CA1103009A - Well-treating process and aqueous solution for use in said process - Google Patents

Abstract

ABSTRACT

WELL-TREATING PROCESS AND AQUEOUS SOLUTION FOR
USE IN SAID PROCESS

A process for stimulating a hydrocarbon containing underground formation comprising siliceous material, by treating the formation with an aqueous solution in which a mud acid is generated from a fluoride salt and a relatively slowly-reactive acid-yielding material formed by chloro-carboxylate material from which chlorine is relatively easily hydrolyzable.

Description

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WELL-TREATIN~ PROCESS AND AQUEOUS SOLUTION
FOR USE IN SAID PROCESS
The invention relates to a process, such as a well-treating process, for dissolving siliceous rnaterials which are remotely located but are fluid-contactable, such as permeability-impairing particles of sand or clay, in or around the borehole of a well.
More particularly, the invention relates to dissolving siliceous materials with a self-acidifying solution of a salt of a hydrofluoric ac;d and a relatively slowly-reacting acid-yielding material, which solution can be flowed into contact with the siliceous materials before the solution becornes strongly acidic.
The solution can dissolve siliceous material while maintaining a relatively high pH, and can convert dispersible clay or clay-sized particles of siliceous materials to non-dispersible solids.
A well-treating process of the above type is known from the USA patent specification No. 3,828,854 (C.C. Templeton; E~A. Richardson; granted 13th August, 1974).

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Object of the invention is to improve the efficiency of the well-treating process referred to above.
The well-treating process according to the invention includes the step of injecting into a siliceous material-containing region an aqueous solution of a salt of hydrofluoric acid and an acid-yielding material comprising a dissolved chlorocarboxylate material being substantially a weak acid or a weak acid salt.
The aqueous solution according to the invention comprises a salt of hydrofluoric acid and at least one dissolved chlorocarboxylate material from which chlorine is easily hydrolyzable.
As used herein, "dissolved chlorocarboxylate material" refers to an aqueous solution of at least one mono or polychlorinated carboxylic acid or carboxylic acid salt or mixtures of one or more of such acids or salts.
In numerous situations, such as treatments of oil-producing wells, it will be desirable to adjust the composition of the injected solution so that substantially all of the dissolved chlorocarboxylate material that may contact the reservoir oil contains only the salts of any chlorocarboxylic acids that are used. This will ensure that substantially none of C

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the chlorinated organic material can become dissolved in the oil, since such material can cause significant damage to the catalysts used in processes to which the produced oil is eventually subjected in the S refinery.
Unobviously, it has now been discovered that a self-generating mud acid in which the acid-yielding reactant is a dissolved chlorocarboxylate material, can undergo a second stage reaction in the presence of an excess clay or other siliceous material. In the second stage reaction, more clay than the stoichiometric limit for the amount of fluoride ions present (if they react as they do in a regular mud acid system) is converted from clay or clay-sized dispersible solid particles to non-dispersible solid materials. The non-dispersible materials occupy only relatively small portions of the pore spaces in the reservoir formations and thus enhance the permeability and/or degree of consolidation o~ the treated ~orm-ations.
In addition, in a self-generating mud acid system in which the acid-yielding reactant is a chloro-carboxylate rnaterial, the rate of hydrolysis (to form HCl and a hydroxycarboxylic acid) is significantly increased by increases in pH. In certain situations this is advantageous in that, where the reservoir temperature is too low to provide a desirable rate of ~ -, ~i~13~6~9 hydrolysis in an acid solution, a substantially neutral or alkaline solution can be relatively slowly injected into the reservoir at a rate such that the relatively high rate of hydrolysis (due to the high pH) is kept high by the alkaline-reacting components of the reservoir rock. Such an injection can be intermittently interrupted to ensure that sufficient hydrolysis is occurring; but, when the hydrolysis within any portion of the solution begins to lower the pH to an extent reducing the rate of hydrolysis~ flow should be resumed so that portion is displaced into contact with fresh portions of the : reservoir rock, which neutralizes the acids and raises the pH.
In general, the present invention provides mud acidization treatments that are particularly useful for treating conduits or formations in or around bore-holes of wells in which permeability ls impaired by dispersed or adhering particles of clay or siliceous material. The high pH and relatively slow reaction of the present systems tends to (A) minimize their damage to acid-sensitive cement sheaths, resin-consolidated reservoirs, or the like, and (B) to increase the depth of penetration of their permeability-improving effects.
Aqueous liquids suitable for forming the presentself-generating mud acids can comprise substantially --, 1~3~9 any relatively soft, brackish, fresh, or pure water.
Since multivalent cations tend to precipitate fluoride ions, and increasing concentrations of total dis-solved salts tend to decrease the solubility of siliceous materials in a hydrofluoric acid containing solution, a soft water, at least as pure as fresh water, is preferred. Where desirable, chelating or sequester-ing agents, such as the polyaminopolyaCetic acid salts, e.g., ethylene-diamine-tetraacetic acid, citric -acid, or the like, can be used to increase the tolerance to an aqueous liquid containing multivalent cations.
In at least some situations, substantially any water-soluble fluoride salt can be used in the present process. But, the ammonium salts of~hydrofluoric acid, i.e., ammonium fluoride and ammonium bifluoride, are particularly suitable. As known to those skilled in the art, in using ammonium bifluoride, enough ammonia can be added to provide substantially equivalent proportions of ammonium and fluoride ions. Al~er-natively, an excess or deficiency Or ammonia (or ammonium hydroxide) can be used to increase or decrease the initial pH, and thus to increase or decrease the extent to which the chlorocarboxylic acid is converted to an ammonium salt.
In general, chlorocarboxylic acids suitable for use in the present invention comprise substantially any such acids which are relatively water-soluble, or form water-solukle salts, and are hydrolyzable (with respect to at least one, and preferably all, chlorine atoms) at moderate temperatures (such as about 40-150C
or 310 420 Kelvin), at moderately slow rates which are adapted to provide half-lives (for the unhydrolyzed chlorocarboxylic acids or salts) in the order of fror.
about 0.5 to 10 hours. Such acids can suitably contain one or a plurality of chlorine atoms attached to aliphatic, alicyclic or aromatic, or the like, car-boxylic acids. Examples of suitable acids includemonochloroacetic acid, dichloroacetic acid, 2-chloro-propionic acid, ortho, meta, or para-chlorobenzoic acid~ 2,3-dichlorobenzoic acid, para-chlorohexahydro-benzoic acid, and the like. Particularly suitable chlorocarboxylic acids (which have hydrolysis react on half-lives of suitable duration for many reservoir temperatures) comprise monochloroacetic acid, 2-chloro-propionic acid, and dichloroacetic acid.
The concentrations of the f`luoride salt and dissolved chlorocarboxylate material used in the acidizing systerns of the present invention can ~e varied relatively widely. It is ~enerally desirable that the hydrolyzing of the chlorocarboxylic ac-d icns generate sufficient HCl to make the system at least 0.1 normal in HCl and that the system contains sufficient fluoride to become at least 0.1 norm-1 in hydrofluoric acid.

