BRINE STABILIZATION F. W..JESSES ANI)
J. L. BATTLE Hiinihlc Oil and Rcfining Company, Ilouston, Tcxns
RISES protluccd w i r h oil ~isually contnin high hicnrbonate miceiitr:~tioii~.‘Thc solulJility of these bicarbonates depends upon the quantity of carbon dioxide held in solution which, in turn, is largely dependent on the temperature, pressure, and composition of the water. As the water is produced, much of the carbon dioxide is liberated as a result of the reduction in pressure; the bicarbonates decompose to form carbonates, particularly the calcium salt, in excess of its solubility:
B
Ca(HCO&
+
CaC03 (solid) 4 COz t
+ HzO
I n a relatively quiet reservoir this excess calcium carbonate does not precipitate readily and results in a solution supersaturated with calcium carbonate. Upon agitation or contact with solid material, a portion of Two Oil Field Brine Subsurface Disposal Systems in Texas; Both Include Oil Skimmer, 12,000-Barrel Settling Pit, and Gravity-Type Sand Filter the calcium carbonate precipitates. This precipitate is a cementing material and has been found to be a good plugging material. The present study investigates the possibility of using sodium hexametaphosphate in the stabilization of calcium carbonate in oil-field brines to be returned to subsurface The use of sodium hexametaphosphate is formations. Although applicable in many instances where studied for stabilizing the condition called subsurface disposal is being practiced, the brine studied was “calcium carbonate supersaturation” of produced from the Edwards limestone of Southwest Texas. an oil field brine prior to injection into subA typical analysis of this water follows:
surface formations. Results indicate that brines treated with 2 to 5 parts per million are stable with respect to precipitation of calcium carbonate for 7 t o 14 days. This period should permit sufficient time for surface storage and subsequent disposal into the formation without the plugging of the interstices of the disposal sand body. A modified carbonate stability test is presented; it is based on pretreatment of the solid calcium carbonate with a concentrated solution of sodium hexametaphosphate to saturate the surface area of the solid particles prior to its use for stabilizing brines.
Sodium Calcium Magnesium Chloride Sulfate Bicarbonate Total solids Hydrogen sulfide Calcium carbonate supersstn. PH
Milliequivalents per Liter
Parts per Million
534.0 140.0 40.0 680.0 23.0 11.0
12,282 2,800 486 24,113 1.104 671
... ... ...
41,456 320 168 7.4
Treatment of this water for disposal presents several difficult problems. Upon aeration, the high calcium carbonate supersaturation may be readily reduced to 5 p. p. m.; but in so doing, the hydrogen sulfide is oxidized to colloidal sulfur, which is highly dispersed and therefore difficult to remove by filtration. I n addition, such treatment leaves a water saturated with oxygen; and when it comes in contact with the highsulfide water in the formation, colloidal sulfur again prec,ipitates and tends to plug the formation. These factors seem
with
Sodium Hexametaphosphate carbonate supersaturation to the extent expected when the usual carbonate stability test was applied (Figure 1). Since the inhibiting effect of the hexametaphosphate was assumed to be a surface adsorption phenomenon, it was suspected that the small quantity of hexametaphosphate present in the sample under test was insufficient to saturate the large surface area of the extremely small particles of solid calcium carbonate added to the sample; considerable precipitation resulted. Rice and Hatch (6)indicated that, when the same solid calcium carbonate is used repeatedly with successive increments of the same threshold-treated water in the stability test, the amounts of calcium carbonate precipitated become progressively less until constant results may be obtained. Therefore it was decided to saturate the surface area of the solid calcium carbonate to be used in the normal stability test by pretreatment with sodium hexametaphosphate, and to determine the extent to which the presence of hexametaphosphate influenced the standard carbonate stability test. The reprecipitated calcium carbonate used in stabilizing the hexametaphosphate-treated brines was permitted to soak, with frequent stirring, for 4 hours in a concentrated sodium hexametaphosphate solution and allowed to settle. The supernatant liquid was decanted and the solid allowed to airdry. The treated calcium Carbonate, free of any excess
to justify the use of a closed system, provided some means is found to stabilize the high calcium carbonate content of the water to prevent its precipitation on return of the water to the formation. A “closed system” in this case may be defined as a plant designed to exclude air (oxygen) from contact with the fluid in storage. In practice this is frequently obtained b y maintaining an oxygen-free gas blanket (such as methane) over t h e surface of the fluid at 2 t o 5 pounds per square inch gage pressure. An open reservoir covered with a 6-12 inch layer of crude oil is a practical means of approaching such conditions in the field. In contrast, an (‘open system” is one in which air is in contact with the fluid surface at all times.
