NO813177L - PROCEDURE FOR THE MANUFACTURE OF BURNER TREATMENT LIQUIDS - Google Patents

Description

The present invention relates to the production of polymer-containing salt solutions, which are suitable for various applications where increased viscosity, filtrate control or other functional properties are achieved as a result of the polymer composition.

Polymer-containing salt solutions are suitable as well service fluids, e.g. drilling fluids, revision fluids, completion fluids, sealing fluids, treatment fluids for wells and underground formations, for distance maintenance and well abandonment as well as for other applications where there is a need for thickened, aqueous media. It is known to use hydrophilic polymers, such as hydroxyethyl cellulose (HEC), as thickeners for aqueous media, e.g. those used in well service fluids. But such polymers do not readily hydrate, solvate, or disperse in aqueous solutions containing one or more water-soluble salts of polyvalent cations such as the strong oilfield salt solutions with a density greater than 1.39 g/cm , which are preferred for the production of bronze- the vice fluids. Elevated temperatures and/or mixing with high shear force for a longer time are required for effective thickening of such salt solutions with hydrophilic polymer materials in order for a homogeneous mixture to be achieved. In many cases, e.g. in revision operations, the available equipment for producing the well treatment fluids is not really suitable for such conditions. Consequently, it is usually necessary to prepare such thickened salt solutions in a place other than the well area itself or to circulate the liquid in the hot borehole, if it is desirable to use such thickened salt solutions.

The present invention therefore aims to provide a method for the production of thickened, polymer-containing salt solutions, particularly strong salt solutions with a density greater than 1.39 g/cm when mixed with low shear and without heat input.

Other purposes and advantages of the invention will be apparent from the following description, together with the following claims. According to the invention, a suspension of a hydrophilic polymer and water is formed by general uniform dispersion of the polymer in the water. An inorganic salt with a positive heat of dissolution is then added to the suspension without external heating, as the added amount of salt is sufficient to raise the temperature of the dispersion to above 71, 1°C due to the salt's heat of solution.This suspension can be used directly as a well treatment fluid, depending on the desired density.

In another embodiment of the invention, a sufficiently strong, aqueous salt solution is added to the liquid suspension of the polymer to produce a well treatment fluid of the desired density.

The hydrophilic polymers that are suitable for carrying out the invention are finely divided organic polymers, which are generally water-soluble or dispersible in water and which, after dissolution or dispersion in an aqueous medium, increase the viscosity of the system, but do not easily allow themselves to be hydrated, solubilized or disperse after addition to strong salt solutions which have a density greater than 1.39 g/crn^ and contain soluble salts of polyvalent cations. Such polymers are selected from the group consisting of cellulose derivatives, water-dispersible starch derivatives, polysaccharide gums and mixtures thereof. Examples of cellulose derivatives are the carboxyalkyl cellulose ethers, such as carboxymethyl cellulose and carboxyethyl cellulosep hydroxyalkyl cellulose ethers, such as hydroxyethyl cellulose and hydroxypropyl cellulose? and mixed cellulose ethers, such as carboxyalkyl-hydroxyalkyl-cellulose, ie carboxymethyl-hydroxyethyl-cellulose? alkyl-hydroxyalkyl-cellulose? i.e. methyl-hydroxyethyl-cellulose, methyl-hydroxypropyl-cellulosej alkyl-carboxyalkyl-cellulose, i.e. ethyl-carboxymethyl-cellulose, cf. US-PS 4 110 230. Examples of starch derivatives are carboxyalkyl-starch ethers, such as carboxymethyl-starch and carboxyethyl- starch; hydroxyalkyl starch ethers, such as hydroxyethyl starch and hydroxypropyl starch? and mixed starch ethers, such as carboxyalkyl-hydroxyalkyl starch, ie carboxymethyl-hydroxyethyl starch? alkyl-hydroxyalkyl-starch, i.e. methyl-hydroxyethyl-starch? alkyl-carboxyalkyl-starch, i.e. ethylcarboxy-methyl-starch. Examples of polysaccharide gums include: the bio-polymers, such as Xanthomonas (xanthan) gum? galactomannan gum, such as guar gum, locust bean gum, tara gum, glucomannan gum and derivatives thereof, especially the hydroxyalkyl derivatives, cf. US-PS 4,021,355 and 4,105,461. Other polymers that can be used include pre-gelatinized starch powder and stabilized, partially dextrinized polysaccharide powder, which is toxic non-ionic.

