This invention relates to sulfate scale dissolution. More particularly the invention relates to ∞rηpositiαns and methods for dissolving an alkaline earth metal sulphate scale, especially strontium and barium sulphate scale. The invention is particularly useful for the removal of such scale from oilfield equipment including dσ rihole pipe, tubing and casing as well as subterranean formations. It is also applicable to the removal of these scale deposits from other equipment such as boilers and heat exchangers.

Reference is hereby made to our expending patent application of even date entitled "Method of Decontaminating Earth an Or Natural Gas Processing Equipment". Many waters contain alkaline earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulfate, bicarbonate, carbonate, phosphate, and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when the concentrations of the barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. Solubility products are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature and the introduction of additional ions which can form insoluble cαπpσunds with the ions already present in the solution.

As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expensive problem in many

industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in ether systems, causing delays and shutdowns for cleaning and removal.

Barium and strontium sulfate scale deposits present a unique and particularly intractable problem, under most conditions, these sulfates are considerably less soluble in all solvents than any of the other ccπmcnly encountered scale-fonning cxxπpounds, as shewn by the σemparative solubilities given in Table 1 belcw.

Table 1 Comparative Solubilities. 25°C in Water. Scale Solubility. mcτ./l.

Gypsum 2080.0

Strontium sulfate 140.0 Calcium Carbonate 14.0 Barium sulfate 2.3

It is generally acknowledged that barium sulfate scale is extremely difficult to remove chemically, especially within reasonably short periods of time: the solvents which have been found to work generally take a long time to reach an equilibrium concentration of dissolved barium sulfate, which itself is usually of a relatively low order. Consequently, barium sulfate must be removed mechanically or the equipment, e.g.pipes, etc., containing the deposit most be discarded.

The incidence of barium sulfate scale is worldwide, and it occurs principally in systems handling subsurface waters. Because of this, the barium sulfate scale problem is of particular concern to the petroleum industry as water is generally produced with petroleum and as time goes on, more petroleum is produced by the waterflooding method of secondary

recovery, implying even greater volumes of produced water. The scale may occur in any different places, including production tubing, well bore perforations, the area near the well bare, gathering lines, meters, valves and in other production equipment. Barium sulfate scale may also farm within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lcwer flew rates, higher pump pressures, and ultimately abandonment of the well.

Barium sulfate scale is particularly troublesome when sulphate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. This particular aspect of the barium scale problem is severe in seme U.S. oil fields as well as seme older North Sea oil fields. Scaling of this nature is also expected to occur during advanced production stages in other North Sea fields particularly after seawater breakthrough has taken place.

Another problem associated with the formation of barium and strontium sulfate scales is that radium, another member of the alkaline earth group of metals tends to be deposited at the same time so that the equipment becomes radioactive, and may eventually have to beccme unusable far safety reasons alone. At present, a considerable amcuπt of oilfield tubular goods are in this condition and cannot be readily restored to visable condition because of the difficulty of removing the radioactive scale.

Various proposals have been made in the past far removing barium sulfate scale chemically. Most of these processes have utilised chelating αr cc plexing agents, principally the polyεrarincpolycarrΛκylic acids such as ethylenediaminetetraacetic acid(K)TΑ) αr diethylenetrώminepenfcaaσetic acid(DTPA) .

US-A-2877848 discloses the vise of HDTA in αarribinatiαn with various surfactants far this purpose. US-A-3660287 discloses the vise of EDTA and DTPA in the presence of carbonate ion at

relatively neutral pH (6.5-9.5) and US-A-4,708,805 discloses a process for the removal of barium sulfate scale by sequestration using an aqueous solution of citric acid, a polycarbαxylic acid such as carbazic acid, and an alkylene-ρolyaminαpolycarbακylic acid such as ΕΣfJΑ αr DTPA. The preferred aqueous sequestering solutions have a pH in the range of about 9.5 to about 14, provided by a base such as potassium hydroxide αr potassium carbonate.