Previous Studies Hall (8) is credited with the initial work on the application of molecularly dehydrated phosphates to boiler water conditioning, flotation processes, etc. In those processes in which the water is completely softened, he reported that it required “four formulaweights of sodium hexametaphosphate to one formula-weight of calc5um a t a pH of 8.5 and seven formula-weights of sodium hexametaphosphate for this weight of calcium a t a pH of 10”. In such cases the calcium is apsumed to be present in the form of a stable complex ion and cannot be precipitated by soap or carbonate. The exact composition of this complex is not known, but from the above statement it apparently varies with the pH __ of the solution. More recently Rosenstein (8) discovered that minute concentrations of sodhm hexametaphosphate (1 to 2 p. p. m.) would prevent the precipitation of calcium carbonate from irrigation water when ammonia is added as a source of nitrogen for cro s. The work of Rice and Hatch (5, 6) and Rice and Partridge has extended the use of metaphosphate to industrial water treatment, scale removal, and corrosion control. However, since the minute uantities required to prevent precipitation are far too small tosbe explained on the basis of complex-ion formation as postulated by Hall (S), considerable difficulty was experienced in finding a satisfactory explanation until the recent work of Reitemeier and Buehrer (n, 4). The results of their quantitative and microscopic study of the inhibition process as applied to ammoniacal solutions of calcium bicarbonate strongly suggest that the true mechanism of the process is primarily a surface adsorption phenomenon; they state that the action “is largely an indirect one, involving either a stable electrostatic attraction between the calcium and metaphosphate ions or a marked decrease in the activity of calcium ion due t o the presence of the metaphosphate”. Whatever the effect may be, their experimental evidence indicates that there is a definite interference or retardation of the growth of calcite crystals when a water is treated by quantities of sodium hexametaphosphate in the order of 1 to 2 p. p. m . 4 . e., “threshold treatment”. In the presence of these low concentrations of hexametaphosphate the crystals formed grow more slowly, are less numerous, are larger, and are quite distorted from the usual rhomboid form of calcite crystals; in concentrations in excess of these values, precipitation may be prevented entirely. (Since the preparation of this article for publication, a pamphlet covering research work along similar lines by the Dearborn Chemical Company has come to the attention of $he authors.)
7‘)
Modified Carbonate Stability Test An attempt was made to compare the extent to which calcium carbonate in brines may be stabilized when treated with various concentrations of sodium hexametaphosphate and the effect of atmospheric contact on solutions so treated. It was anticipated that the tests would be allowed to go to complete equilibrium conditions, and the basis of comparison was to be the carbonate stability test (1) applied to aliquot portions of all samples a t regular intervals. Earlier tests involving the treatment of brines with various concentrations of sodium hexametaphosphate failed to provide stabilization of the condition referred to as calcium
Treating System (2500 Barrels per Day) in East Texas for Disposal of Oil Field Brine by Injection into Subsurface Formations; Injection Well in Background
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sodium hexametaphosphate solution, was used in testing the treated brines. Table I and Figure 2 present the results obtained by the use of untreated and treated calcium carbonate in making the usual carbonate stability tests.
HASTINGS-
CONCENTRATION S O D I U M HEXAMETAPHOSPHATE ADDED W.M.
Figure 1.
Effect of Sodium Hexametaphosphate
on Calcium Carbonate Stability
PROCEDURE.The effect of sodium hexametaphosphate in the stabilization of calcium carbonate in both open and closed systems was studied by the following procedure: The brine \vas transferred by siphon from the original airtight container to eight 2-liter glass bottles, four of which were covered by a Binch layer of SAE-20 lubricating oil. The four remaining samples were left open to the atmosphere. Two of the oil-covered samples and two of the open samples were treated with a standard solution of sodium hexametaphosphate to the extent of 2 to 5 p. p. m., respectively. TWO hundred milliliter samples were remosred by pipet from each sample daily for the modified carbonate stability tests. Figures 3 and 4 indicate the results of each series.
Vol. 35, No. 6
For the open untreated samples, Figures 3 and 4 both show an initial rapid increase in the amount of calcium carbonate precipitated to maximum values of 254 and 258 p. p. m., respectively, followed by a decrease until slight solubility is obtained after approximately 15 days. Visual observation showed noticeable turbidity developing in both the 2 and 5 p. p. m. treated samples on the third day. The precipitate appeared to be crystalline and settled completely. Microscopic examination indicated the crystals to be the typically rhombic form characteristic of calcite. The initial rapid increase in precipitation is undoubtedly due to the fact that the rate a t which carbon dioxide is initially liberated is substantially greater than the rate of precipitation of the calcium carbonate formed. After the formation of a nucleus, however, complete precipitation occurs readily. The closed untreated samples also indicate an initial increase in the amount of calcium carbonate Precipitated, although the maximum value is far less than in the case of the open untreated samples. This maximum is followed by an extended period during which the course of decomposition is somewhat erratic. Tests on successive days indicate a slight decrease in the water treated with 2 p. p. m., while a slight increase was a t first noted in the sample containing 5 p. p. m.
