DISSOLUTION ANALYSIS IMPROVES OIL FIELD SCALE TREATMENT
A. B. Carel, J. W. Wimberley
Conoco Inc.
Ponca City, Okla.
An analytical method has been developed for the complete dissolution and analysis of oil field production scales.
The results of the analysis allow for selecting more appropriate scale treatments. The method is adaptable to a wide range of scale sample types. Complete dissolution of the scale metals is accomplished by successive chelation in acid and basic solution.
The chelation product is analyzed by inductively coupled plasma analysis (ICP).
Analysis for total anions is also made. This allows an anion assignment for the solubilized metals. Some findings were:
- Oil field scales can be solubilized and complexed by successive chelation in hydrochloric (HCI) acid solution at pH 1.0 and ammonium hydroxide (NH4OH) solution at pH 10.8 with cyclohexanedia-minetetraacetic acid (CyDTA).
- The complexed metals, including barium and strontium, can be analyzed by ICP.
- Other instrumental techniques, namely ion chromatography (IC), Coulometrics Inc.'s carbon analyzer, and X-ray diffraction can be used to determine sulfate on the complexed liquid samples, organic and inorganic carbon on the solid scale sample, and carbonate and other salt quantitation and identification.
- These techniques provided near 100% solution of five scales, and 98 and 96% solution of two remaining scale samples.
- Summation of total cations and anions for six samples gave values within the range of 96 to 106% for five samples and 111% for the sixth sample analyzed.
METHODOLOGY
A complete and reliable method for analysis of a broad range of oil field scales has been needed, but to date has not been available in the literature. Reference 1 summarizes available analytical methods for analyses of scales.
The paper includes techniques for analysis of most metal ions in scales, mainly AA (atomic absorption) analysis of solutions.
We recommend ICP over AA for analysis of metals because the detection limit for many elements is lower by a factor of 10-100 for ICP. ICP also has reduced matrix effects and the ability to analyze multiple ions simultaneously.
Reference 1 did not mention or cite the analysis of scales for barium or the direct analysis of metal chelates.
Chelation of the metals in the scale sample is the key to complete dissolution of the scale.
The use of the acid form of CYDTA allows for chelation of metals without sample contamination and, therefore, makes anion analysis of the metal chelated solution possible.
The analysis of the metal chelates by ICP is a very important part of the method. Even barium, strontium, and calcium are solubilized and analyzed as the chelates.
Complete analysis of the scale sample includes grinding, drying, and analysis for percent organic and inorganic carbon, metal carbonates, silica, metals, metal sulfates, and sodium chloride.
ANALYSIS OUTLINE
The scale sample is finely ground and dried to remove moisture. Analyses for organic and inorganic carbon are first made with the Coulometrics Inc.'s carbon analyzer.
Samples containing significant inorganic carbon are analyzed by X-ray diffraction to obtain the carbonate ratio and identify the metal species of the inorganic carbonates. The sample is heated to 500 C. and the weight loss determined.
This weight loss includes organic carbon, water of hydration, and weight loss due to carbonate decomposition from CaCO3, FeCO3, and other species such as Ca(Mg0.67Fe0.33)(CO3)2.
Heating with HCI acid is used to solubilize acid-soluble scale components. Excess CYDTA acid is added and dissolved with heat to complex acid solubilized metals.
NH4OH is then added to a pH of 10.8. The sample is covered and stirred for 24 hr to chelate and solubilize other metals such as barium and strontium. This will dissolve most scale samples completely except for silica.
Most of the excess ammonia is removed with heat and the pH of the solution adjusted to 8.0.
The sample is filtered, made to volume, and analyzed for metals by ICP and for sulfate by IC.
The filter residue is analyzed for silica by evolution with HF. Calculations of the total scale composition are made from these combined data.
The two boxes list the material and reagents needed, and the procedure for doing the analysis.
The procedure is intended to give an estimate of the total cation and anion composition of the scale. When sulfate and carbonate values have been obtained and anion assignment made, the values obtained give a material balance that generally deviates less than 1 0% from the theoretical sample balance.
Tables 1-5 contain the data collected in the analysis of six scale samples.
ORGANIC CONTENT
The ignition loss on heating to 500 C. for 1 hr was obtained for each sample. These data, Table 1, consist of loss due to organic carbon, water of hydration, and that due to carbonate decomposition.
