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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22 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C reflects the lower extremity of the casing and agreement with cur rent requirements indicated by the sur- face electrode shows proper posi - tioning and manipulation of sur- face equipment. Selection of minimum protective current from the current-potential curves is based on comparison of field data with curves obtained for labora tory systems exhibiting simi- lar polar ization behavior and hav- ing known current requirements. Ground Bed Location Ewing and Bayhi 1 reported no appreciable effect of ground-bed lo - cation on current distribution on ce mented casings to 3000 feet, but information on deeper wells with un cemented casing is lacking. This fac tor was studied here by observ- ing variations in the protective cur- rents indicated by surface and bot- tom hole reference electrodes as the ground bed location was varied. Experimental Procedures, Conditions and Apparatus Field Study Procedure for obtaining current - potential (polarization) curves in the field is best described by refer- ence to the diagram of Figure 1. The poten tial of the casing with respect to t he reference elec t rodes was taken first with no current flowing to the casing. Then with the well casing as cathode, a small direct current (measured on an ammeter, and regulated by adjust ing the volt- age of the d-c power sup ply and/or varying the series resistor) was ap- plied between the ground bed and casing. After a time, t he current was interr upted, and t he casing con nected to the reference electrode through a high impedance voltme- ter. The casing-to-reference poten- tial was read on the meter, and the current application resumed. Inter- rupt ion of current, voltage mea- surement and re application of cur- rent were accom pl i shed w it h a double-throw switch, the entire op- eration requiring about five sec- onds. After each potential reading, the current was increased by a fixed increment and t he operat ion re- peated. When the interval between potential readings was less than 20 minutes, intervals were timed to with in five seconds of the indicated time. In order to study the effects of ref erence electrode location, ground bed location and time interval be- tween potential readings, several current- potential curves were run on each well. After each run the well was "de polarized" by making the casing the anode and by apply- ing about 20 amperes until the po- tential was 50 to 100 millivolts more positive than the original non-po- larized value. The po tential then was allowed to stabilize with no c urrent appl ied for at least one hour, or preferably longer, be fore beginning the next run. Although the absolute values of the potentials were not reproduced in duplicate runs, the relative potential shifts and curve shapes apparently were not af fected by previous runs. In accordance with standard pro - cedure, casings were either discon - nected or insulated from flow lines, gas lines, etc., during the study. Test Wells: Seven wells, each com pleted with open hole below the cas ing shoe, were selected for test. Information pertinent to these wells is listed in Table 1. Cur rent Supply a nd Reg u la- tion: For shorter runs, current was sup plied from a welding generator. It was regulated by variable resis- tances in series in the circuit and measured with 0-10 and 0-100 am- pere am meters. When greater preci- sion was desired, a more sensitive m i l l ivolt m e t e r wa s c o n n e c t e d across a 0.0005- ohm shunt. For six of the permanent installa - tions, a rectified commercial a-c power supply was used. At deep Well G, because commercial power was not economically available, a 110-volt, 500-watt motor generator was used. On these installations current was regulated by adjusting rectifier voltages and by variable series resist ances and measured by the volt age drop across 0.001-ohm shunts mounted on t he rect if ier poles. A double-throw switch for current in terruption and potential measurement also was mounted on each rectifier pole. Ground Bed: Temporary ground beds of heavy duty aluminum foil in 2 by 2 by 100 foot trenches were lo cated at various distances from the well head along a line perpen- dicular to a line from the well head to the center of the trench. Graphite a node s a nd met a l lu rg ic a l cok e backfill were used to construct per- manent ground beds. Where possible, the ground beds were located opposite the flow lines and as far from other buried metal a s prac t ic a l. I n eac h c a s e, t he perma nent beds were installed a minimum of 200 feet and in t he same direction from the well head as the temporary ones. Voltage Measurement: Accurate voltage measurement in the field re - quires a h igh impeda nce meter, com bining sensitivity and stability with ruggedness and rapid reading. A bat tery operated vacuum t ube voltmeter, accurate to +0.002 volt over t he 0 to 1.5 volt range, was used in this study. Reference Electrodes: Reference e l e c t r o d e s u s e d f o r s u r f a c e measure ments were standard cop- per-copper sulfate field electrodes. For the bot tom hole reference, how- ever, a spe cially designed electrode was built for temperatures to 225 F and hydro static pressures to 6000 psig. A single conductor, armored cable was used to spot the electrode in position im mediately below the casing shoe. Laboratory Apparatus and Procedure Laborator y pola r i zat ion data were obtained using coils of Num- ber 20 AISI 1010 wire as cathodes

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