3~9 If equal molar proportions of ammonia and acid are present in a situation where a chlorocarboxylic acid is mixed with water, ammonium fluoride and ; ammonia, the pH will be the substantially neutral pH
set by the ionization constant of the particular chlorocarboxylic acid (such as about 6.5 for mono-chloroacetic acid). While the solution is flowing ~ into greater depths and is within the well conduits, ; the rate of the hydrolysis reaction is increasing as the temperature increases. The hydrolysis can occur with either or both the chlorocarboxylic acid and its anions and can yield a mixture of hydrochloric and hydroxycarboxylic acids. The acid-generation reduces the pH of the solution and converts the hydrofluoric acid salt to hydrogen fluoride or hydrofluoric acid.
In the injected solution, the composition and concentration of the chlorocarboxylic acid can be correlated with the reservoir temperature and fluid injection rate to cause a selected degree o~ hydroly~is within the well conduits. When the injected solution begins to flow through a clay-containing reservoir, the acids are quickly spent and the pH is increased, since the rate Or the clay-dissolving reaction is much greater than the rate of hydrolysis. Subsequently, the rate of the clay-dissolving reaction becomes limited by the rate at which the acids are generated by the hydrolysis reaction within the self-generating ~1~`3~

mud acid solution.
Where, in the above situation, the ammonia (or ammonium hydroxide) is used in a stoichiometric excess relative to the amount of chlorocarboxylic acid, the pH of the solution is increased in proportion to the amount of that excess. The hydrolysis rate (for a given temperature) is higher due to the higher pH, while the solution is in the well conduits. Although this causes the acids to be hydrolytically released at a higher rate, more acid is needed (to lower the higher pH) before any hydrofluoric acid is released.
Within a clay-containing reservoir formation the rate of clay-dissolving becomes the rate established by the rate of hydrolysis at a substantially neutral pH, since the hydrolytically released acids are then spent substantially as quickly as they are released.
Where, in the above situation, a stoichiometric excess of the chlorocarboxylic acid is used, the solution may have at least two distinct phases of clay-dissolving reaction behaviour. If the reactants consist essentially of chlorocarboxylic acid and ammonium fluoride, the system is initially a regular mud acid comprising a mixture of the weak chloro-carboxylic acid and hydrofluoric acid. Where the system also contains at least a significant proportion-of ammonia, preferably enough to neutralize a 30~