Particularly preferred are the HEC polymers which are generally obtained in high yield, are water-soluble and non-ionic materials, prepared by treating cellulose with sodium hydroxide followed by reaction with ethylene oxide. Each anhydro-glucose unit in the cellulose molecule has three reactive hydroxy groups. The average number of moles of ethylene oxide that is linked to each anhydroglycose unit in the cellulose is called moles of bound substituent. In general, it can be said that the greater the degree of substitution, the greater the water solubility. In general, the use of HEC polymers with the highest possible molar substitution is preferred.

Upon addition of one of the dry, finely divided hydrophilic polymers discussed above to aqueous media, such as salt solutions, the polymer particles typically undergo a surface hydration that prevents the interior of the particle from being hydrated, solvated, or otherwise readily dispersed in the aqueous medium . Consequently, a high shear force, long mixing periods and/or elevated temperatures must be used for a homogeneous system to be achieved. By means of the present invention, the hydrophilic polymers will be easily hydrated, dissolved or dispersed in the aqueous salt solution at relatively low shear forces and ambient temperatures.

In the initial step of the method, the hydrophilic polymer and water, e.g. fresh water, distilled water etc. mixed under conditions that give a uniform dispersion of the polymer in the water. The term "uniform dispersion" in this context means a state where polymer and water form a generally homogeneous system, be it a solution or a mixture, where the individual polymer particles are generally evenly distributed throughout the suspension of polymer and water . The polymer and the water can be mixed together by conventional mixing techniques and without special conditions in terms of temperature, mixing times or other parameters. It is sufficient that the polymer and the water are mixed sufficiently to give a uniform dispersion of the polymer suspension in the water.

In the next step of the method, one or more inorganic salts are added in dry form to the suspension of polymer and water. The salt is of a type that has a positive heat of solution and where salt dissolution in water generates heat. The amount of inorganic salt added to the polymer suspension will be such that a temperature of more than approx. 71.1°C as a result of the heat of dissolution of the salt and without the addition of external heating. Dissolution of the salt in the polymer suspension can be carried out using usual mixing techniques.

The inorganic salt(s) that can be used in the second step of the method are water-insoluble salts that generate heat when dissolved in water and that preferably form salt solutions that are appropriate in hydrocarbon extraction operations. Preferred salts are those selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof. As mentioned, the added amount of salt should preferably be such that the temperature of the polymer/water suspension is raised above approx. 71.1°C. However, it is preferred that the added amount of salt is such that the temperature is raised to at least 82.2°C and preferably at least 93.3°C. It will be obvious that different salts have different positive heats of solution. Therefore, the added amount of salt will depend on the particular salt(s) chosen.

The polymer/water suspensions produced as stated above can in themselves be used as well service fluids, provided the amount of added inorganic salts is sufficient to achieve the desired density. In one case, the amount of polymer, water and mixed inorganic salt can e.g. be sufficient for a thickened salt solution with the desired density to be formed. But most often, an aqueous salt solution with a given density will be added to the polymer/water suspension containing the soluble salt" The aqueous salt solution is added in such a quantity that a well treatment liquid with a fixed density is obtained. In this latter embodiment of the method according to the present invention, the polymer, the water and the inorganic salt are mixed as mentioned above to hydrate the polymer and form the polymer/water suspension. After this, the aqueous salt solution is mixed into the polymer/water suspension containing the inorganic salt and the well treatment fluid is thus prepared. The aqueous salt solutions that can be mixed in polymer/water suspensions generally contain soluble salts, such as a soluble salt of an alkali metal, an alkaline earth metal, a group Ib metal, a group IIb metal, as well as water-insoluble salts of ammonia and other cations. Generally speaking, such aqueous salt solutions contain soluble salts of polyvalent cations, e.g. Zn and Ca. Aqueous salt solutions comprising a salt selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof are thus particularly preferred. The aqueous salt solutions will generally have a density that varies from approx. 1.39 g/cm to approx. 2.30 g/cm<3>.

