Precipitation and Dissolution of Calcium−ATMP Precipitates for the

Mar 19, 1997 - The significance of these observations was evident when the release characteristics of each precipitate from porous media were studied ...
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Langmuir 1997, 13, 1791-1798

1791

Precipitation and Dissolution of Calcium-ATMP Precipitates for the Inhibition of Scale Formation in Porous Media R. Pairat and C. Sumeath The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand

F. Henry Browning and H. Scott Fogler* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 Received August 27, 1996. In Final Form: November 20, 1996X The precipitation reaction between aminotri(methylenephosphonic acid) (ATMP), a phosphonate used for scale prevention in high-water-volume industrial processes such as petroleum production, and calcium was systematically studied. By varying the precipitating conditions, three distinct precipitates formed: a crystalline, sheetlike, 1:1 calcium-ATMP precipitate; an amorphous, spherical-shaped, 2:1 calciumATMP precipitate; and an amorphous, spherical-shaped, 3:1 calcium-ATMP precipitate. Corresponding batch dissolution experiments showed that as the precipitate calcium-ATMP molar ratio increased from 1:1 to 2:1 to 3:1, the rate of dissolution and the equilibrium solubility limit decreased significantly. The significance of these observations was evident when the release characteristics of each precipitate from porous media were studied as related to ATMP use in oil-recovery systems. The 3:1 calcium-ATMP precipitate was released from porous media in a much slower manner than the other two precipitates, strongly suggesting that the 3:1 precipitate is most suitable for use in oil recovery.

Introduction The loss of oil well productivity resulting from undesirable scale formation is a major problem facing the petroleum industry today. Most of the scaling problems occur as a result of changes in fluid conditions (i.e., calcium carbonate) or are due to the mixing of incompatible brines (i.e., barium sulfate). In addition to limiting oil production, scale formation can also foul/damage production equipment. These problems resulting from scale formation are also prevalent in other industrial systems involving large quantities of brines (i.e., cooling water towers). Because these problems often cost millions of dollars in terms of production downtime and equipment replacement, it becomes extremely important to prevent scale formation and maintain relatively “clean” production systems. The most commonly used method to prevent scale formation in an oil well is to inject (i.e., “squeeze”) threshold scale inhibitors into the reservoir during production downtime (i.e., the shut-in period) where they are capable of interacting with the formation rock. This injection process is referred to as a “squeeze treatment”.1 Once production is resumed, the scale inhibitor prevents scale formation in the produced fluids at substoichiometric amounts until the inhibitor is exhausted (usually 1-10 ppm). Once the scale inhibitor is depleted, the well must be reinjected (i.e., resqueezed) which, in turn, is undesirable due to the costs incurred during production downtime. Hence, it becomes imperative to design squeeze treatments where the inhibitor is released from the porous media slowly, resulting in desirable long treatment lifetimes. To achieve this goal, one must have a fundamental understanding of the governing retention/release mechanisms from porous media. The two major retention/release mechanisms that have been identified are adsorption/ * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Vetter, O. J. An Evaluation of Scale Inhibitors. JPT 1972, 24, 997.

S0743-7463(96)00842-6 CCC: $14.00

desorption and precipitation/dissolution. This study is focused on studying precipitation squeeze treatments because previous research has shown that these treatments offer longer squeeze lifetimes.2,3 Precipitation processes occur as a result of the threshold scale inhibitor reacting with available divalent cations (such as calcium or magnesium) in the reservoir during the shut-in period. Upon continued production, the scale inhibitor precipitates are slowly dissolved back into the produced fluid where they prevent scale formation. Extensive precipitation studies have been performed with phosphonate scale inhibitors, a family of scale inhibitors commonly used in the field today.3,4 These studies (performed with the phosphonates HEDP and DTPMP, respectively) have shown that by varying the precipitating conditions, distinct calcium-phosphonate precipitates with markedly different properties (i.e., morphologies, dissolution rates) result and adversely affect the rate at which they are released from porous media.3,4 Aminotri(methylenephosphonic acid) (ATMP) is a different phosphonate that has proven highly effective in preventing calcium carbonate scale formation, although studies focused on examining its precipitation capabilities with calcium are insufficient. Hence, the goals of this study are (1) to understand how (or if) ATMP interacts with calcium to form distinct precipitates, (2) to determine the properties of distinct calcium-ATMP precipitates, and (3) to elucidate the release mechanisms of these distinct calcium-ATMP precipitates from porous media. (2) Carlberg, B. L. Scale Inhibitor Precipitation Squeeze for NonCarbonate Reveroirs. Paper SPE 17008 presented at the Production Technology Symposium, Lubbock, TX, 1987. (3) Browning, F. H.; Fogler, H. S, Precipitation and Dissolution of Calcium-Phosphonates for the Enhancement of Squeeze Lifetimes. Paper SPE 25164 presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, 1993. (4) Kan, A. T.; Oddo, J. E.; Tomson, M. B. Formation of Two Calcium Diethylenetriaminepentakis(methylene phosphonic acid) Precipitates and Their Physical Chemical Properties. Langmuir 1994, 10, 1450.