Another approach which has recently been made is to use a polyether in combination with the amincpolycarbαxyliσ acid. DS-A-4,190,462 discloses that barium sulfate scale can be removed frαn remote locations extending into a subterranean earth fαrmation b contacting the scale with an aqueous solution consisting essentially of water, a mαnovalent cation salt of a monocyclic macrcydic polyether containing at least two nitrogen-linked carbco^methyl groups and enough mαnαvalent basic compound to provide a solution pH of about 8. Similar disclosures are to be found in US-A-4,215,000 and US-A-4,288,333. These polyether materials have, hcwever, the disadvantage of being costly which is a severe drawback for oilfield use where cost is a major factor.

Although many of these known compositions will remove scale, the rate of dissolution is slew and the amount of scale dissolved is small.

We have new found a way of removing barium sulfate scale using various novel combinations of scale-rεmσving agents. These combinations are capable of removing scale at markedly higher speeds than prior scale-removing compositions and are also capable of removing relatively more scale far a given quantity of solvent. They are, moreover, relatively cheap and are therefore well suited to use in oilfield operations.

Broadly, the invention relates to a composition and method for dissolving an alkaline earth metal sulfate scale. In a broad aspect the composition comprises:

(a) an aqueous solution having a pH of from substantially 8 to substantially 14;

(b) a chelating agent comprising an aminocarboxylic acid, polyaminocarboxylic acid, polyamine, salts thereof, or mixtures thereof; and

(c) a scale-dissolving quantity of a synergist or catalyst.

Furthermore, in a broad aspect the method comprises contacting the scale with an aqueous solution having a pH of substantially 8 to substantially 14, which solution comprises: a chelating agent comprising an aminocarboxylic acid, a polyaminocarboxylic acid, a polyamine, a salt thereof, or mixtures thereof, and a scale-dissolving quantity of a synergist.

The aqueous solution is contacted with the scale for a time and at a temperature sufficient to dissolve it.

In one aspect of the invention the catalyst is selected from a member of the group consisting of anions of organic or inorganic acids and mixtures thereof, having an ionization constant greater than substantially a pK of l. The ionization constant advantageously has an ionization constant (Ka) of less

-2 -2 . . . than substantially 10 (ka<10 ) . This enables a composition to be provided which can dissolve substantially more of the scale within a substantially reduced time period than is possible with the chelating agent alone. In this aspect of the invention, the catalyst may include fluoride anions, oxalate anions, persulfate anions, dithionate anions, hypochlorite anions, formate anions, thio anions, amino anions, hydroxyacetate anions. The preferred

anions in the catalyst are oxalate, thiosulphate, nitriloacetate and monocarboxylic acid anions.

In another aspect of the invention the synergist αr catalyst comprises oxalate anions, thiosulphate anions, nitriloacelate anions or oncarboxylic acid anions. m this aspect of the invention the chelating agent is advantageously a polyaminocarboxylic acid αr a salt of such an acid.

In both aspects of the invention scale the constituents are preferably selected such that about 80% to about 90% of a solution level of barium, strontium αr calcium sulfate is dissolved from powdered scale in about 10 minutes at a temperature of about 100°C.

In another aspect the invention provides a method for selecting a catalyst suitable far enhancing the dissolution of an alkaline earth metal sulphate when vised in combination with a chelating agent selected frcm a member of the group consisting of an aminocarboxylic acid, a polyaminocarboxylic acid, a polyamine, salts thereof αr mixtures thereof, comprising: selecting said catalyst fr n anions having a negative free energy of reaction for the conversion of barium sulfate to barium compound which catalyst provides for substantially increased rates of dissolution of said scale than is possible with said agent alone.

The invention can also be applied to the reduction of the radioactivity of oilfield tubular goods contaminated with radium-containing scale of deposited alkaline metal sulfates. This method involves contacting the scale σr the aqueous solution with one of the aqueous compositions described above far a time sufficient to remove the scale frαn the tubular goods.