Optimum Conditions for Stabilization The curves of Figures 3 and 4 have the same general form. I n both instances the open and closed treated samples indicate relatively stable solutions in the earlier stages. The plots of the closed treated fiamples in each instance are characterized by a gradual increase in the amount of calcium carbonate precipitated with time. It is thought that the rate of increase in precipitation may be closely correlated with the rate of decomposition (or hydration) of the hexametaphosphate. The pyrophosphate is only slightly less efficient than the hexametaphosphate in preventing precipitation of calcium carbonate while the orthophosphate offers little inhibition to its precipitation; therefore the fact that the rate of precipitation is relatively slow tends to indicate that the pyrophosphate may be one of the intermediate products of decomposition. There is considerable controversy in the literature as to what the intermediate decomposition products are. However, the recent work of Williams and Barnette (9) on the rate of decomposition of hexametaphosphate provides evidence that, under the conditions outlined here, one of the intermediate products is the pyrophosphate.
TABLE I. RESULTS OF CARRONATE STABILITY TESTS P. P. M. CaCOa Pptd. from Closed Untreated Water MetaphosphateTime Washed treated CaCOa Day: CaCOa
P. P. M. CaCOa Pptd. from
Closed Water Treated with
2 P. P. M.Hexametaphosphate
Washed CaC03
Metaphosphatetreated CaCO8
T I M E I N DAYS
Figure 2. Results of Testing Brine with Treated and Untreated Solid Calcium Carbonate
It is possible that this fluctuation may be due to the fact that the %inch layer of oil offers sufficient resistance to the diffusion of the carbon dioxide from solution to cause it to be liberated in stages; there are thus alternate periods during which the rate of liberation of carbon dioxide exceeds the rate of precipitation of calcium carbonate and vice versa. Following the extended period during which the carbonate stability fluctuates a t a relatively high concentration, it again approaches a maximum value and then decreases rapidly and in much the same manner as in the open untreated samples. This decrease in the amount of calcium carbonate precipitated is accompanied by considerable turbidity which, as in the open untreated sample, is crystalline in nature. It is noteworthy that treatment with 2 p. p. m. sodium hexametaphosphate produced greater stability than treatment with 5 p. p. m. Whereas the crystalline precipitate from the untreated sample of brine was distinguished by its rather large, typically
June, 1943
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4. Calcium Carbonate Stability of Brines, Untreated and Treated with 5 P. P. M. Sodium Hexametaphosphate (Using Treated Solid Calcium Carbonate)
Figure 3. Calcium Carbonate Stability of Brines, Untreated and Treated with 2 P. P. M. Sodium Hexametaphosphate (Using Treated Solid Calcium Carbonate)
rhomboid structure, the turbidity which developed in both the open and closed treated samples was extremely fine; it remained in a dispersed form throughout the solution and was relatively slow in settling. Microscopic examination revealed highly distorted crystals similar to those pictured by Buehrer and Reitemeier ( 8 ) . The change in alkalinity with time of these samples is shown in Figure 5. As expected, the alkalinity of the closed treated samples decreased least rapidly, folIowed by the closed untreated, the open treated, and the open untreated. I n addition, Figure 5 shows the change in pH of two open Samples; one was treated with 2 p. p. m. sodium hexametaphosphate, and the other untreated. The variation in the p H of the two solutions is best explained on the basis of the relative rates of liberation of carbon dioxide and precipitation of calcium carbonate. I n the untreated sLmples, these two rates are approximately equal as evidenced by the early precipitation of calcium carbonate; as a result the p H remains relatively constant. I n the treated sample, the rate of liberation of carbon dioxide is far in exces~of the rate of precipitation of calcium carbonate. Since the metaphosphate present prevents calcium carbonate precipitation, there is naturally an increase in the pH of the solution. This continues until the metaphosphate loses its inhibiting effect as a result of decomposition. When this point is reached, calcium carbonate precipitates and the p H of the treated solution rapidly approaches that of theuntreated solution. No pH measurements were made on closed treated and untreated solutions since, with very little change in total alkalinity over 2 weeks, the pH change would be small. 1NTERPRETATION OF CARBONATE STABILITY RESULTS
The results obtained by the modified carbonate stability test require careful interpretation. One of the objectives in
0
653
the treatment of brines is reduction of the calcium carbonate supersaturation. General methods of treating water cause precipitation of calcium carbonate to a great extent and result in solutions quite stable with respect to calcium carbonate supersaturation. However, when a water is treated with sodium hexametaphosphate to obtain calcium carbonate stabilization, the standard stability test is no longer applicable, as noted by Rice and Hatch (6). It becomes necessary to use the modified stability test, so that its interpretation will be consistent with the findings in regard to calcium carbonate stabilization. Whereas the standard carbonate stability test purports to show the amount of calcium carbonate potentially precipitable, it is not satisfactory when sodium hexametaphosphate has been employed as the stabilizing agent. First, there is a
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TIME I N D A Y S
Figure 5.
