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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27 MATERIALS PERFORMANCE: VOL. 57, NO. 9 SEPTEMBER 2018 Feasibility Studies on Cathodic Protection of Deep Well External Casing Surfaces to be free in locating areas of low soil re sistivity for the permanent installa tion. A distance of 100 feet seems suf ficient in most cases. In East Texas, some current loss to sur- face struct ures was indicated by changes in the pipe- to-soil poten- tials of the flow lines. Because this effect increased as the ground bed approached t he well, cur rent re- qu i rement s a nd pos sible i nter - ference effects should be deter- mined with the temporary ground bed at the same location as the per- manent bed. Reference Electrode Location Proper positioning of the surface reference electrode with respect to the well head and ground bed was studied on Well A in the East Texas Field and on three wells in West Texas. Results summarized in Table 4 show that there is no apparent re - lat ionsh ip bet ween t he value of indi cated protective current and position of t he surface reference electrode with respect to the well head or ground bed. Protective cur- rent value obtained with a surface reference is in good agreement with the value in dicated by the bottom hole reference, provided the ground bed is located properly and time in- terval is ade quate. Position of the reference elec- trode with respect to the ground bed affects the shape of polarization curves. In Figure 10 the curve with a reference electrode near the ground bed is dis placed to the right and is more sweeping than that taken with a reference electrode f urt her re- moved from the ground bed. There is no indication that the absolute value of the casing-to-soil potential is significant in selecting the protective current. The poten- t ial wit h no c urrent f low varies with location of the reference elec- t rode, a nd refer ence elec t rodes matched to wit h in one millivolt may indicate differences as great as 100 millivolts, depending on loca- tion. Total shift in potential varies with the conditions of the run and a l s o mu s t b e d i s r e g a r d e d i n se lecting protective currents. Fi gure 12—Ef fec t of time inter val on Test Well E. Fi gure 11—Ef fe c t of t ime inter val on Test Well A.

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