In both the above aspects of the invention the concentration of synergist αr catalyst is usually about 0.01 M to about 1.0 M, preferably about 0.5 M, with similar concentrations

being appropriate for the primary chelant (the polyaminopolycarboxylic acid) . Substantially improved scale dissolution rates are obtained when the aqueous solution containing the composition is at a temperature of about 25°C to about 100°C, but higher temperatures are obtainable dcwhhole because at greater formation depths higher existing pressures will raise the boiling point of the aqueous solution, and consequently greater scale removal rates may be attained. m accordance with the present invention, alkaline earth metal sulfate scales, especially barium sulfate scale, are removed by the use of a combination of chemical scale-removing agents. The method is particularly useful far the removal of such scale from oilfield equipment used to bring oil and/αr water from subterranean formations to the surface. The method may, however, also be used to remove scale from the formations themselves, especially in the regions surrounding production and injection wells,as mentioned above. The method may also be vised to remove scale from above- ground equipment both in the oilfield and elsewhere, for example, frαn boilers and heat exchangers and other equipment exposed to scale-farming conditions.

The scale itself is usually in the farm of an adherent deposit of the scale-faming mineral an metal surfaces which have been exposed to the water containing the scale-fanning components. These components comprise alkaline earth metals including calcium, strontium and barium, together with variable amounts of radium, depending upon the origin of the waters. As noted above, barium sulfate scale is particularly difficult to remove by existing chemical methods in view of its very lew solubility.

The present scale removal is effected with an aqueous solvent which comprises a polyaminopolycarboxylic acid such as BETA, αr DIPA as a chelant αr chelating agent whic is intended to form a stable complex with the cation of the alkaline earth

scale-forming material. Of these chelants, DTPA is the preferred species since it farms the most soluble complexes at greatest reaction rate. EETA may be used but is somewhat less favorable and, as noted belcw, may be less responsive to the addition of the catalyst αr synergist. The chelant may be added to the solvent in the acid farm ar, alternatively, as a salt of the acid, preferably the potassium salt. In any event the alkaline conditions used in the scale removal process will convert the free acid to the salt.

The concentration of the chelant in the solvent should normally be at least 0.1M in order to achieve acceptable degree of scale removal. Chelant cαncentratiαns in excess of 1.0 M are visually not necessary and concentrations from about 0.3M up to about 0.6M will normally give good results; although higher concentrations of chelant may be used, there is generally no advantage to doing so because the efficiency of the chelant utilisation will be lower at excess chelant concentrations. This economic penalty is particularly notable in oilfield operations large volumes of solvent may be used, especially in formation scale removal treatment.

As noted above, the catalyst is preferably an oxalate, thiosulfate, nitriloacetate or mαnocarbαxylic acid aniαn.

The αxalate may be added as the free acid or the salt, preferably the potassium salt. If the free acid is used, addition of the potassium base to provide the requisite solution pH will convert the acid to the salt farm under the conditions of use.

Thiosulfate salts, especially sodium thiosulfate or potassium thiosulfate provide a suitable source of the thiosulfate aniαn since the acid itself is unstable; these salts are, moreover, readily available ccπmercially at reasonable cost.

Nitriloacetate anions may be supplied by the acid itself, which is converted to the salts under the pH cαnditiαns employed,

or by the addition of a nitriloacetate salt. Nitriloaσetic acid, otherwise known as triglycine, N(C2LO0CH) ₃ , is an acid which is insoluble in water but which readily farms water soluble mono-, di-, and tribasic salts from various cations including the alkali metals, especially sodium and potassium. The addition of the free acid to the solvent will therefore result in dissolution under the pH conditions prevailing. Alternatively, the nitriloacetate anions may be added by addition of the nitriloacetate salts, especially the sodium αr potassium salts. In either case, the alkaline cond tions will result in the formation of the tribasic nitriloacetate aniαn, NfOLOOO), , in the solvent. The potassium salts are preferred in view of their greater solubility and far this reason, the solvent should preferably be brought to the desired pH value with a potassium base, preferably potassium hydroxide.

When the catalyst is an anicn αr anions of at least one mαnocarbαxylic acid, the acid may be substituted with various functional groups, especially electronegative groups such as hydrαxyl, amino, halo or mercapto αr may be vmsubstituted. The lcwer substituted fatty acids such as the C.-G, substituted fatty acids where the subεtituent is an electronegative group such as hydrαxy, mercapto, or amino are suitable since they have good solubility in oilfield waters, are readily available and are relatively inexpensive. Suitable acids of this type include mercaptoacetic acid, aminoacetic acid and hydαxyacetic acid. The vmsubstituted fatty acids such as acetic acid and formic acid have not been found to provide any major improvement in scale removal with DTPA as a chelant and are therefore not preferred. The ar natic carbαxylic acids may also be used when they have an adequately high solubility in water. The acid may have substitueπts other than the carboxyl group on the aromatic nucleus, for exairple, hydroxyl as in salicylic acid whic is a preferred acid of this type. Other arαnatic carbαxylic acids