Alkalinity and pH US. Time of Brines, Untreated and Treated with 2 P. P. M. Sodium Hexametaphosphate
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INDUSTRIAL AND ENGINEERING CHEMISTRY
relation between the calcium and metaphosphate ions, described by Buehrer and Reitermeier ( 2 ) as an electrostatic attraction, which prevents the precipitation of the normal calcium carbonate; secondly, in a water treated with 2 p. p. m. sodium hexametaphosphate, insufficient metaphosphate is present t o saturate the surface area of the solid calcium carbonate used in the normal stability test and still maintain the new equilibrium between the calcium and metaphosphate ions in solution. Thus, when testing a mater treated with sodium hexametaphosphate, it becomes necessary to provide conditions which will not disturb the calciummetaphosphate equilibrium. This is accomplished by the use of solid calcium carbonate treated so that the surface of the solid has been rendered inactive or coated with a film of the sodium hexametaphosphate. When a solid is used, the surfaces of which have been saturated by previous treatment, no sodium hexametaphosphate is lost from the water to the surface of the solid. The modified test-that is, the use of treated solid calcium carbonate-no longer gives the total potentially precipitable calcium carbonate, but rather the amount of calcium carbonate which will precipitate, even though the largest portion of the total is maintained in solution by the inhibiting effect of the sodium hexametaphosphate. The results of this study indicate conclusively that treatment of brines for disposal with 2 to 5 p. p. m. sodium hexametaphosphate, in a closed system, will result in a solution which is relatively stable with respect t o calcium carbonate precipitation for a t least a week. This period is sufficient to allow for any kind of treatment or protracted shutdown of the disposal system.
Conclusions 1. The ordinary calcium carbonate stability test is not applicable to metaphosphate-treated waters.
Vol. 35, No. 6
2 . A modification of the standard calcium carbonate stability test has been developed which is suitable for testing waters previously treated with sodium metaphosphate. 3. It appears that waters treated with sodium hexametaphosphate produce a calcium carbonate stability that remains constant for about 15 days. 4. The most satisfactory treatment of brines for calcium carbonate stabilization is 2 p. p. m. of the metaphosphate. 5 . Under the conditions covered by the tests, a closed system is more suitable than an open system for treating brines that contain high concentrations of hydrogen sulfide and of bicarbonates, qrovided the bicarbonates are stabilized by 2 to 5 p. p. m. sodium hexametaphosphate.
Acknowledgment The authors wish to express their appreciation to B. I. Thorngren of the Maintenance Engineering Corporation, of Houston for helpful suggestions in the interpretation of the experimental data. They wish also to thank the Humble Oil and Refining Company for permission to publish the paper.
Literature Cited Am. Pub. Health ilssoc., Standard Methods of Water Analysis, 8th ed., p. 122 (1936). Buehrer, T. F., and Reitemeier, R. F., J . Phys. Chem., 44, 552-74 (1940) Hall, R. E., L'. S. Patent 1,966,615 (April 24, 1934). Reitemeier, R . F., and Buehrer, T. F., J. Phgs. Chem., 44, 53561 (1940). ENG.C H E X ~31,51-7 , (1939). Rice, Owen, and Hatch, G. B., IND. Rice, Owen, and Hatch, G. B., J . Am. Water Works Assoc., 31, 1171-85( 1939). Rice, Owen, and Partridge, E. P., IND.EXG.CHEW,31, 58-63 (1939). Rosenstein, L., U. S. Patent 2,038,316 (1936); reissues 20,360 (1937) and 20,754 (1938). Williams, M., and Barnette, L. A . M., personal communication.
Courtesy, Chicago Bridge (e Iron Company
These All-Welded Oil Storage Hortonspheres at a Refinery Have a Capacity of 25,000 Barrels Each: in the Background Holds 120,000 Barrels
the Noded Spheroid
