be obtained with excellent solubility and high viscosities. The water content of the reacting mass can be from 3 to 20% by weight, but 10 to 1550 is especially preferred. Since the reaction takes place in nonhomogeneous stages, it is necessary to observe the above mentioned conditions for each reaction procedure very carefully in order to guarantee a constant reproduction of the process.For the same purpose, the operation of the equipment should also be scheduled accordingly. Hereinafter certain examples concerning the formulation and the procedure to obtain the above described polymers are described. Other examples compare the differences between the products of the invention with other traditional products, which examples point out the special properties of the additives of the invention.

These examples shall be considered as illustrative and are not limitative of the invention.

EXAMPLE 1 A reaction is carried out with guar meal and acrylonitrile in such a manner as to obtain a degree of substitution DS = 1 The reaction is carried out in the presence of sodium hydroxide present in an amount of about 10% by weight of the polygalactomannan. The water content of the total reaction mass is 13%. The reaction is carried out in a closed reactor and at a temperature of 650C for 3 hours, the pressure in the reactor being kept below 0.6 to 0.8 atm.

A vacuum of 700 mm Hg was then applied and the atmosphere of the reactor was saturated with nitrogen. Apparatus is set up for the distillation process and, applying a moderate nitrogen stream, the reacting mass is kept at 700C for 3 hours.

The characteristics of the product can be checked by means of successive drawings. Flnally the product is neutralized with formic acid to obtain a pH = 6.5; it is cooled and unloaded on the desiccation and grinding line. The ground product is sieved in order to obtain particles which must not exceed 200 micron. The resulting powder in aqueous solution of 2% solids gives a viscosity of 18,000 cps, measured at 250C with Brookfield viscosimeter model RVT.

EXAMPLE 2 A reaction is carried out with guar meal and acrylonitrile to obtain DS = 0, and methyl chloride to reach DS = 0.7, in the presence of sodium hydroxide in a ratio of 12% in respect of the polygalactomannan. The water content is 1 6% of the total reacting mass. The reaction in a closed reactor is carried out for a first stage of 3 hours at 400C. The reaction is completed at 650C for 2 hours and then at 80"C for 2-4 hours, keeping the reacting mass at this temperature until the pressure in the reactor reaches 0.5 atm. During the whole reaction, the pressure must never exceed 4 atm.The vacuum of 700 mm Hg is applied and the atmosphere of the reactor is saturated with nitrogen while keeping the temperature at 70-750C. The characteristics of the product can be checked by means of successive drawings until the desired values are reached. Finally, the product is neutralized and the subsequent operations carried out in the same way as for Example 1. The resulting product shows good solubility in water and in solution at 2% gives a viscosity of 14,000 cps, measured at a temperature of 250 C, with Brookfield viscosimeter, model RVT.

EXAMPLE 3 Guar meal is reacted with acrylonitrile to obtain DS = 0.3 and with propylene oxide to obtain DS = 0.6. The reaction is carried out in the presence of a quantity of ethyleneglycol dimethylether of 50% based on guar quantity, using triethylamine, in a quantity of 2% in respect of the amount of guar, as catalyst. The water content is 7% of the total reacting mass. The reaction is carried out in a closed reactor for a first stage of 3-4 hours at 600C. An amount of sodium hydroxide equal to 5% of the guar quantity, is added by means of an aqueous solution, in such a way as to bring the content of water in the reacting mass to 20%. The temperature is increased up to 800C and kept for 2-4 hours, until the pressure goes below 0.8 atm. During the whole reaction the pressure must never exceed 3 atm.A vacuum of 700 mm Hg is applied and the distillation of the recovered solvent (ethyleneglycol dimethylether) is started through the condenser. Recovery is complete by operating at a temperature of 70-750C under vacuum. The atmosphere of the reactor is saturated with nitrogen. The characteristics of the product can be checked by drawing small quantities of it until the desired values are reached. The product is neutralized and the process is completed as for Example 1. The resulting product has a good water solubility and its solutions at 2% give a viscosity of 16.500 cps, measured with Brookfield viscosimeter model RVT at 250C.

EXAMPLE 4 A reaction is carried out with guar meal and acrylonitrile in order to obtain DS = 0.7 in the presence of ethyleneglycol dimethylether (20% of the guar quantity), using potassium hydroxide (8% of guar quantity). The content of water in the reacting mass is 1 6%. The reaction is successively carried out In the same conditions described in Example 3. The resulting product shows good solubility in water and in solution at 2% it has a viscosity of 24,000 cps measured at 250C with Brooltfield viscosimeter model RVT.