The amount of hydrophilic polymer used in the method according to the present invention will be such that it gives a final concentration from approx. 0.29 to approx. 28.5 g/l, regardless of whether the final well treatment fluid comprises (a) the polymer/water suspension formed by mixing water, the polymer and the inorganic salt or (b) polymer, water, inorganic salt and an amount of aqueous salt solution.

Although the mechanism of the method according to the invention is not fully understood, it has been shown that salt solutions produced by the method have improved rheological and filtration properties, compared to salt solutions produced by simple dispersion of the hydrophilic polymer in dry form in a salt solution and then heating of the mixture to solvate the polymer. The addition of artificial heat to the mixture of a hydrophilic polymer and a salt solution does indeed improve the result, but does not give the remarkable results that are obtained if the polymer is first dispersed in water and the suspension is then given an elevated temperature by means of the natural heat of dissolution from inorganic salt which dissolves in the polymer/water suspension. In order to provide a more detailed illustration of the invention, the following examples are presented, which are not intended as a limitation. Unless otherwise stated, all measurements of physical properties were carried out in accordance with test procedures as specified in Standard Procedure for Testing Drilling Fluid, API RP 13B, 7th oppi. April 1978. In the examples below, the following hydrophilic polymers were used

Carboxymethyl cellulose (Hi Vis CELLEX)

phtane gum (BARAZAN)

Hydroxyethyl cellulose (NATRASOL 250 HHR)

Polyanionic cellulose powder (DRISPAC)

Cross-linked hydroxyethyl starch (BOHRAMYL)

Pregelatinized: : starch powder (IMPERMEX)

Stabilized partially dextrinized polysaccharide powder, toxic non-ionic (DEXTRID)

Example 1

Several hydrophilic polymers were used to prepare thickened aqueous salt solutions as described below. About.

2 g of the polymer was mixed in 204.4 g of water using a Multimixer for approx. 10 minutes. Next, 114.0 g of CaCl2 pellets (94-97%) and 280.5 g of CaBr2 (91%) were added to the prehydrated polymer, followed by 16.8 ml of a 2.30 g/cm<3>CaB ^/ZnB^ brine so that the density of the resulting brine would be 1.82 g/cm<3>. The heat of solution of the added salts was brought to boiling (100°C) of each sample. The resulting thickened mixtures were left overnight at ambient temperature and then the rheological and filtration properties of each mixture were determined. The rheological properties were measured using a Fann Model 35A Visconer and a Brookfield RVT Viscometer. The filtration properties were measured on an API filter press. The stated properties

is plastic viscosity (PV) cp, yield strength (YP) kg/m 2 , apparent viscosity (AV) cp, 10-sec. gel strength (GEL 10 s) kg/m and API filtrate (API-FIL) ml. All filtration tests were carried out after 28.5 g/l CaCo^ had been added as bridging agent. The results of the measurements carried out are shown in table 1. Table 2 gives the same information for identical samples after they had been rolled for 16 hours at 65.6°C.

Example 2

Control thickened aqueous salt solutions were prepared by adding 2 g of each of the dry polymers used in Example 1 to a hand-beaten 1.82 g/cm3 salt/peak solution prepared by mixing 214.2 g of H90 , 119.5 g CaCl 2 , 294.0 g CaBr 2 and 16.2 ml of a 2.30 g/cm CaBr 2 /ZnBr 2 salt solution. Data for the rheological and filtration properties of the control samples after overnight standing are given in Table 3 below. Corresponding data for identical samples which had been rolled for 16 hours at 65.6°C are given in Table 4. Compared with the data given in Tables 1 and 2, the data given in Tables 3 and 4 show that salt solutions produced according to the invention (Example 1), where<*>the polymer is first hydrated and dry salts are added, have superior viscosity and give lower filtrates in each individual case for rolling and in almost all cases after rolling at an elevated temperature .