These inorganic carbonates were identified by X-ray diffraction as being present in the scale samples: FeCO3, CaCO3, and Ca(Mg0.67 Fe0.33)(CO3)2. Instrumental analysis is preferred as the least time-consuming method for determining both organic and inorganic carbon,
The data from the solvent extraction include organic carbon values for the six scale samples obtained by xylene and chloroform washing (extraction). These data can be compared to the organic carbon obtained instrumentally. Because of carbonaceous materials insoluble in the organic solvents, the values resulting from the organic carbon analyses, in most cases, are higher than from solvent extraction.
The percentage of inorganic carbonates is obtained with a Coulometrics Inc.'s instrument. Analysis of Samples 2, 5, and 6 by X-ray diffraction showed the carbonates to consist of these species at these percentages:
- Sample 2: CaCO3, 61 wt %, Ca(Mg0.67 Fe0.33)(CO3)2 38 wt %
- Sample 5: FeCO3 dominant species
- Sample 6: CaCO3 dominant species
Upon heating to 500 C., all of these carbonates decompose to the oxides (this was confirmed by X-ray analysis). The percent weight loss in the decomposition was calculated. The water of hydration value is calculated by:
Average ignition loss - (Corrected organic carbon + Weight loss due to carbonate decomposition).
INORGANIC CARBONATE
To obtain the percent total organic carbon, the carbon analyzer converts carbon in a sample to CO2 for coulometric detection. Carbon is often differentiated as total organic carbon (TOC) and total inorganic carbon (TIC). Total carbon (TC) is the sum of TOC and TIC.
Total carbon can be determined by combusting the sample at a high temperature in an oxygen atmosphere, oxidizing all carbon to CO2.
Total inorganic carbon can be directly determined by acidifying the sample which converts all carbonate to CO2. The CO2 is then purged into the coulometer for measurement. The difference of these two results yields TOC.
The particular unit used in our laboratory for inorganic carbon analysis was a Coulometrics Inc. CO2 analyzer model. The unit uses a 10% perchloric/20% phosphoric acid mixture combined with heat to decompose carbonate to CO2,
The CO2 is titrated by the coulometer and read out in percent C or carbonate. Several other models and differing types of analyzers are available for organic and/or inorganic carbon analysis.
The carbonate content of the scale samples is needed for a complete understanding, analysis, and anion assignment (Table 5) for cations.
The percent carbonate in Scale Samples 1-6 is shown at the bottom of Tables 2-5. Significant percentages of carbonate are found in Scale Samples 2, 5, and 6. The values are 6.3, 9.4, and 3.3% carbonate, respectively.
Percent carbonate species is obtained by X-ray diffraction. One of the important benefits of scale analysis by X-ray diffraction is the assignment of the percent and type of carbonate species. These assignments and percentages were obtained by this analysis on Scale Samples 2, 5, and 6.
- Sample 2: The carbonate present consists of 61 wt % CaCO3 and 38 wt % Ca(Mg0.67 Fe0.33)(CO3)2. The presence of NaCl, ZnS (as the dominant Zn compound), and CaSO4 were also confirmed.
- Sample 5: The dominant species is reported as FeCO3 with a trace of CaCO3. Calcium was found to be present mainly as CaSO4 and Zn as ZnS.
- Sample 6: Calcium carbonate was the dominant carbonate. Calcium sulfate was reported with traces of ZnS and NaCl.
All three of the metal carbonates identified decompose to the oxides on heating to 500 C. These data were used to account for and assign the types and percent of carbonate in the scale, the metal associated with it, and the amount of metal remaining for assignment with other anions such as sulfate or as the oxide. The chemical factors for these calculations are readily available from handbook data.2
If the main purpose of scale analysis is to predict an appropriate treatment necessary to dissolve the scale, then X-ray and IC analyses may not be needed. From examination of carbonate and metals data (Table 2), one should be able to predict what type of treatment is required.
For example, Samples 1, 3, and 4 are low in carbonate and high in strontium and/or barium. This indicates that HCI treatment to decompose carbonates is not required. Furthermore, since the samples contain high levels of strontium or barium, the chances are extremely high that the anion involved is sulfate. Therefore, treatment with ammoniacal CYDTA is indicated.
Samples 2 and 5 contain both carbonates plus appreciable strontium and/or barium. Our experience has been that the carbonates of strontium/barium and, to a lesser extent, calcium are not soluble in CYDTA. But acid treatment to decompose the carbonate followed by treatment with ammoniacal CYDTA will dissolve them.