EXAMPLE 5 The product which is obtained by Example 1 is prepared with a DS = 1.5 by increasing the quantity of sodium hydroxide up to 12% in respect of the amount of guar. After the addition of acrylonitrile, a forced saponification of the cyanoethyl group takes place, while keeping the product under a moderate nitrogen stream and in a controlled steam stream at a temperature of 800C and operating in order to obtain a final water content of 1 6-1 8% in the reacting mass. The temperature of 800C and the above conditions are kept until a solution of 1% of the product will give a pH value of 7.7-8.2. The product is successively neutralized and the process is completed as per Example 1.The solution at 2% of the resulting product will have a viscosity of 12,000 cps at 250C and measured with Brookfield viscosimeter model RVT.

EXAMPLE 6 In this example, different samples of cellulosic derivatives and modified water soluble galactomannans are compared with the polygalactomannan derivatives of the invention. The behaviour of aqueous solutions at 0.48%, after degradation with breaking agents at different intervals are examined, and the results are shown in the following Table. Timings are measured from the preparation of the solutions.

TABLE Insoluble Viscosity Viscosity product after 40 after 100 after 40 Viscosity hours Sediment hours Sediment hours Product A 30 38 30% 2 32 12% B 24 25 00 12 00 00 C 26 30 18.8% 5 19.2 0.63% D 28 30 15.3% 4.5 16 0.55% E 32 34 00 8 00 00 F 24 26 00 10 00 00 G 27 31 00 5 00 00 H 36 40 2% 6 3 0.7% Product A is surface treated guar; Product B is methylhydroxypropyl cellulose with methyl of DS = 0.5 and hydroxypropyl of DS - 1 and has a viscosity of 10,000 cps measured in a solution at 2% in water; Product C is a hydroxypropyl guar with DS = 1.2 and with viscosity of 12,000 cps measured in a solution at 2% in water; Product D is a methylhydroxyothyl guar with methyl of DS = 0.8 and having a viscosity of 14,000 cps measured in a solution at 2%; Product E is the product described in Example 1; ; Product F is the product described in Example 2; Product G is the product described in Example 3; Product H is the product described in Example 4.

The above Table shows that the products made of ethylcyano-substituted guar in accordance with the invention, permit the preparation of fracturing solutions which, after hydrolysis, have a considerably lower content of insoluble products as compared with the content of those products prepared by prior art techniques.

This characteristic meets with the requirement of helping an easier flow of the petroliferous fluid (oil or gas) towards the pump at the base of the well, after biochemica! degradation of the water soluble polymer solutions; therefore, they allow an improved recovery by means of fracturing treatment.

Claims (14)

1. A water soluble polymeric additive for a solution to be utilised in the recovery stage of an oil or gas well fracturing process wherein said additive comprises an ethylcyanoderivative of a polygalactomannan.

2. An additive as claimed in claim 1, having a degree of substitution of from 0.3 to 3.0.

3. An additive as claimed in claim 2, having a degree of substitution of from 0.5 to 1.5.

4. An additive as claimed in claim 1,2 or 3, in which the ethylcyano radical is substituted by other radicals such as alkyl, hydroxyalkyl, carbamil and alkaline carboxyethyl up to a maximum of 70%.

5. An additive as claimed in any preceding claim in which the polygalactomannans is derived from guar or locust seeds.

6. A water soluble polymeric additive substantially as hereinbefore described and exemplified.

7. A process for the preparation of a water soluble additive for a fracturing solution, comprising the reaction of a polygalactomannan with acrylonitrile, in the presence of an alkaline catalyst.

8. A process as claimed in claim 7, in which the alkaline catalyst is selected from sodium hydroxide, potassium hydroxide, a tertiary amine or an organic quaternary ammonium base.

9. A process as claimed in claim 7 or 8, in which the reaction is also carried out in the presence of water.

10. A process as claimed in claim 9, in which the amount of water is present in an amount of from 3 to 20% by weight.

11. A process as claimed in any one of claims 7 to 10, in which the polygalactomannan reacting with acrylonitrile contains different radicals substituting the ethyicyano radical up to a maximum of 70%.

12. A process as claimed in claim 11, in which, besides acrylonitrile, the polygalactomannan reacts with one or more substances which are suitable for the introduction of radicals different from ethyloyano, in the galactomannan molecule.

1 3. A process as claimed in claim 11 or 12, in which the possible radical substituting the ethyloyano radical are selected from alkyl, carbamil, hydroxyalkyl and alkaline carboxyethyl.

14. A process as claimed in any one of claims 7 to 13, in which the reaction is carried out at temperatures from 200C to 800C and at pressures ranging from O to 8 atm.

1 5. A process as claimed in any one of claims 7 to 14, in which the reaction is carried out in the presence of a solvent selected from the dimethyl, diethyl, dipropyl or dibutyl ether of ethyleneglycol.

1 6. A process as claimed in any one of claims 7 to 1 5, in which the final product is neutralized with an inorganic or organic acid.

1 7. A process for the preparation of a water soluble additive for a fracturing solution, substantially as hereinbefore described and exemplified.