Example 3

Using the method indicated in Example 1, polymer-containing, aqueous salt solutions with different densities were prepared. Five grams of carboxymethyl cellulose (Hi Vis Cellex) was prehydrated in water by mixing for 10 minutes. Torre CaCl 2 pellets were added to the prehydrated polymer and mixed to obtain a carboxymethyl cellulose concentrate with a density of 1.39 g/cm 3 , 140 ml of the concentrate was added to 210 ml of an aqueous salt solution with a density of 1.39 g/cm 3 to achieving a polymer concentration of 5/7 g/l. In the same way, aqueous salt solutions of o 1.70 g/cm 3 and 2.04 g/cm 3 were prepared. The composition of each aqueous salt solution is given in table 5 below.

Control samples of thickened, aqueous salt solutions were prepared by mixing 2 g of the dry carboxymethyl cellulose (Hi Vis Cellex) with ready-made salt solutions with a density of 1.39, 1.70 respectively. 2.04 g/cm . Rheological and filtration measurements of the sample which was prepared with the method according to example 1 and the control samples were carried out as indicated in the preceding examples. The results are shown in table 6. Table 7 shows the data obtained for all samples after rolling at 65.6°C for 16 hours. From these data it appears that the apparent viscosities of salt solutions produced according to the invention have twice as high and higher values than those obtained for the control samples. The superior filtering properties of the samples produced according to the invention will also be conspicuous in a comparison of data.

Example 4

Salt solutions with a density of o 1.85 g/cm 3 and with a content of 2.85 g/l carboxymethyl cellulose, either in the form of Hi Vis CELLEX (with high viscosity) or polyanionic cellulose powder (DRISPAC), were prepared by that the polymer was added and mixed into 176 ml of water to dissolve the polymer. Then, 114 g of CaC3 were added while mixing

(95%) and 180 g CaBr2 (91%). The heat of dissolution of these salts raised the temperature to 100°C. The API rheology and liquid loss were determined for these viscous solutions after cooling to room temperature. The data obtained are shown in table 8.

Example 5

Salt solutions with different densities and a content of 4.28 g/l hydroxyethyl cellulose (NATROSOL 250 HHR) were prepared by mixing the polymer with the amount of water indicated in table 9. As the polymer hydrated in the water, increase the viscosity. Then, the specified amount of CaCl2 (95% active) was added while mixing. The heat of solution from CaCl2 raised the temperature to about 82.2°C and the solution became more viscous. The indicated amount of a 1.70 g/cm CaBr 9 solution was slowly added, followed by the indicated amount of a 2.30 g/cm ZnBr 2 /CaBr 2 solution. After cooling to room temperature for one hour, the API rheology and liquid loss were determined. The solutions were then rolled at 65.6°C for 16 hours, cooled to room temperature and the API rheology and liquid loss determined. The obtained data are shown in table 9.

Example 6

Three carboxymethyl cellulose (Hi Vis Cellex) polymer concentrates were prepared by dispersing 2 g of polymer in 3.89 ml of water. A concentrate (control sample) was diluted with 155.5 ml H 2 O followed by the addition of 114.0 g CaCl 2 , 280.5 g CaBr 2 and 16.8 ml 2.30 g/l CaBr 2 /ZnBr 2 salt solution. The temperature of this sample reached 100°C.

Another concentrate (sample A) was added to a salt solution at room temperature. The salt solution was prepared with 155.5 ml H 2 O, 114.0 g CaCl 2 , 280.5 g CaBr 2 and 16.8 ml of a 2.30 g/ZnBr 2 salt solution. Mixing in the concentrate caused heat generation, so that the sample reached a temperature of 45.6°C. It was observed that insoluble clumps of carboxymethyl cellulose immediately formed. When the sample was cooled overnight, a large amount of suspended threads formed which eventually floated on the surface. It did not appear that any of the polymer concentrate was included in the salt solution.

A third concentrate (sample B) was prepared as sample A, and heat was again generated when the concentrate was mixed in, so that the temperature rose to 43.9°C. Sample B was then rolled at 100°C for 3 hours in an aging cell. The temperature of the sample was measured at 71.1°C after aging. Although some suspended polymer threads formed as floats on the surface, most of the polymer concentrate appears to have been dispersed.

