Preparation of Viscosifiers for Zinc Salt Workover and Completion Brines

To avoid damage to the production stratum, clear water fluids as distinguished from drilling muds are used in workover and completion operations as a standard oil well drilling practice. The clear water fluids are concentrated water solutions of common metal salts, such as NaCl, CaCl.sub.2, CaBr.sub.2, and the like. The salt selected and the concentration employed are determined by the desired density of the fluid. To obtain very high densities, zinc salts are used, such as ZnBr.sub.2 and mixtures thereof with CaBr.sub.2.

Additives are used with clear water fluids to control and increase their viscosity. Increased viscosity enhances the cuttings carrying capacity of the fluid, reduces fluid loss, and tends to promote conditioning of the formation to minimize caving and water damage to water-sensitive formation. Polysaccharide gums, such as guar gum, locust bean gum, and hydroxyethyl cellulose have been used or proposed for use in clear water workover and completion fluids. In particular, hydroxyethyl cellulose (HEC) and guar gum have been employed, but HEC is the only polysaccharide gum which has been used with some success in high density zinc salt brines.

Hydroxyethyl cellulose (HEC) and other polysaccharide gums are supplied in dry powder form. Their addition to workover and completion brines requires special mixing and dispersing equipment. HEC, guar gum, and similar additives tend to hydrate too rapidly or non-uniformly, and form clumps referred to as "fish eyes". Such fish eyes can reduce the effectiveness of the viscosifier, and, in extreme cases, cause formation damage. It has been found particularly difficult to add a viscosifier to a high density zinc brine. One procedure now used in oil field practice is to prehydrate the HEC in an alcohol-water slurry, and then slowly add the slurry to the zinc brine accompanied by turbulent mixing. This practice requires the use of a separate tank for the prehydration, and involves additional time and expense.

It is desired to develop viscosifiers which give a greater viscosity increase for a given amount of the additive, especially with reference to zinc brines. Although HEC is used with zinc brines, the viscosity effect is less than desired, and as the density of the brine increases (with increasing zinc content), the viscosity effect of the HEC additive decreases.

The present invention is based in part on the discovery that the dispersibility and viscosity effect of polysaccharide gum viscosifiers can be substantially improved by a chemical treatment in which polysaccharide gum is contacted with an organic solvent solution of a basic nitrogen compound, such as quaternary ammonium compound, hexamethylene tetramine and dimethylol urea. For example, with particular reference to high density zinc brines, good dispersibility is obtained without prehydration and while avoiding the formation of fish eyes by ordinary mixing equipment. For a given concentration in the brine, such as 1% by weight, the viscosity effect, as determined by apparent viscosity, can be increased as much as 200% or more. Natural polysaccharide gums which have not heretofore been useable with zinc brines can be modified by the treatment of this invention so that they become advantageous viscosifiers for zinc brines. Such desirable treated viscosifiers include carrageenan in its usual salt forms and locust bean gum. In particular, it is believed that the treated carrageenan viscosifier, either alone or in admixture with treated locust bean gum, can provide improved results over the present standard HEC viscosifier. The carrageenan viscosifiers of this invention have been found to give greater viscosity effects at the same additive level than the standard HEC viscosifiers. Further, it appears likely that the modified carrageenan viscosifier may deposit less residue in the oil well formation.

Polysaccharide gum viscosifiers include natural vegetable gums, such as carrageenan, locust bean gum, and the like. Similar gums can be produced as fermentation products of bacteria or yeast, and include gum dextran, xanthan gum and the like. The synthetic cellulose-derived polysaccharide gums include particularly hydroxyethyl cellulose. Other synthetic gums of this type are known and are commercially available, such as carboxymethyl cellulose, but have not been used as viscosifiers for clear water fluids. The process of this invention is preferably applied to the class of the natural polysaccharide gums, and particularly to carrageenan and locust bean gum. The dispersibility and/or viscosity effect of all such gums is substantially improved by the treatment. Carrageenan may be employed in any of its usual salt forms, such as sodium, potassium, or calcium carrageenan. In relation to costs and the results obtained, the sodium carrageenan salt is particularly desirable. Such carrageenan salts can be obtained from Marine Colloids, Inc. Springfield, N.J.