with carbαxyl groups attached directly to the arcmatic nucleus may also be used. The preferred acids have been found to enhance the rate of barium sulfate scale dissolution using polyaminσpolycarixjxylic chelarrts, especially DTPA, to a significant and useful degree, so that dissolution of oilfield scales is usefully accelerated by the vise of these compositions.

It has been found that the action of -the synergist may be selective for the chelant. Far example, salicylate produces a significant increase in scale removal with the chelant DTPA but αily a slight improvement with IDTA. The use of DTPA is therefore favored not only because it generally shows an improved propensity in itself to remove the alkaline earth metal sulfate scales but also because it exhibits better response to a number of these synergists.

The carbαxylate synergist may be added as the free acid or the salt, preferably the potassium salt. If the free acid is used, the additiαi of the potassium base to provide the requisite solution pH will convert the acid to the salt farm under the conditions of vise.

The cxsweπtratian of the catalyst αr synergist in the aqueous solvent will be of a similar order to that of the chelant: thus, the amount of the synergist aniαn in the solvent should normally be at least 0.1M in order to achieve a perceptible increase in the efficiency of the scale removal, and concentrations frαn about 0.3M up to about 0.6M will give good results. Although higher concentrations of the synergist e.g. above 1.0 M may be used, there is generally no advantage to doing so because the efficiency of the process will be lcwer at excess catalyst concentrations. Again, this economic penalty is particularly notable in oilfield operations.

The scale removal is effected under alkaline conditions preferably at pH values of frαn about 8.0 to about 14.0, with optimum values being from about 11 to 13, preferably about 12.

The preferred solvents comprise about 0.1 to about 1.0 M of ethylenediaminetetraacetic acid (ΕΣfIA) αr diethylenetriaminepeπtaacetic acid (DTPA) , αr salts of these acids, as a chelant. In addition, the oxalate, thiosulfate, nitriloacetate or carbα^lic aniαn as the catalyst αr synergist is added to the aqueous solution in about 0.01 to about 1.0, preferably about up to 0.5 M. The pH of the solvent is then adjusted by the addition of a base, to the desired value, preferably to about pH 12. The base may be a hydroxide of lithium, sodium, potassium or cesium, but potassium is preferred. We have found that it is important to avoid the use of sodium cations when operating at high pH values, above pH 8, and instead, to use potassium or, alternatively, cesium as the cation of the scale-removing agent. Potassium is preferred far economy as well as availability. Thus, the normal course of making up the solvent will be to dissolve the chelant and the acid synergist (αr its potassium salt) in the water to the desired concentration, after which a potassium base, visually potassium hydroxide is added to bring the pH to the desired value of about 12. This aqueous composition can be used to remove scale from the equipment, αr alternatively, pumped into the subterranean formation vΛien it is the formation vΛiich is to be subjected to descaling.

The mode of operation of the synergist αr catalyst is not presently understood. While not desiring to be bound to a particular theory σαnoerning the actual mechanism of its activity in converting or dissolving the scale, it is believed that adsorption of the synergist αr catalyst an the barium sulfate surface may modify the surface crystal structure in such a way that the barium in the modified crystal is easily removed by the chelating agent.

The aqueous solution containing the composition can be directed down a wellbore to remove barium sulfate scale which has