© 1997 American Chemical Society

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Figure 1. Molecular structure of the scale inhibitor ATMP.

Figure 2. Batch apparatus used for precipitate synthesis.

Experimental System Materials. The phosphonate scale inhibitor used in this study was ATMP, as shown in Figure 1. This inhibitor was supplied by Monsanto Chemical Co. as an aqueous solution (50% by weight). The ATMP molecule contains three active phosphate groups which offer three potential reacting sites with calcium cations. The advantages that the scale inhibitor ATMP offers include (1) it effectively inhibits calcium carbonate scale formation, (2) it is stable in both acids and bases, (3) it can disperse existing solids, and (4) it is soluble in most brines.5,6 The commercial ATMP solution was diluted to a titrating concentration of 5% in deionized water. Calcium chloride anhydrous (CaCl2) was a coprecipitating solution which was prepared to 0.5 M. Potassium hydroxide and hydrochloric acid were used to control the precipitating pH in the system. Methodology. Synthesis of the Calcium-ATMP Precipitates. A schematic diagram of the experimental apparatus used to prepare the calcium-ATMP precipitates is shown in Figure 2. All experiments were performed at room temperature. First, the desired amount of 5% ATMP solution was placed into a beaker where it was continuously stirred using a magnetic stirrer. A pH electrode was placed into the solution to measure the pH over the course of the titration. Concentrated KOH and HCl were added into the solution to adjust the pH to the desired value (1.5-7.0 in this study) and for maintaining a constant pH during the titration. Next, a prepared calcium solution was introduced into the solution in 5-mL increments until the titration was completed. After the titration, the resulting solution was left for 24 h in the beaker where the precipitate formed. The resulting precipitate was filtered using 0.22-µm filter paper and left to dry in an oven at 100 oC until the weight was constant. Characterization of Calcium-ATMP Precipitates. The dried precipitates were characterized by three different techniques as described below. (5) Information on Dequest 2000 and 2006 Phosphonates, Monsanto Technical Bulletin. Publication 9023. (6) Lewis, A. L.; Raju, K. U. Some Important Chemistry of Aminotrimethlyene Phosphonic Acid. Bahrain Society of Engineers and Saudi Arabian Interest Group American Chemical Society International Conference of Chemistry in Industry, 1992.

Pairat et al. Precipitate Morphology. The precipitate morphologies were determined using a scanning electron microscope (SEM) and X-ray diffraction (XRD). Molar Composition of Calcium and ATMP. The molar compositions of calcium and ATMP in the precipitates were determined using the AAS (Perkin-Elmer 3100) and Hach techniques. With the Hach technique, 25 mL of ATMP solution was oxidized in the presence of persulfate under UV light for 10 min to orthophosphate. The resulting orthophosphate was then reacted with a molybdate blue reagent where the concentrations could then be determined colorimetrically using a UV spectrophotometer. Precipitate Equilibrium Solubility Limit. The equilibrium solubility limit for each precipitate was determined using batch dissolution. A weighed amount of precipitate was placed into a beaker that contained a predetermined amount of deionized water. While the solution was stirred continuously, samples were taken periodically and analyzed to determine the ATMP concentration as a function of time until equilibrium was approached. Etched Glass Micromodel Experiments. A schematic diagram of the micromodel apparatus is shown in Figure 3. The micromodel experiments offer the advantage that the precipitation and dissolution mechanisms of calcium-ATMP precipitates could be visually observed through a stereo zoom microscope and a high-resolution video recorder and monitor. The procedure used to perform the micromodel experiments was similar to that used in an actual squeeze treatment. First, a supersaturated calcium-ATMP solution, prepared in the same manner as in the batch experiments, was injected into the micromodel via a syringe pump (Harvard 22) until complete micromodel saturation (3-5 PV). Next, the supersaturated solution was left to shut-in for a desired time period (usually 24 h) during which precipitation occurred. After the shut-in period, deionized water (at pH 6.0) was pumped into the micromodel to dissolve the precipitate and elute the micromodel. During this elution process, the dissolution mechanisms were visually observed and recorded while effluent samples were collected and analyzed for ATMP to obtain a characteristic elution curve.

