Materials Performance

SEP 2018

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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25 MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 Pola r i zat ion c u r ves obta i ned with this solution are shown in Fig- ure 6. The data for Curve 1 were taken without a consistent time in- terval between readings. It is in- cluded here to demonstrate the im- portance of the time interval on the break point. The break at 0.24 ma is due to a time interval of 75 minutes at 0.24 ma compared to 16 hours at 0.30 ma. Th is ef fec t completely masks any break which might occur at the true protective current of 0.35 ma. The break at 0.8 ma is also at- t r i b ut e d t o t h e e f f e c t of t i me interval. Data for Curve 2 were taken at essentially constant potentials, but the total elapsed time is not as long as for the slow curve for the sodium acetate solution (Curve 2, Figure 4). The curve corresponds in shape to Curve A, Figure 2, and to several of the field curves taken with moder - ately long time intervals. The pro - tective current is clearly indicated as the base of the upper straight line segment (0.35 ma) rather than Feasibility Studies on Cathodic Protection of Deep Well External Casing Surfaces the intercept of straight line seg- ments (0.21 ma). Field Study— Interpretation of Data To extend the interpretation of laboratory polarization curves to field curves, it is necessary to an- ticipate some of the results of the field study. In Figure 12, the base of the upper straight line segment oc- curs at the same current value, re- gardless of the manner in which the curve was taken, provided a mini- mum time interval was used. How- ever, the intercepts of extensions of straight line segments may lead to rather large variations in the indi- cated protective current for each well. Although both single- break and double-break curves were ob- tained for each West Texas well, de- pending on conditions of the run, the base of the upper straight line seg ment for single-break c ur ves and the point of inflection for dou- ble -brea k c u r ve s were i n good agreement for all four wells. Table 2, the protective current is listed for eac h of t he seven test wells, and compared to the total area of casing, the estimated area of un cemented casing and the area of cas ing in the major corrosive zones. For the three wells in the East Texas Field, the current per square foot of bare casing averages 0.82 ma, with a range in individual values of only 0.05 ma. Protective current densi- ties for t he Spraberr y wells also show t he best ag reement when based on area of uncemented cas- ing. The low value for Well D may be due to t he greater amount of cement in the major cor rosive zone. Although there is good agree- ment in current densities among wells which are in the same field and which have similar comple- tions, data from field to field are not comparable. Therefore, current re- quirements probably can be deter- mined from polarization curves for i ndividual wells a nd applied to other wells in the same field on the Figure 6—Polarization curves for calcium carbonate-sodium sulfide-hydrazine solu- tion with 8.7 pH. Curve 1 is variable time interval; Curve 2 is slow time interval-con- stant potentials. Figure 7—Polarization curves with bottom hole reference for Test Well A with varying ground bed location.

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