As used in the process of the present invention, the gum to be treated should be in particulate form, such as in the form of a dry powder. The moisture content of the gum should be low, since it is desirable to avoid hydration during treatment. However, the treatment can be carried out in an organic solvent solution so as to maintain the gum in substantially unswollen condition. Some water can be present in the solvent carrier for the treating agent, but it should be less than the amount of water which will cause the gum to hydrate. Polar organic solvents are particularly desirable as the carrier for the treating agent, such as the lower aliphatic alcohols, including particularly the monohydric alcohols containing from 1 to 4 carbon atoms. A particularly desirable solvent is isopropanol or a mixture of isopropanol with a minor proportion of water. Where the treating agent is obtained commercially as a water solution, it can be added to the isopropanol or other alcohol, and the resulting solvent mixture can contain up to 10% or more water. Since the presence of water is not required for the desired reaction, however, the amount of water can be kept to a minimum. Other organic polar solvents can also be used, such as methyl ethyl ketone, diethyl ether, and the like, either alone or in admixture with an alcohol.

The treating agents are basic nitrogen compounds, that is, they contain one or more strongly basic nitrogen group. Such reagents include hexamethylene tetramine, dimethylol urea, and quaternary ammonium compounds, such as dimethyl dialiphatic quaternary ammonium salts or dimethyl benzyl quaternary ammonium salts. Such quaternary ammonium compounds are usually supplied in their chloride or sulfate salt forms, but other salts can be used. The quaternary ammonium salts and the treating agents in general should be soluble in the carrier solvent. For example, suitable quaternary ammonium compounds include the chloride or sulfate salts of dimethyl dialiphatic quaternaries in which the aliphatic groups contain from 8 to 22 carbons. When such quaternaries are prepared from vegetable oils, the aliphatic groups usually contain from 12 to 18 carbons. Instead of two long chain aliphatic groups, the quaternaries can contain an aromatic group, such as a benzyl group. For example, dimethyl benzyl quaternary ammonium chloride or sulfate can be used. One particularly suitable quaternary is prepared from hydrogenated tallow, the aliphatic groups being predominantly saturated C.sub.16 and C.sub.18 aliphatic groups. A specific example of such a compound is dimethyl di-(hydrogenated tallow) quaternary ammonium chloride. Such quaternaries are available commercially.

The carrier solvent may contain from 0.25 to 15% by weight of the treating reagent, such as, for example, from 0.5 to 10% of the reagent. While higher reagent concentrations might be used, there does not appear to be any advantage in using reagent concentrations above 10-15%. In general, a sufficient amount of the treating agent should be used to provide the desired viscosity improvement.

The treatment of the gum viscosifier can be carried out in a very simple manner. All that is required is the contacting of the treating solution with the viscosifier, which is in the form of a particulate solid, the amount of the treating solution applied at least being sufficient to thoroughly wet the viscosifier. To assure completion of the reaction, the viscosifier can be mixed in the presence of the treating solution for at least 15 minutes, such as for a period of 1 hour. The reaction can be carried out at ordinary room temperature, such as a temperature from 20.degree. to 30.degree. C., and the amount of the treating solution used does not appear to be critical. For example, from 0.5 to 5.0 parts by weight of the treating solution can be used per part of gum. In practice, good results can be obtained using from 0.8 to 2.0 parts of the treating solution per part of gum. Since the solvent is removed after the treatment, in general, it is desired to employ a minimum amount of the treating solution, such as approximately one part of the treating solution per part of the gum.

Although the treated viscosifier could be used as produced, that is, the viscosifier and treating solution could be added directly to the clear water brine, it is preferred for commercial purposes to recover the treated viscosifier in dry form. This can be done without difficulty since the particulate viscosifier has been maintained in essentially unswollen condition during the contacting with the treating solution. The treated viscosifier can be recovered essentially free of the solvent carrier by conventional drying procedures. For example, drum-drying or flash-drying can be used. If sufficient treating solution is present to form a slurry, it may be desirable to first concentrate the treated viscosifier by centrifugation or filtration, and thereafter to carry out the drying of the separated viscosifier. One convenient apparatus for the drying step is a rotary vacuum dryer. Alternatively, flash-drying can be carried out by atomizing the particulate carrier into a heated airstream. The use of a rotary vacuum dryer will be further illustrated in the following examples.

The treated viscosifier in dry particulate form can be readily incorporated in clear water brines by standard mixing procedures. The equipment at the drilling site includes a tank equipped with a jet hopper-mixer referred to as a "Lightening Mixer". The apparatus utilizes propeller-type agitation, and the liquid is recirculated through a venturi in the feed hopper. Although no liquid heating means are usually provided, the temperature of the brine can be raised by recirculating the brine through the hopper. For example, 110.degree. F., is a desirable temperature to use for addition of the treated viscosifier, and this can easily be obtained by recirculation without additional heating means. With the brine recirculating through the jet hopper, and after the appropriate temperature has been developed, such as 105.degree.-115.degree. F., the viscosifier powder is gradually added to the hopper, and thereby incorporated in the brine. The viscosifier hydrates gradually and uniformly, and little or no fish eyes are formed. It is therefore not necessary to use a prehydration procedure, as is presently employed with untreated HEC. The amount of viscosifier to be added will depend on the desired viscosity of the brine. In general, it will usually be desirable to add from 1 to 6 pounds of viscosifier (dry basis) per barrel of brine (42 gallons). For example, the most desirable level may be about 3 to 4 pounds per barrel of brine.