Experimental Results and Discussion Batch Synthesis and Characterization Experiments. The batch synthesis experiments were performed by systematically varying the precipitating solution’s pH and calcium-to-ATMP molar ratio from 1.5 to 7.0 and 1:1 to 10:1, respectively. Each of the resulting precipitates were subsequently characterized as described above, and a summary of the results is shown in Table 1. Precipitate Composition. Examination of Table 1 clearly shows that by varying the solution pH, three distinct calcium-ATMP precipitates with different molar compositions were formed. At a pH of 1.5, a 1:1 calciumATMP precipitate formed, while at pH values of 4.0 and 7.0, the calcium/ATMP molar ratios changed to 2:1 and 3:1, respectively. In addition, at pH 6.0, when the precipitating solution’s calcium/ATMP molar ratio was increased from 1:1 to 10:1, the resulting precipitate molar ratio increased from 2:1 to 3:1. These observations indicate that increasing the pH and the calcium/ATMP molar ratio in solution serves to increase the number of calcium cations that can attack and bond with the ATMP molecule. These results are consistent with previous studies with calcium-phosphonate precipitates and can be readily explained by closely examining the structure and pKa values of the ATMP molecule.4,5 As the pH is increased, the solution equilibrium shifts and the ability of the ATMP molecule to deprotonate increases, as shown in Figure 4. At pH 1.5, 1-2 hydrogens are capable of deprotonating, while at pH 7.0, 4-5 hydrogens can deprotonate. This deprotonation, in turn, greatly enhances the number of available sites for the calcium cations to attack. In a similar manner, increasing the precipitating solution’s calcium/ATMP molar ratio can effectively reduce

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Figure 3. Schematic diagram of the micromodel apparatus. Table 1. Summary of Batch Synthesis and Characterization Experimentsa exptl conditions molar product [Ca2+][ATMP] 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.1672 0.08 0.08 0.08 0.08 0.08

precipitation

pH

Ca/ATMP molar Ratio in soln

Ca/ATMP molar ratio in precipitate

precipitate morphology

equilib concn,b ppm

1.5 1.5 4 4 4.5 4.5 6 6 7 7 7 7 7-6.432 7-4.888 9

1:1 10:1 1:1 10:1 1:1 12:1 1:1 10:1 1:1 1:1 5:1 10:1 1:1 10:1 1:1

1:1 1:1 2:1 2:1 2:1 2:1 2:1 3:1 2:1 2:1 3:1 3:1 2.14 2.85 no precipitate

platelet platelet spherical particles spherical particles spherical particles spherical particles spherical particles spherical particles spherical particles spherical particles spherical particles spherical particles platelet and particles platelet and particles X

2900 2960 960 490 420 640 450 330 X X 240 230 490 380 X

a CaCl , 0.5 M; ATMP, 5% molar product ) 0.08 M2. CaCl , 0.5 M; ATMP, 10%; molar product ) 0.1672 M2. X, no analysis at this 2 2 condition. b Strong function of final pH.

Figure 4. Effect of pH on the deprotonation of ATMP and the resulting species composition.

the pKa values by chelation reactions involving calcium and ATMP, resulting in an increase in the ability of the ATMP molecule to deprotonate.7-9 This observation is illustrated in Figure 5. By plotting the calcium/ATMP

molar ratio of the precipitate as a function of solution pH, it is evident that increasing the precipitating solution’s molar ratio from 1:1 to 10:1 changes the pH value at which the transformation between distinct precipitates occurs.

(7) Martell, A. E.; Calvin, M. Chemistry of the Metal Chelate Compounds; Prentice-Hall: Englewood Cliffs, NJ, 1956.

(8) Cilley, W. A.; Grabenstetter, R. J. Complexation Between Ca++ and Ethane-1-Hydroxy-1, 1-Diphosphonic Acid. J. Phys. Chem. 1971, 75 (5), 676.

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Figure 5. Effect of pH and calcium-ATMP molar ratio in solution on the resulting precipitate composition.

Precipitate Morphologies. To further distinguish how the precipitating conditions can affect the resulting properties of calcium-ATMP precipitates, the morphologies of the precipitates were studied. The SEM micrographs of each distinct precipitate are shown in Figure 6. The 1:1 calcium/ATMP precipitate is comprised of platelike sheets, while the 2:1 and 3:1 precipitates were comprised of powdery spherical particles. An XRD analysis on these precipitates verifies these differences in that the 1:1 precipitate has distinct peaks, signifying a crystalline

Figure 6. Morphologies of the distinct precipitates.