fouled the tubular equipment e.g. piping, casing etc., and passage ways. Prior to being directed into the wellbore, the composition may be heated to a temperature between about 25°C to about 100°C., although the temperatures prevailing dcwrihole may make pre-beating unnecessary. Once within the tubular goods and the passageways requiring treatment, the composition is allowed to remain there for about ten minutes to about 7 hours. After remaining in contact with the equipment for the desired time, the composition containing the dissolved scale is produced to the surface and may be disposed of as required, possibly by re-injection into the subsurface formation. This procedure can be repeated as often as required to remove scale from the equipment. m one procedure for circulating the solvent through the tubular goods in the well the solvent is pumped down through the production tube and returned to the surface through the annular space between the production tubes and the casing (or vice versa) . Also, the cleaning solution may be pumped down through the production tubing and into the formation, thereby cleaning the well, including the well casing, and the formatiαn pore space by dissolving barium sulfate present as it flews over and along the surfaces that need cleaning. The spent composition containing the dissolved, ccmplexed barium together with any other alkaline earth metal cations which may have been present in the scale, especially radium, can be subsequently returned to the surface, for example, by displacement or eπtraiπmeπt with the fluids that are produced through the well after the cleaning operation. In an alternative manner, the cleaning solution may be applied batchwise fashion, for example, by flowing the solution into the well and optionally into the pare spaces of the adjacent earth formation and there keeping the solution in contact in non-flowing condition with the surfaces that are

covered with barium sulfate scale, for a period of time sufficient to dissolve the scale.

The present scale removal technique is very effective far lowering residual radioactivity of pipe contaminated with radium-containing barium sulfate scale. As noted above, radium is frequently precipitated with barium in scale with the result that scaled pipe is often radioactive to the point that it cannot safely be used. Using the present scale removal compositions, activity can be reduced to an acceptable level in comparatively short times without further treatment. Seme residual activity arises from lead and other radio-isotcpes which are not dissolved in the solvent: these isotopes are decay products of radium and have originally been incorporated in the scale with the barium and the radium sulfates. Although they are not removed chemically by the present scale removal technique, the dissolution of the barium scale together with the other alkaline earth metal sulfates enables these other components of the scale to be removed by simple abrasion, for example, by scrubbing with or without a detergent/water scrub solution. In this way, the residual activity level may be reduced to a very low value, belcw the appropriate regulatory standards. Thus, by vising the present chemical scale removal technique in combination with a simple mechanical removal of loose, nαn-adhereπt material, previously radioactive pipe may quickly and readily be restored to useful, safe condition.

Reference is new made to the accompanying drawings in which:

Figure 1 is a graph which depicts the dissolution efficiency of barium sulfate as a function of ligand cαnceπtratiαn;

Figure 2 is a graph which depicts the free energy of reaction for barium sulfate conversion at 25°C;

Figure 3 is a graphical representation of the rate of barium sulf te dissolution in the presence of a solution having a pH of 12, 0.5 M DTPA, and varying concentrations of an oxalate catalyst at a teπperature of 100°C.

Figure 4 is a graph whic shows the effect of chelant concentration on the rate of barium sulfate dissolution;

Figure 5 is a graphical representation of the rate of barium sulfate dissolution in 0.5 M EDTA with catalysts of 0.5 M oxalate, 0.5 M potassium fluoride and 0.5 M potassium acid tartrate;

Figure 6 is a graph depicting the rate of barium sulfate dissolution in 0.5 M DTPA with catalysts of 0.5 M potassium fluoride, 0.5 M oxalate, and 0.5 M potassium acid tartrate at 25°C.

Figure 7 is a graph which shows the rate of dissolution of barium sulphate in various solvents;

Figure 8 is a graph which depicts the rate of barium sulfate dissolution in 0.5 M DTPA with catalysts of 0.5 M potassium fluoride and 0.5 M oxalate at various temperatures;

Figure 9 s a graph which shews effect of temperature on the rate of barium sulfate dissolution.

Figure 10 is a graph which shows the respective rates of dissolution of various sulfate species on a chelant-cαntaining solvent.

Figure 11 is a graph which shows the respective rates of dissolution of varicus barium sulfate species in a chelaπt-σontaining solvent.

Figure 12 is a graph which shews the lew residual rates of radioactivity which may be achieved for contaminated oilfield pipe by vise of the present scale removal process.

Figure 13 is a graph vhich shows the rate of dissolution of barium sulfate in solutions of DTPA containing varicus substituted acetic acids as synergists.

Figure 14 is a graph vhich shews the rate of dissolution of barium sulfate in solutions of DTPA containing salicylic acid as a synergist; and

Figure 15 is a graph which shews the rate of dissolution of barium sulphate in solutions of DTPA containing thiosulphate αr nitriloσelate aniαn as a synergist.