predominant proportion of the chlorocarboxylic acid, the system is initially a buffer-regulated mud acid (of the type described in USA patent specification No. 3,889,753; inventor E.A. Richardson; granted 17th June, 1975). And, where, for example, the system consists essentially of significant proportions of hydrochloric acid, chlorocarboxylic acid, and the ammonium salt of the chlorocarboxylic acid mixed with ammonium fluoride, the system is initially a regular H~l-HF mud acid, then becomes a buffer-regulated mud acid, and then becomes a self-generating mud acid in which the acid-yielding reactant is a chlorocarboxylic acid and/or its salt.
In the present process, whenever the mud acid ~ 15 solution contains less than enough ammonia (or other ; alkali) to neutralize substantially all of the acid that is initiallypresent, it is desirable that the amount of ammonium fluoride be significantly greater than the stoichiometric equivalent for acid-base interaction of the unneutralized (and unhydrolyzed) portion of acid. This ensures that, after the free acid has been spent, the reservoir is subjected to a significantly extensive treatment with a self-generating mud acid in which the acid-yielding reactant is a chlorocarboxylic acid salt.
- The process of the present invention is also useful for well treatment processes (of the type described in United States patent s.pecification No. 3,868,998;
inventors: J.H. Lybarger; ~.F. Scheuerman, G.T. Karnes; granted 4th March, 1975) for placing a slurry of particles that form ; sand or gravel packs in wells and/or fractures ~ithin a subterranean earth formation, for displacing a ~iscous low-fluid loss, slow-acting acidic solution along and into the walls of such a fracture, for temporarily diverting fast-acting acid away from a zone that tends to act as a thief zone in a permeability profile improving treatment for an interval of inhomogeneously permeable earth formati.ons, or the like.
Particularly suitable water-thickening materials for use in such treatment operations comprl.se water-soluble cellulose ethers which are relatively acid-sensiti.ve. Examples of such ethers include hydroxyalkyl, carboxyalkyl or lower alkyl cellulose ethers typified by h.ydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and the like, which are themselves substantially completely aqueous liquid soluble materials that form substantially completely soluble hydrolysis products when they are hydrolyzed in an acidic aqueous liquid. Hydroxyethyl cellulose such as Natrisol (.trade mark) available from Hercules Powder Company, or ~164 (.trade mark) available from Dow Chemical Co., or WG-8 (trade mark~ available from Halliburton Services Company are particularly suitable.

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3()~9 The invention will now be described in more detail with reference to the drawings.
Figure 1 of the drawings illustrates a graph indicating the amount of clay (in grams/litre) along 5 the I-axis, that is dissolved or converted with time (in hours) along the X-axis.
Figure 2 illustrates a graph of variation of rate of hydrolysis (half time life in minutes along the I-axis) with change in temperature (in C as well 10 as Kelvin along the X-axis).
Figure 1 shows the results of suspending an excess of sodium bentonite clay in a mud acid F solution prepared in accordance with the present invention. The solution tested was a water solution 1.5 15 normal in monochloroacetic acid, 1.5 normal in ammonium hydroxide, and 1 normal in ammonium fluoride. The proportion of the initially suspended clay was 25.77 grams per litre and the suspension was maintained for the indicated times at 93C (366 K). The experimental 20 quantities which were observed included the amount of total suspended solids and the percent of clay remaining in those solids (as determined by X-ray diffraction analysis). From those quantities and the initial concentrations, calculations were made of the 25 amounts (in grams per litre) of (1) unreacted clay (2) reacted clay, and (3) solid products.

3~9 The data points used in the preparation of Figure 1 and the formulas used in the calculations are shown in TABLE 1. Curves A and ~ in Figure 1 indicate the amount of clay reacted (in grams/litre) ; 5 and the amount of net dissolved solids (in grams/litre), respectively. The stoichiometric limit is indicated by line C.

