Seawater Injection Experience An Overview
Authors: McCune Conwell C. 
Published: December 2007

Wells completed from offshore platforms have unique cathodic protection (CP) problems because they are shorted electrically to the platforms. Thus, there is no convenient way to determine how much galvanic or impressed current flows in each well casing system. Also, most platforms are protected with sacrificial anodes, and the depth to which they protect well casings is unknown. Furthermore, there are no established protection criteria applicable to offshore well casings attached to platforms. This study was performed to investigate the effects of galvanic current and impressed current CP on an offshore platform and six wells, and some CP guidelines were established.

Introduction

Casing failures have occurred in wells completed offshore in the Arabian Gulf, and there is concern about the apparent inability of galvanic anodes to protect wells attached to platforms. This study was done to determine the depths to which galvanic current is effective, to evaluate the effectiveness of impressed current, and to define CP criteria for offshore wells and platforms.

Facilities and Tests

A six-well platform in 40 ft (12 m) of water was used in this study. Fig. 1 shows the locations of the platform, drilling barge, anode beds, and silver/silver chloride (Ag/AgCl) reference electrodes. A variable output, 50-V, 400-A rectifier supplied direct current between the anode bed(s) and the platform during the impressed current tests. Silicon iron and platinized titanium anodes were installed at 350, 550. and 700 ft (107,168, and 213 m) to the west of the platform. The Ag/AgCl reference electrodes were located at similar distances north of the platform. Toward the end of the study. all the anodes were retrieved and reinstalled 1,600 ft (488 in) south of the platform.

Testing consisted of measuring casing voltage profile (CVP) logs and potential (P) logs. IR* voltages also were measured at various casing depths between the wellhead and the bottom contacts on the CVP tool. All measurements were done with current applied and momentarily interrupted.

CVP logs were obtained with a logging tool that had two sets of knife-edged electrical contacts spaced 18 ft (5.5 m) apart and cable connected to a microvolt meter at the wellhead. When contacting the inside of the casing, each reading was a measure of the IR voltage across 18 ft (5.5 m) of casing, which, when divided by the casing resistance, indicated the current flow in that segment of the casing. The polarity was such that positive readings indicated current flow up the casing (noncorrosion), and negative readings indicated current flow down the casing. Negative slopes in the log indicated current discharge from the casing (corrosion). The erasure of negative slopes and negative readings was presumed to indicate the cessation of corrosion, which can be accomplished by use of CP.

P logs were obtained by measuring the potential between the bottom contacts on the CVP tool and the remote [700-ft (213-m)] reference electrode. The difference between two successive readings 18 ft (5.5 m) apart is the same as the CVP reading across the same 18-ft (5.5-m) interval. Protection is indicated when the log increases positively from wellhead to bottom because positive current flows from regions of higher to regions of lower potential. P logs also can be generated by algebraically adding the wellhead potential to the IR voltage drops measured at various depths between the wellhead and the bottom contacts on the CVP tool.

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