Pairat et al.

precipitate, while the 2:1 and 3:1 precipitates (the 2:1 pattern is shown) have essentially no peaks, indicating more amorphous precipitates (see Figure 7). These results are not surprising given the differences in the molar compositions and follow directly from previous results obtained from other calcium-phosphonate precipitates.3,4 Precipitate Solubilities. The solubility curves of each distinct precipitate, determined from batch experiments, are shown in Figure 8. Again, distinct differences are evident between the precipitates. As the molar composition of the precipitates increased from 1:1 to 2:1 to 3:1, the rate of dissolution and the final solubility limit decreased significantly. This phenomenon is once again consistent with previously published results on calcium-phosphonate precipitates. These significant differences in solubility are important with respect to oil-field applications because the rate of dissolution (or equilibrium solubility limit) plays an important role in dictating the rate at which the phosphonate scale inhibitors are released from porous media and, hence, govern the squeeze lifetime, as will be shown shortly. A closer examination of Figure 8 and Table 1 reveals one final interesting phenomenon. At pH’s of 4.5 and 6.0 (precipitating calcium/ATMP ratio of 1:1 for each), the resulting molar compositions are 2:1 in each case but the dissolution curves are markedly different (the equilibrium concentrations are approximately 820 and 450 ppm,

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Figure 7. XRD patterns of the 1:1 and 2:1 precipitates.

Figure 8. Effect of precipitating solution pH on the dissolution of resulting calcium-ATMP precipitates.

respectively). These results seem to suggest that two distinct 2:1 precipitates are capable of forming, possibly resulting from chaining between calcium and ATMP molecules. However, further research is necessary to clarify this phenomenon. Performance of Precipitates in Porous Media (Micromodels). The batch synthesis experiments showed that three distinct precipitates (possibly four) can be formed by varying the precipitating conditions. The next step is to elucidate how the formation of different precipitates affect the release of ATMP from porous media. The remainder of this paper will focus on studying the release characteristics of each distinct precipitate from porous media and what factors govern the release of ATMP following a precipitation squeeze treatment. To carry out this study, etched glass micromodels were used. Micromodels are valuable tools for this type of work because through the use of a stereo zoom microscope and a VCR, the release mechanisms of the precipitates can be visually observed. As mentioned earlier, the procedure used to perform these experiments mimicked that of an actual squeeze treatment process in that calcium-ATMP precipitates were grown in situ in the micromodel (at conditions defined in the batch synthesis experiments) (9) Grabenstetter, R. J.; Quimby, O. T.; Flautt, T. J. The Acid Dissociation Constants of Substiuted Methanediphosphonic Acids: A Correlation with P31 Magnetic Resonance Chemical Shift and with Taft s*. J. Phys. Chem. 1967, 71 (13), 4194. (10) Shuler, P. J. Mathematical Model for the Scale-Inhibitor Squeeze Process Based on the Langmiur Adsorption Isotherm. Paper SPE 25162 presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, 1993. (11) Walton, A. G. The Formation and Properties of Precipitates; Robert E. Krieger Publishing: Huntington, NY, 1979.

Figure 9. Elution curve resulting from the dissolution of a 1:1 calcium-ATMP precipitate in simulated porous media.

during the shut-in period and subsequently eluted by deionized water. Precipitate Release Mechanisms. During the elution process of each distinct precipitate, the dissolution mechanisms were visually observed and recorded while effluent samples were collected and analyzed for ATMP. By comparing the micromodel images with the resulting elution curve, it was possible to elucidate how each calcium-ATMP precipitate interacted with the porous media (i.e., micromodel) to govern the shape of the elution curve as well as to determine the resulting squeeze lifetimes. For each precipitate, the elution curve was broken down into distinct regions where time-lapse micromodel images could be utilized to describe the governing release mechanisms in each region. Because the 2:1 and 3:1 precipitates dissolved in a similar manner, only the 2:1 release process will be discussed. 1:1 Calcium-ATMP Precipitate. The elution curve of the 1:1 precipitate was broken down into four distinct regions (see Figure 9). A comparison between the elution curve with the time-lapse photographs for the 1:1 precipitate made it possible to describe the release mechanisms occurring during each region of the elution curve (see Figures 9 and 10): Region 1: An Initial Flat Region. The precipitate was comprised of platelike sheets preferentially situated in pore throats. In this region, the saturated Ca-ATMP solution initially in contact with the precipitate was swept from the micromodel, resulting in the high ATMP concentration seen in the initial portion of the elution curve. Region 2: A Slow Declining Region. The release of ATMP in this region was governed by external hydrodynamic dissolution of the precipitate by surrounding flowing fluid. In addition, precipitate loosely packed in pore bodies migrated and dissolved as well.

