Materials Performance

MAR 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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49 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 position. The prepared solutions were fil- tered through 0.45-µm filter paper and then the EOR chemicals were added into both cation and anion solutions. Fresh anion solution was prepared by adding so- dium bicarbonate (NaHCO 3 ) salt on the day of the experiment. The EOR chemicals and scale inhibitors used in this study were commercial materi- als supplied by service companies. They were used as-received without further puri- fication . Three inhibitor products—tri- phosphonate (Inhibitor A), penta-phospho- n a t e ( In h i b i t o r B ) , a n d p o ly a c r y l a t e (Inhibitor C)—represented the most com- mon inhibitor types used in the oil indus- try. The EOR polymer used in this study was a par ti al ly hydrolyzed p olyacr yl ami d e (HPAM), while the surfactant was a mix- ture of cationic and nonionic components. Tests were conducted first with the sur- factant/polymer mixture and then with the surfactant and polymer separately. Throughout this study, the concentrations were kept at 150 mg/L for the surfactant and 200 mg/L for the polymer. These were the predicted peak breakthrough concen- trations in the produced water as identified during field trials. Scale inhibitors were evaluated at 2, 5, 10, and 20 mg/L. Experiments were conducted with the static bottle method. The test temperature was set at 71 °C and test duration was 24 h. The test protocol was adopted from NACE International TM0374-2007 10 with modifi- cations described previously. 11 Consider- ing the calcium concentration was signifi- cantly higher than bicarbonate alkalinity in the test solution, the percentage change in calcium due to CaC O 3 precipitation would be small (<3%) and difficult to mea- sure accurately. Thus, the efficacy of scale i n h i b i t o r s w a s c a l c u l a t e d b a s e d o n changes in alkalinity instead of calcium concentration as is often used, as illus- trated in Equation (1): × Inhibition (%) = Alk. – Alk. Alk. – Alk. 100 isample f sample i blank f blank (1) where Alk. sample = alkalinity in the treated sample, Alk. blank = alkalinity in the blank sample (without additives), and the sub- scripts "i" and "f " denote initial (t = 0) and final (t = 24 h) values, respectively. Alkalinity was measured by the titra- tion method with the end point of pH 4.5. The pH meter (Orion Star A216 † ) was cali- brated with standard buffer solutions of pH 4.01, 7.00, and 10.01 before use. At the end of the experiment, solutions with visible amounts of precipitates were filtered for morpholog y and cr ystalline phase characterizations with an environ- m ent al scannin g el e ctron micro sc op e (ESEM) (Quanta 400 † ) and an x-ray diffrac- tometer (Rigaku Ultima IV † ). Results and Discussion Chemical analysis results indicated that the additives at the applied dosages had little effect on the initial alkalinity and pH values of the test solution. Variations in each test run were <0.25 meq/L for alkalin- ity and within 0.03 for pH readings. These results suggest that the additives would have negligible effects on the supersatura- tion state of test solutions with respect to CaCO 3 . Rather, the obser ved changes in CaCO 3 formation and inhibition were asso- ciated with precipitation kinetics. Effect on CaCO 3 Formation On the average of three test runs, the change in alkalinity without the EOR chem- icals was 3.93 meq/L. With the EOR poly- mer added, this change decreased to 3.58 meq/L, suggesting less CaCO 3 formation. In comparison, the change increased to 4.08 meq/L with surfactant alone and to 4.10 meq/L when the surfactant and polymer were combined. Although the differences were relatively small, the same trends were confirmed in all three repeated runs. These results suggest that the surfactant in this study might enhance the CaCO 3 formation, while the polymer has a small degree of in- hibition efficiency. Effect on CaCO 3 Inhibition Test results showed that EOR chemicals could significantly deteriorate the scale in- hibitor performance. The calculated inhibi- tion (%) results are summarized in Table 1. Inhibitor A was able to inhibit >80% of CaCO 3 formation at 2 mg/L and 100% at 10 and 20 mg/L. When the surfactant/poly- mer mixture was added, its inhibition (%) was reduced to <10% at 2 mg/L; <50% at 10 mg/L; and <70% at 20 mg/L, where its ef- ficacy was much less than the 2 mg/L dos- age provided in solution without the sur- factant/polymer mixture. Similar results were observed for Inhib- itor B. Its effectiveness at 2 and 5 mg/L was almost eradicated by the surfactant/poly- mer mixture; however, ~50 and ~95% inhi- bition was achieved at these levels, respec- t i v e l y, i n s o l u t i o n s f re e o f t h e E O R chemicals. At 10 and 20 mg/L of Inhibitor B concentrations, the inhibition (%) was di- minished from 98 to 12% and from 100 to 20%, respectively, by the addition of the EOR chemicals. † Trade name. TABLE 1. CaCO 3 INHIBITION (%) RESULTS SHOWING THE IMPACT OF EOR CHEMICALS AT DIFFERENT INHIBITOR CONCENTRATIONS Inhibitor EOR Chemicals 2 mg/L (%) 5 mg/L (%) 10 mg/L (%) 20 mg/L (%) Inhibitor A Tri-phosphonate Absent 84 97 100 100 Present 9 17 48 68 Inhibitor B Penta-phosphonate Absent 49 96 98 100 Present 1 2 12 20 Inhibitor C Polyacrylate Absent 15 26 40 84 Present –6 –5 –2 0

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