The compositions and methods according to the invention are particularly useful for more efficiently removing barium αr strontium sulfate scale frαn wells, wellstrea processing equipment, pipelines and tubular goods used to produce oil from a subterranean formatiαn.

In order to demonstrate the barium sulfate scale-dissolving capacities of the composition, several aqueous solutions have been tested in laboratory tests the results of which, are described in the discussions which follow. The experiments described below were, except as noted belcw, carried cut in a cylindrical glass vessel having a height of 10 cm and an internal diameter of 7.5 cm. Barium sulfate, αr where applicable, other solid scale components were added to the test solution and agitated with the selected solvents and the rates of dissolution and final dissolved concentrations determined. The results are reported graphically in the figures.

As shewn in Figure 1, varying concentrations of DTPA and ECTft. are compared with 2.2.2-cryptand which is described in US-A-4,215,000. As described, varicus concentrations of DTPA with oxalate and EDTA were compared with the barium sulfate dissolution of 2.2.2-cryptand. The results were obtained at 25°C and demonstrate that DTPA/αxalate complexes more barium sulfate (49 g/1) than 2.2.2-cryptand (37 g/1) . As the dotted lines in the graph reveal, DTPA/αxalate is substantially more efficient than either 2.2.2-cryptand αr an ΕETA chelant at all concentrations. Efficiency of a chelant αr solvent is defined as

the fraction of chelant that is σc plexed with barium divided by the total conoentratiαi of chelating agent.

The amount of oxalate catalyst utilized in combination with DTPA is not critical, within the limits described above. This is illustrated in Figure 3 vhich shews that all concentrations of oxalate catalyst frαn 0.1 to 0.5M contribute to the dissolution of 80 to 90 percent of the saturation level of barium sulfate within ten (10) minutes of contact. Additionally, as demonstrated in Figure 3, the fast rate of dissolution is a significant feature of the present scale removal technique. In practical applications of the methods therefore, contact times of less than about 4 hours, eg. 1 αr 2 hours, may be sufficient, depending an the scale thicikness. Another significant feature of the technique is the high equilibrium (saturation) levels of dissolved barium, strontium and calcium sulfate scales which are obtained in the aqueous solution, making the process particularly efficient in terms of solvent utilisation.

Barium sulfate αr other scales dissolved in the solvent are influenced by the concentration of chelant used. The effect of varying the DTPA concentration (100°C) is depicted in Figure 4 at chelant concentrations from 0.IM to 0.6M. Increased DTPA concentration causes an increase in the rate of barium sulfate dissolution and the amount of barium sulfate held in the solvent. It should be noted in particular that the final equilibrium concentration of barium sulfate is 60 g/1 vhich is far in excess of the solubility in water alone.

Figure 5 illustrates the rate of barium sulfate dissolution when 0.5 M IETA is used with 0.5 M catalysts includingoxalate, potassium fluoride and potassium hydrogen tartrate. Figure 5 also illustrates the barium sulfate dissolution rate when 0.5 M ΕETA is vised alone. The temperature of the solvent in which the catalyst is used affects the rate of barium sulfate αr scale dissolution. This is further shewn in

Figure 6. Here differences in the barium sulfate or scale dissolution rate of a solvent containing 0.5 M DIPA is shown when the temperature is maintained at 100°C and 25°C with designated catalysts. These catalysts comprise 0.5 M potassium fluoride, 0.5 M oxalate and 0.5 M potassium acid tartrate.

Effectiveness of an oxalate catalyst is shewn with DTPA when compared to DTPA alone at 25°C and 100°C. This is illustrated in Figure 7. DTPA with oxalate at 25°C has nearly the same barium sulfate dissolution rate as DTPA only at a temperature of 100°C. Examining Figure 7, it is readily apparent that the oxalate catalyst caused the difference. This is apparent since the barium sulfate dissolution rate of DTPA with oxalate at 25°C is much greater than the dissolution rate of barium sulfate by DTPA alone at 25°C. As illustrated in Figure 7, about 90 percent of the scale is dissolved in the lab within the first ten minutes of contact, using Powdered BaS04. Much slower rates of barium sulfate di solution are shewn when EDTA and DTPA are utilized without a catalyst at 25°C. The results show that the DTPA/αxalate combination complexes more barium sulfate than DTPA alone and that DTPA is more effective than IDTA. at both temperatures. Furthermore when the escalate is present with the DTPA, the equilibrium concentration of di solved barium sulphate is reached far more quickly than with either the EDTA αr the DTPA, *Λιich have not achieved equilibrium after 7 hours at the termination of the experiment.