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Figure 10. Release Regimes of a 1:1 calcium-ATMP precipitate from a micromodel.

Figure 11. Elution curve resulting from the dissolution of a 2:1 calcium-ATMP precipitate in simulated porous media.

Figure 12. Elution curve resulting from the dissolution of a 3:1 calcium-ATMP precipitate in simulated porous media.

Region 3: A More Gradually Declining Region. This region was governed by both external and internal hydrodynamic dissolution. The remaining precipitate in pore bodies was easily dissolved and shrunk rapidly. Region 4: A Long Tailing Region. At this point in the dissolution process, the precipitate had receded inward to the point where mass transfer of the ATMP from the precipitate-fluid interface to the free-flowing fluid was dominating the phosphonate release. The dissolution of precipitate was extremely slow and steady in this region. 2:1 and 3:1 Calcium-ATMP Precipitate. The elution curves of the 2:1 and 3:1 precipitates were broken down into two distinct regions (see Figures 11 and 12). By

comparing these elution curves with the time-lapse photographs (of the 2:1 precipitate), the release mechanisms in each region could be determined (see Figures 11, 12, and 13): Region 1: A Rapidly Declining Region. In this region, the saturated calcium-ATMP solution initially in contact with the precipitate was swept from the micromodel. At this point, the ATMP concentration rapidly declined, resulting in the sharp drop evident in the elution curve. Region 2: A Long Tailing Region. Following the sharp drop in the elution curve, slow and steady dissolution took place, likely due to the low solubilities of the 2:1 and 3:1 precipitates. After a long period of dissolution, the

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Figure 13. Release regimes of a 3:1 calcium-ATMP precipitate from a micromodel.

2:1 and 3:1 calcium-ATMP precipitate elution curves are similar, although the long tailing region is much more extensive in the 3:1 elution curve. These observations are extremely important with respect to oil-field applications because, as previously discussed, the time in which a scale inhibitor is exhausted from the porous media (i.e., the squeeze lifetime) plays an important role in determining the success of a treatment. Based on the micromodel experiments, the 1:1 precipitate had the shortest lifetime, while the 3:1 precipitate had the longest. Hence, oil producers would likely want to design squeeze treatments with ATMP so as to ensure that the 3:1 precipitate forms. Knowing the precipitating conditions at which a slower dissolving precipitate can be induced will give producers a powerful tool with which to improve future squeeze treatments with desirable lifetimes. Figure 14. Effect of precipitate type on the release of ATMP from a micromodel.

Conclusions

precipitate situated in a porous media transformed from the amorphous to the crystalline form and packed strongly. Squeeze Treatment Comparison. A comparison of the elution curves for each distinct precipitate is shown in Figure 14. It is evident that an increase in the calcium/ ATMP molar ratio decreases the rate at which ATMP is released from the micromodel. This observation can be directly related to the solubility limits as determined above in the batch dissolution experiments. The elution curve of the 1:1 precipitate, which had the highest solubility limit (∼1200 ppm) and highest dissolution driving force for precipitate dissolution, initially shows higher ATMP concentrations than the 2:1 or 3:1 precipitates, which lends itself to the relatively rapid depletion of the ATMP. The

Two parameters that affected the molar composition of precipitates were the precipitating solution’s pH and calcium/ATMP molar ratio. High pH’s and large calcium/ ATMP molar ratios in solution increased the ability of the ATMP molecule to deprotonate and increased the resulting precipitate’s calcium/ATMP molar ratio. The resulting Ca-ATMP precipitates had markedly different dissolution patterns and morphologies. The placement of a supersaturated solution in a micromodel showed how each distinct precipitate was released from the porous media. The release of a 1:1 calcium-ATMP precipitate from porous media was broken down into four distinct regions, with the long tailing region being governed by mass-transfer dissolution.

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The release of 2:1 and 3:1 precipitates from a micromodel had two regions of dissolution. The particles situated in a micromodel were very small and densely packed in pore throats. During the elution, the precipitate extended into the pore bodies, which were slowly dissolved. The 3:1 calcium-ATMP precipitate had the longest squeeze lifetime. After eluting for a long period, the precipitates remaining in the porous media transformed into a crystal form which was less soluble than the amorphous phase. The 3:1 calcium-ATMP precipitate seems to be most

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ideally suited for actual squeeze treatments because it gives the longest squeeze lifetimes. Acknowledgment. We express sincere gratitude for the financial support provided by The Petroleum and Petrochemical College, Chulalongkorn University, and The University of Michigan. In addition, Monsanto and Daika Company supplied the chemicals needed for this study. LA9608425