While the viscosifiers prepared in accordance with the present invention can be advantageously used with low or moderate density brines, such as brine formed from NaCl or CaCl.sub.2, or brines containing CaBr.sub.2 either alone or in admixture with CaCl.sub.2, the treated viscosifiers provide the greatest advantage when used with high density zinc brines. Such brines contain zinc bromide in admixture with a calcium salt, such as calcium bromide alone or in combination with calcium chloride. The relative amounts of the zinc bromide to the calcium salt determine the density of the brine, which can range from about 15 pounds per gallon to around 19 pounds per gallon. Since a density up to 15.1 pounds per gallon can be obtained with calcium bromide-calcium chloride mixtures, the zinc brines containing zinc bromide in admixture with calcium bromide are employed at densities greater than 15.1 pounds per gallon, such as 15.2 lbs/gal. and higher. With such brines, greatly improved results are obtained with the treated viscosifiers of this invention as compared with the prior art untreated hydroxyethyl cellulose. For example, treated HEC can be added more easily to the zinc brine, and a greater viscosity can be obtained.

This invention in its various aspects is further illustrated by the following examples.

Treating solutions were prepared from quaternary ammonium compounds, hexamethylene tetramine and dimethylol urea and isopropyl alcohol (100%). The quaternaries were dimethyl di-(hydrogenated tallow) quaternary ammonium chloride (Arquad 2HT) and dimethyl benzyl quaternary ammonium chloride (Arquad B). One percent and 10% by weight solutions of the quaternaries were prepared, and 10% by weight solutions of the other two reagents. Approximately 10 parts by weight of this treating solution was used per part of the gum being treated, the gum, which was in the form of a fine powder, being thoroughly mixed with the treating solution. The treating solution was left in contact with the gum for approximately 1 hour at room temperature (20.degree.-25.degree. C.). The treated gums were then dried in a 200.degree. F. oven. After drying, the dried material was ground to assure that it was in the form of a fine powder. (In commercial practice, this grinding step can probably be omitted.)

The zinc brine used in the comparative tests contained zinc bromide and calcium bromide in the proportions giving a density of 16.0 pounds per gallon. The treated and untreated gums were added to specimens of the zinc brine at a level of 1% by weight (35 lbs./barrel). The measurements of apparent viscosity in centipoise (cps) were determined using a Fann motor-drive viscosimeter as described in "Standard Procedure for Testing Drilling Fluids", Section 2, Viscosity and Gel Strength, "API Recommended Practice, American Petroleum Institute, API RP 13 B", April, 1969, pages 5-6. When this apparatus is operated at a speed of 600 rpm, the apparent viscosity in centipoises equals the 600 rpm reading divided by 2.

The comparative data is summarized below in Tables A and B:

For commercial production of treated viscosifiers in accordance with the present invention, the treatment and drying steps may be carried out continuously in a drum-drying apparatus. The following is illustrative:

(1) To a rotary vacuum dryer which has been purged of air with nitrogen, add 1,000 parts by weight of the polysaccharide gum viscosifier to be treated, such as Na Carrogeenan, and 500 parts of isopropyl alcohol (99-100%).

(2) Tumble 15 minutes to wet gum powder, then add a solution comprising 500 parts of isopropyl alcohol (90-100%) and 50 parts by weight of the basic nitrogen treating reagent.

(3) Without heat or vacuum tumble mixture for 1 hour.

(4) Apply heat with hot water to the dryer jacket and reduce the internal pressure to less than 100 mm. Collect through the vapor condenser the evolved alcohol and water, for example about 720 parts of isopropyl alcohol and 33 parts of water over 4.5 hour period.

(5) Continue to apply heat and maintain vacuum for 1 hour to remove final traces of volatiles.

(6) Final product will be fine powder of treated viscosifier.

The procedure decribed in Example II is followed, except that a mixture of Na Carrageenan with locust bean gum is used. For example, from 25 to 75 parts by weight of carrageenan (dry basis) can be admixed with 25 to 75 parts of locust bean gum, the gums being in the form of fine powders. For example, 50 parts by weight each of Na Carrageenan can be mixed with 50 parts of locust bean gum. The mixture is then treated and dried as described in Example II.