The above three samples were then rolled for 64 hours at 65.6°C. After cooling, they were tested on a Brookfield Viscometer at 50 rpm. The control sample maintained a reading of 1590 cp and showed a smooth consistency, as for rolling.

Most of the insoluble clumps and all suspended threads dissolved in sample A. However, the Brookfield reading showed only 180 cp.

Sample B, which appears homogeneous, resulted in a Brookfield reading of 250 cp.

From the aforementioned results^ it appears that simple prehydration

of the polymer in water alone (sample A) is not the same mechanism as the salt activation method according to the present invention. In other words, it is necessary that dry salt with a positive heat of solution is added to the polymer concentrates after the polymer has been dispersed in water. Although the application of external heat (rolling at 100°C) leads to some response (250 cp Brookfield reading for sample B, compared to 180 cp Brookfield reading for sample A), the effect is still far from the 1590 cp Brookfield reading which was obtained for the control sample prepared according to the invention.

The invention can be carried out in other special embodiments without deviating from the scope of the invention. The mentioned examples are therefore to be regarded as illustrative, but not limiting, in all respects. The scope of the invention is indicated in the following claims, rather than in the above description, and all changes that fall within the meaning and scope of the claims are to be considered covered by the invention.

Claims (10)

1. Process for producing a liquid suspension of a hydrophilic polymer in an aqueous salt solution, characterized in that a hydrophilic polymer and water are mixed under conditions that give a generally uniform dispersion of the polymer in the water, that an inorganic salt with positive heat of dissolution, where the salt is added in such an amount that a temperature above 71.1°C, preferably above 82.2°C and most preferably above 93.3°C is achieved in the dispersion as a result of the salt's heat of dissolution. 2. Method as stated in claim 1, characterized in that the hydrophilic polymer is selected from a group consisting of carboxyalkyl cellulose ethers, hydroxyalkyl cellulose ethers, mixed cellulose ethers, carboxyalkyl starch ethers, hydroxyalkyl starch ethers, mixed starch ethers, polysaccharide gum, pregelatinized starch powder, stabilized partially dextrinized polysaccharide powder, toxic and nonionic, and mixtures thereof, and preferably is hydroxyethyl cellulose. 3. Method as stated in claim 1 or 2, characterized in that the inorganic salt is selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof. 4. Method as stated in claims 1-3, characterized in that the hydrophilic polymer is present in an amount of 0.29-28.5 g/l in the suspension. 5. Process for the production of an aqueous salt solution, which is suitable as a well treatment liquid, characterized in that a hydrophilic polymer and water are mixed under conditions that give a generally uniform dispersion of the polymer in the water, that an inorganic salt with a positive heat of dissolution is dissolved in the dispersion , where the salt is added in such an amount that a temperature above 71.1°C is achieved in the dispersion as a result of the heat of dissolution of the salt, - preferably above 82.2°C and preferably above 93.3°C, and that to said dispersion an aqueous salt solution with a given density is added in sufficient quantity to produce a well treatment fluid with a fixed density. 6. Method as stated in claim 5, characterized in that the hydrophilic polymer is selected from the ,:j . group consisting of carboxyalkyl cellulose ethers, hydroxyalkyl cellulose ethers, mixed cellulose ethers, carboxyalkyl starch ethers, hydroxyalkyl starch ethers, mixed starch ethers, polysaccharide gum, pregelatinized starch powder, stabilized partially dextrinized polysaccharide powder, toxic, nonionic, and mixtures thereof, preferably hydroxyethyl- cellulose. 7. Method as stated in claim 5 or 6, characterized in that the inorganic salt is selected from the group consisting of calcium chloride, calcium bromide, zinc bromide and mixtures thereof. 8. Method as stated in claims 5-7, characterized in that the aqueous salt solution contains at least one salt selected from the type consisting of water-soluble salts of alkali metals, alkaline earth metals, metals of group Ib, metals of group Ilb and mixtures thereof , preferably a water-insoluble salt selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof. 9. Method as stated in claims 5-8, characterized in that the polymer is present in an amount from 0.29 to 28.5 g/l. 10. Method as stated in claims 5-9, characterized in that the well treatment liquid has a density of 1.39 to 2.30 g/cm <3> .