Although catalysts greatly enhance the rate of barium sulfate dissolution by DTPA αr EDTA, this enhancement varies with the particular catalyst employed. Figure 8 illustrates graphically the difference in barium sulfate dissolution vhen selected catalysts are utilized with DTPA. These catalysts comprise 0.5 M potassium fluoride at 100°C and 25°C, and 0.5 M oxalate at 25°C.

As mentioned above, the rate of barium sulfate scale dissolution varies with the solvent composition utilized. In order to determine candidates for use as a solvent, free energy calculations for the conversion of barium sulfate to barium carbonate were utilized. This is a well known conversion using concentrated sodium carbonate solution and solid alkaline earth metal sulfates.. Free energy of conversion of barium sulfate to barium carbonate is calculated as essentially zero, meaning that the conversion is energetically favorable. Hcwever, the conversion is expected to reach equilibrium with less than full conversion of barium sulfate to barium carbonate. This is the actual reaction situation with only about 75-80 percent of the barium sulfate being converted. Further calculations were made using common anions, both organic and inorganic. Seme of the anions considered are shewn in Figure 2 of the free energy graph. Those anions with a negative free energy of reaction are considered very reactive toward conversion of alkaline earth sulfates to the respective barium compounds. Many of the anions are by nature strong oxidizing agents, for example, persulfate (S_0 ) , dithionate (S ₂ 0 _g ) , and hypochlαrite (OCl) , normally would not be considered practical for use in a hydrocarbon environment. Fluoride (F~) and oxalate (C D. -) anions are found to be very active catalysts for ethylenediaminetetraaσetic acid (ΕETA) and diethylenetxiaminepeπtaacetic acid (DTPA) αr their salts for dissolution of barium sulfate, respectively. Experimentally, it was determined that the catalysts alone (without EETA OR DTPA) have no scale dissolution properties of their own.

Oxalic acid αr salts are used to obtain oxalate anions. Salicylic acid, the precursor to aspirin, is also an effective catalyst far dissolution of barium, strontium and calcium sulfates in combination with DTPA. Experiments indicate that it has a higher rate of dissolution and saturation level than oxalate aniαns. Both oxalic and salicylic acids are commercially

available and are relatively inexpensive. For 0.1 to 0.5 molar concentrations, salicylic acid would add US$5 to US$24 per barrel for chemical cost, whereas oxalic acid would contribute from about US$2 to US$7 to the cost per barrel.

Salicylic acid is used to obtain salicylic anions. Effective catalyst anions are also obtainable from aminoacetic acid (glycine) , glycolic acid (hydrαxyacetic acid) , and thioglycolic acid (mercaptoacetic acid) .

One example of a preferred aqueous solvent which can be used comprises 0.5M DTPA and 0.3M Oxalic acid adjusted to a pH of 12 with potassium hydroxide.

Another example of a preferred aqueous solvent for use herein comprises 0.5M EDTA and 0.5M KF adjusted to a pH of 12 with potassium hydroxide.

Figure 9 shows that the rate of dissolution of the barium sulfate scale is related to temperature, with faster rates of dissolution being attained at the higher temperature (0 C.)

Figure 10 shows the results of a batch test carried with scale material removed from field tubing similar to that used in a continuous flow loop test in which the test solution was circulated. The samples of scale material were added to the solvent (0.5M DIPA,0.5M oxalate pH=12, 0°C.) in a concentration equivalent to 60 g./l. of scale. The concentrations of the different species dissolved in the solvent at different times were determined. The results of the batch tests are shown in figure 10 and indicate that in addition to the dissolution of the barium, the strontium sulfate also reaches equilibrium concentration in a very short time. The results from the flow loop tests are similar but with much lower final concentrations.

Figure 11 shows that the scale removal process is effective both with barium sulfate in the powder form and also with actual pipe scale/tar mixtures and with barite ore (BaSO.) .