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1 1`~ 3 V ~9 It will be apparent from Figure 1 and TABLES T and II that, for times up to about 15 hours, the clay-dissolving behaviour of a mud acidizing solution prepared by the present process is similar to that of a self-generating mud acid system prepared as described in patent No. 3,828,854. ~y about 15 hours the net dissolved solids amount to about 2/3 of the clay reaeted.
But, at longer times, sueh as 48 hours, a treatment with the present systems provides a reaction whieh does not oeeur when the hydrolysis produets are mainly a weak aeid, such as f.ormic acid, and an aleohol sueh as methanol. The present system, whieh hydrolytieally generates both a strong aeid, hydro-ehlorie aeid, and a weak acid, a hydroxycarboxylic aeidg eontinues to reaet long after a peak is seen `
in the net dissolved solids. This is a seeondary reaetion proeess in whieh additional elay solids are eonverted almost eompletely into solid produets (which eontain si:Liea, aluminium and fluorine). Inobservations with a scanning electron microscope, these final solid produets can be seen to be well-defined small erystals that are deposited on the grains of the sand paeks. As indieated on Figure 1, by 48 hours appreeiably more clay has disso].ved than the 15 grams per litre that would be considered to be the stoichiometrie limit (see line C in Figure 1) for an 11~30l:39 analogous self-generating mud acid solution, such as one 2 molar in methyl formate, and 1 molar in ammonium fluoride (which behaves as shown in Figure 2 of patent No. 3,828,854~. Although the second stage reaction with the present system represents no gain in net dissolved solids, the replacement of the dispersible clay with a non-dispersible product solids further improves the formation permeability and/or stability. As indicated in TABLE II, similar second stage reaction results have been obtained at 107C
(380 K) and such results have also been obtained at 80C (353 K).
In the present process a clay-dissolving reaction occurs after any free acids that were initially present have become spent and the rate at which clay or siliceous materials are being dissolved is limited by the rate at which a hydrogen fluoride-containing solution is being formed by the hydrolysis of the salt of a chlorocarboxylic acid. The reaction appears to be typified by the following equation. However, it should be noted that the present invention is not dependent upon this particular mechanism. Where a clay-dissolving solution comprises an aqueous solution of ammonium chloroacetate and ammonium fluoride, the hydrolysis and ionization reactions appear to be:

3~ 9 Cl-CH2-COO + H20 = H0-C~2-COOH + Cl chloroacetate glycolic acid and HO-CH2-COOH = HO-CH2-COO + H , Pka = 3.83 glycolate -The likelihood of the occurrence of such a mechanism has been indicated by x-ray analyses of precipitates from cooled, spent acid solutions which have dis-solved significant amounts of calcium carbonate.
The x-ray patterns of such solutions correspond to those of a hydrated calcium glycolate. It thus appears that, when a self-generating mud acid solution comprises a mixture Or salts of hydrofluoric and chlorocarboxylic acids, the hydrolyzing of the chlorocarboxylate ions is forming a hydroxycarboxylic-hydrofluoric mud acid at the relatively slow rate that the chlorine atoms are being hydrolyzed from the chlorocarboxylate ions.
Figure 2 shows a graph of variations in hydro-lysis rate (half time life in minutes alon~ the ~-axis) with temperature (in C as well as Kelvin along the X-axis) for the chlorocarboxylic acids 2 chloro propionic acid (curve D), monochloroacetic acid (curve E), and dichloroacetic acid (curve F).
Curve G refers to the behaviour of methyl formate, and has been included for comparison. The half-lives for the ch]orocarboxylic acid anions were determined i~3~g at various temperatures in buffered aqueous solutions, of pH 5 to 5.5, in test tubes, with no sand or clay present, by means of titrating the chloride anions :released by hydrolysis. As indicated in ~igure 2, ;5 particularly suitable half-life values of from about 40 minutes to 400 minutes, the "Desirable Range"
designated by DR on the drawing, can be found for temperatures of from about 77C-121C (or 350K - 394K) which are commonly encountered reservoir temperatures.
In well acidization treatments in which it is generally preferable to use a shut-in time equallingabout 3 half-lives (regarding the acid-generating reactant) of a self-generating mud acid system, such solution provides a range of shut-in times of from about 2 to 20 hours.

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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Well-treating process including the step of injecting into a siliceous material-containing region an aqueous solution of a salt of hydrofluoric acid and an acid-yielding material comprising a dissolved chlorocarboxylate material being substantially a weak acid or a weak acid salt.

2. The process of claim 1 in which substantially all of the cations in the injected solution are ammonium ions.

3. The process of claim 1 in which the injected solution contains hydrochloric acid in a proportion less than the stoichiometric equivalent for acid-base neutralization of the fluoride salt.

4. The process of claim 1 in which the injected solution contains hydrochloric acid and chlorocarboxylic acid in a combined proportion less than the stoichiometric equivalent for acid-base neutralization of the fluoride salt.

5. The process of claim 1 in which the injected solution contains a mixture of chlorocarboxylic acid and chlorocarboxylic acid salt in a combined proportion less than the stoichiometric equivalent for acid-base neutralization of the fluoride salt.

6. The process of claim 1 in which the injected solution further contains a cellulose ether-water thickener.

7. The process of claim 1 in which the dissolved chlorocarboxylate material is selected from the group consisting of monochloroacetic acid, 2-chloropropionic acid, dichloroacetic acid, and the salts of those acids.

8. The process of claim 1 in which substantially all of the dissolved chlorocarboxylate material is the ammonium salt of monochloroacetic acid.

9. An aqueous solution for use in the well-treating process of claim 1, comprising an aqueous solution of a salt of hydrofluoric acid and a dissolved chlorocarboxylate material being substantially a weak acid or a weak acid salt.