Thus, for these two samples, HCI treatment followed by ammoniacal CYDTA would take care of both carbonates and sulfates of strontium and barium.
In the case of Sample 6, it is moderately high in carbonate and calcium but low in strontium and barium. Consequently, treatment with HCI should be adequate since both calcium carbonate and sulfate are soluble in HCI.
Much general information on the composition of scale samples relating to the presence and dominance of the scale components can also be obtained by X-ray diffraction. These include crystalline forms of metal sulfates, oxides, sulfides, and chlorides, mixed metal salts, hydroxides, and other components.
COMPLEXATION
Solution of oil field scale by complexation, ICP analysis for metals, and IC analysis for sulfate, is examined in four different solutions: basic solution with ethylenediaminetetraacetic (EDTA), basic solution with CYDTA, acid solution with CYDTA, and acid and basic solutions with CYDTA.
BASE WITH EDTA
We determined that EDTA acid at a 10:1 weight ratio in a solution of 5% ammonium hydroxide would completely dissolve strontium sulfate, but only partially dissolve barium sulfate. However, CYDTA in the same ratio completely dissolved both compounds.
BASE WITH CYDTA
Experiments were conducted to solubilize barium and strontium sulfate by complexation with the acid form of CYDTA. Complete solution was obtained when 0.1 g of the barium and strontium sulfates were complexed with 2 g of the CYDTA in ammonia solution at pH 10.8. The recovery of the barium was 92.7% and the strontium 104% by ICP analysis.
The effect of CYDTA on recovery of the iron, barium, strontium, and calcium by ICP was determined by adding CYDTA to each of these metals in acid solution at a 4.8:1, CyDTA-to-metal molar ratio, adjusting the pH to 8.0 and analyzing the solutions by ICP using normal metal standards in acid solution at pH 2.0 without CYDTA.
The data showed that iron and barium needed correction factors of 1.09 and 1.10, respectively. No correction factors were needed for calcium and strontium. These correction factors were used throughout this work.
About 0.2 g of Scale Samples 1-6 were accurately weighed into tared 100 ml beakers. The samples were heated to 500 C. for 1 hr and the ignition loss (see Table 1 for ignition loss breakdown) determined.
The residues were transferred to 250 ml Erlenmeyer flasks with water, 2 g of CYDTA added, and the pH adjusted to 10.8 with concentrated ammonium hydroxide. The samples were diluted to about 200 ml, covered, and stirred overnight with a large Teflon stirring bar.
The samples were transferred to 400 ml beakers (the residue could not be quantitatively transferred) and most of the ammonia removed with heat. After cooling, the pH of the solutions was adjusted to 8.0, transferred to 250 ml volumetric flask, and ICP analysis obtained. Analysis for SiO2 was obtained on a Separate 2 g ignited sample.
The residue was treated with concentrated sulfuric acid, ignited to 800 C., and the weight obtained. Hydrofluoric acid was added, the sample reignited to 800 C., cooled, and the percent SiO2 determined.
Table 2 summarizes the data for ignition SiO2, metals, sulfate (determined by IC), and inorganic carbonate. The SiO2 values were determined on separate samples since the residue after complexation with CYDTA could not all be removed from the Erlynmeyer flasks.
All of Samples 1, 3, and 4 appeared to be solubilized except for a small silica-type residue and close to 100% of the sample is accounted for with the basic chelation procedure. The values for strontium, barium, and calcium are close (with the exception of the barium value for Sample No. 2) to those obtained by acid solution and CYDTA chelation in both acid and base (Table 4) which will be discussed later.
A major element not found to be soluble by basic chelation is iron. Separate experiments showed ferrous and ferric iron to be complexed by CYDTA in acid solution (and this is reported in the literature3) but not in basic solution.
The recovery for Samples 2, 5, and 6 (high carbonate containing samples) varies from 66 to 90%. A large portion of this insoluble material is iron, and some associated salts are also insoluble by the ammonia CYDTA complex.
ACID WITH CYDTA
Scale Samples 1-6 were ignited as before except 0.4 g was used and the percent loss on ignition obtained. The acid digestion before complexation was done in this beaker.