Figure 12 shews that the present scale removal technique is very effective far lowering residual radioactivity of pipe contaminated with radium-containing barium sulfate scale. As noted above, radium is frequently precipitated with barium in scale with the result that scaled pipe is often radioactive to the point that it cannot safely be used. A continuous flow loop test was used to remove the scale frαn pipe which was similar to that used with Figure 5 and the radioactivity was determined at successive times during the test. As shewn in the Figure, the activity was reduced to an acceptable level after three hours without further treatment. It appears, however, that seme residual activity arises from lead and other radio-isotopes which are not dissolved in the solvent (see Fig.5) ; these isotopes are decay products of radium and have originally been incorporated in the scale with the barium and the radium sulfates. Although they are not removed chemically by the present scale removal technique, the dissolution of the barium scale together with the other alkaline earth metal sulfates enabled these other components of the scale to be removed by simple abrasion. For Figure 7, the descaled pipe was scrubbed with a soft bristle brush vising a detergent/water scrub solution. The result was to reduce the residual activity level to a very lew value, below the appropriate regulatory standards. Thus, by using the present chemical scale removal technique in combination with a simple mechanical removal of loose, non-adherent material, previously radioactive pipe may quickly and readily be restored to useful, safe condition.

As shewn in Figure 13, DTPA alone and DTPA with varicus substituted acetic acids were compared at 100°C. The results demonstrate that the DTPA/carbαxylate combination complexes more barium sulfate than DTPA alone.

Figure 14 compares the relative rates of barium sulfate dissolution using DTPA alone and DTPA in cαribinatiσn with

salicylic acid. As shown in the Figure, the addition of salicylic acid is effective to almost double the degree of barium sulfate dissolution.

Referring to figure 15, the solvents used comprised 0.5M DTPA in combination with either 0.5M thiosulfate added as potassium thiosulfate, K-S-O., αr 0.1M nitriloacetate, added as nitriloacetic acid, with the required pH of 12 attained by the addition of potassium hydroxide. The results are reported graphically in the Figure.

As shewn in Figure 15, DTPA alone and DTPA with thiosulfate and nitriloacetate synergists were compared at 100 C. The results demonstrate that the DTPA/synergist combination complexes more barium sulfate than DTPA alone, and that thiosulfate is slightly more effective as a synergist than nitriloacetate. Although the nitriloacetate catalyst was initially less effective than the chelant alone, the final equilibrium concentration of the dissolved scale component became greater at contact times longer than about 100 minutes, so that there is a potential for scale removal with this synergist at extended contact times using this low proportion of the synergist.

Distilled water was used in the tests shewn in Figure 15, and with the majority of the other tests (except the continuous flow loop tests) . Minor decreases in efficiency may be observed with tap water and more significant decreases with seawater, e.g. about 20 percent decrease with seawater. This is reasonably to be expected since seawater has interfering ions, e.g. calcium and magnesium. These interfering ions complex with the chelating agent, either DTPA αr EDTA, and reduce the overall dissolving pcwer. Additionally, it has been determined that halide ions have a negative effect en dissolving pcwer as a function of the size of the halide ion. Dissolution rate is increased as the halide ion size is reduced and the charge density is increased.

i.e. in the order of iodide, bromide, chloride and fluoride. Fluoride ion enhances the effect of EETA-based solvents, but not DTPA: fluoride inhibits most DTPA catalyst solvents.

As noted above, the effect of cations is also very important to the success of the scale solvent, especially when added with the sizable portion of caustic required to adjust the pH to 12. Dissolution is enhanced as the size of the cation is increased, i.e. lithium, sodium, potassium and cesium. Lithium and sodium hydroxides in the presence of EDTA, or DTPA, and catalysts are not soluble at a pH of 12, the cptimum value. Cesium is too difficult to obtain, both in quantity and price. Ωierefore, potassium hydroxide, in the form of caustic potash, is the pH adjusting reagent of choice.

Another effective chelant is cyclchexanediaminetetra- aretic acid (CETA) , which may be used with any of the synergists disclosed hereinabσve. CDTA is particularly useful with the oxalate anion at a pH of substantially 13.