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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26 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C basis of average current densities if the wells are com pleted in the same way. When varia tions occur in com- pletion which could give variations in exposed areas of casing in par- ticular formations, cur rent require- ments must be determined from pola r i zat ion c u r ve s for t he i n - dividual wells. Current Distribution— Ground Bed Location Current distribution to the bot- tom was verified for each of t he seven test wells. Data show that, with the ground bed as close as 25 to 50 feet, there is a shift in the bot- tom hole potent ial for all wells. Therefore, dis tribution of protective currents to the entire casing is fea- sible. Distribution is not limited by t he wel l dept h w it h i n prac t ica l limits. Specific effect of ground bed lo - cation on uniformity of current dis- tribution and protective current re- quirements can be considered by comparing bottom hole data with data from a surface reference elec - trode. Data for Well A are illus- trated in Figures 7, 8 and 9; Table 3 sum marizes t he data for all test wells. The probable value of the protec - tive current for Well A is 4 amperes. In Figure 9 the surface reference electrode indicates 4 amperes with the ground bed at 300 and 100 feet, a sl ig ht ly h ig her c u r rent for a ground bed at 50 feet and no clearly defined break for the 25-foot bed. The bot tom hole reference ( Figure 7) shows the same trend, but only the 300-foot bed gives the 4-ampere value. Thus the optimum ground bed location would appear to be be- yond 100 feet. There is evidence, however, t hat cur rent attenuates along the casing with time and that 100 feet may be a prac tical ground bed distance. This is likely in the East Texas field where major corro- sion is near the surface and might correspond most nearly to pipeline conditions. In Figure 8, even with a ground bed at 300 feet, the bottom hole ref - erence indicates protective currents above 4 amperes unless each incre - ment of current is applied for 3 min - utes. The bottom hole and surface r e f e r e nc e s i nd ic at e d t h e s a me protec tive current (4.7 amperes) for Well B with a 175-foot ground bed when t he current was increased slowly. Bottom hole and surface ref- erence data for Well C also showed a sim ilar correspondence wit h a ground bed at only 100 feet, pro- vided t he current was increased even more slowly. For the Spraberry wells in West Texas, the current requirements, as indicated with a surface reference e le c t r o de, a r e not a f f e c t e d by chang ing the ground bed location from 500 to 50 feet. A bottom hole reference showed a sl ig ht ly i n- creased current requirement with a ground bed at 50 feet for Well D. A distance of 300 feet is adequate for the other two wells. Because the current require ment increase at 50 feet is small (0.8 ampere) and be- cause attenuation might be antici- pated, a ground bed location of 100 feet again seems to be a practical figure. For deep Well G, the surface ref - erence electrode data showed no dif ference in current requirements with ground beds at 50 and 500 feet. However, the bottom hole reference showed a higher current require- ment t ha n t he surface reference when the ground bed was at 50 feet. D e t e r m i n at io n of m i n i mu m ground bed distance is of practical impor tance because it is desirable Figure 8—Polarization curves as affected by varying time intervals for Test Well A, taken with bottom hole reference. Figure 9—Polarization curves as affected by varying ground bed locations, taken on Test Well A with surface reference. Figure 10—Effect of reference electrode location on Test Well A.

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