A total of 20 ml of 1:1 concentrated hydrochloric acid and a medium-size Teflon stirring bar was added and covered with a ribbed watch glass. The samples were heated and stirred until near dryness, 10 ml of the same acid mix was added, and the solution reheated (some nitric acid may be beneficial, but is was not investigated).
The solution and residue were transferred to a 600 ml beaker with water and 4 g of CYDTA acid form added. The volume was increased to 400 ml and the samples heated to near boiling (and maintained for 30 min) to dissolve CYDTA and complex soluble metals. The samples were filtered while hot, the residue collected on a No. 40 filter paper, washed, and the filtrate collected in a 500 ml volumetric flask. The filtrate was cooled, diluted to volume, and analyzed by ICP for metals. The SiO2 content was determined on the residue as before after sulfation.
The total recovery data for ignition, SiO2, metals, NaCl, sulfate, and inorganic carbonate are shown in Table 3. Very little barium was found to be complexed, but about half the strontium was complexed and as much as 17% iron in Sample 5 was dissolved and complexed.
Phosphorous was found in all samples except 3 and 4. Sodium chloride was determined from the sodium data. The SiO2 content remains about 42% for Sample 6. Total recovery ranges from 44 to 92%. Barium and strontium in several of the samples (especially 1, 3, and 4) were only partially complexed.
ACID AND BASE WITH CYDTA
The ignition and successive acid and basic digestion were made. ICP and IC analyses were obtained for metals and sulfates. Soluble silica was also obtained by ICP. The total recovery (Table 4) of SiO2 ignition loss, metals, NaCl, sulfate, inorganic carbon, and residue after SiO2 show recovery to range from 96.5 to 111%.
Residue was not found in Samples 1, 3, 4, and 5. Residue values of 2.20% for Sample 2 and duplicate values of 4.52 and 4.20% for Sample 6 were obtained. The recovery data are considered to be quite good considering the complexity of the problem and the many analyses performed.
The major metals found are calcium, iron, barium, and strontium. These, combined with the ignition loss, SiO2 (soluble and insoluble), sulfate, and inorganic carbonate, constitute most of the sample.
Duplicate data for Scale Samples 3 and 6 show only small deviations for the reported duplicate value.
The residue of 4.5 and 4.2% for Sample 6 (Table 5) was examined by X-ray fluorescence and compared to the original sample. The fluorescence spectra are shown in Fig. 1 (original sample) and Fig. 2 (residue samples).
The major elements found in the original sample are silica, sulfur, chloride, calcium, iron, and copper which agrees with the corresponding elements in Table 5 for Sample 6. The insoluble elements for the residue samples (Fig. 2) show aluminum, potassium, calcium, titanium, chromium, iron, and copper to be predominant in addition to platinum from the platinum dish. These elements are a mixture of about equal quantities (other than aluminum).
Titanium and potassium would not be expected to complex with CYDTA. The residue is probably made up largely of insoluble metals from pipe.
ANION ASSIGNMENT
Anion assignment for the scales complexed in acid/base with CYDTA are included in Table 5.
Anion assignment for carbonate and sulfate were first made. Carbonate was assigned based on X-ray diffraction data for Samples 2, 5, and 6 where Ca(Mg0.67 Fe0.33)(CO3)2, calcium carbonate, and iron carbonate were reported. The percent calcium remaining after assignment to the above two calcium compounds was then assigned as calcium sulfate.
All barium and strontium were calculated as the sulfates. The percent sulfate assigned was summed and compared to the IC sulfate value. The sulfate balance is within 4 to 7% (absolute) for Scale Samples 1-4. In these scale samples, the iron was lower than in Scale Samples 5 and 6 and was calculated as the oxide.
The highest iron values are found in Samples 5 and 6. If the iron in Sample 5 is calculated as the sulfate, the values for summed sulfate and IC sulfate are very close.
Also, the sulfate difference value (Sample 6) between the summed and the IC value indicate part of the iron in this sample is sulfate, probably present as a complex mixed salt.
REFERENCES
- Shizuo, Sugita., "The Analysis of Scale," Nippon Kaisui Gakkaishi, Vol. 36, No. 203, 1983, pp. 304-9.
- Chang, Yang C., Solubility Product Constants, Handbook of Chemistry and Physics 64th Ed., 19831984.
- Barnard, A.J., Broad, W.C., and Flashka, H., Nature and Methods of End Point Detection, A Review of Literature, J. T. Baker Chemical Co., November 1957.
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