Application of SP Curves to Corrosion Detection
Authors: Kendall H.A. 
Published: December 2007

Abstract
A correlation was obtained between active internal electro-chemical corrosion occurring in a producing oil well and a standard SP log. The observations were confirmed by extensive laboratory tests in which selected sections of tubing were placed in a corrosive environment. A potential profile was obtained for each tubing joint, after which the joint was sectioned longitudinally and visually examined for active corrosion cells. The anodic areas, as determined by a ferroxyl test, were correlated with the potential profile. The profile was unique for each joint and brine system used, and can be altered by altering the internal surface of the tubing by sandblasting or rubbing with sucker rods.
Introduction
The corrosion and attempts to control corrosion cost the petroleum industry an estimated $2 billion per year. Many methods to detect corrosion and to assess the damage have been introduced and are used extensively. These methods include the study of equipment failures, visual inspections, internal caliper surveys, optical surveys, electromagnetic induction logs, coupon tests, the Corrosometer, down-hole electric resistance measurements and the use of polarization curves. Particular circumstances dictate which of these methods is used. A novel approach to determine the degree of corrosive activity occurring in a producing well has been devised using a toot well known to the logging industry-the SP log. This method is particularly useful when the produced fluids are highly conductive, such as found in most older producing fields lifting a high percentage of saline formation water.
Theory of SP Logging to Detect Corrosive Activity
The SP log is a record of the naturally occurring potential differences between an impolarizable surface electrode and an impolarizable electrode, in this case inside the tubing or casing, surrounded by a conducting fluid medium. In conventional SP logging, the potential differences are caused primarily by an electrochemical effect and secondarily by an electro-filtration effect occasioned by sandshale interfaces and fluids of different salinity. When using this device inside tubing or casing, the potential differences are caused by differences in anodic activity occurring adjacent to those areas where there is active corrosion. Fundamentally, all corrosion is caused by electrochemical processes. Potential differences may be caused when two dissimilar metals are in contact in a conductive liquid environment. Potential differences can also be caused when a metal is immersed in two fluids of different salinities or of different composition. Corrosive effects are primarily a surface phenomenon, as metal ions go into solution at the liquid- solid interface. Consequently, physical or chemical differences in the Surface structure of a metal immersed in a homogeneous fluid medium can give rise to corrosive currents. An example of this is the rubbing of sucker-rod box couplings inside the tubing string, causing local fresh surfaces to be exposed. This area would be anodic to the rest of the tubing. Thus, when an SP sonde is run inside a tubing string filled with a conductive fluid, the log will show a positive reading when adjacent to any local active anodic area.
Field Test of SP Logging Technique
The test well J.E. Allen "A" No. 12 was drilled in the southwest corner of the Luling field on July 20, 1962. This field, located in Caldwell and Guadalupe Counties, Tex., is an old field producing from the Lower Cretaceous (Edward's limestone) formation at a depth of about 2.000 ft. During the next 12 months this well had a sucker-rod failure about every three weeks, and had three tubing failures within the month previous to running the SP log. All the failures were caused by pitting corrosion. The tubing failures were longitudinal splits through badly pitted sections. The well was pumping about 8 BOPD with about 1.250 BWPD at 24 strokes/min. When taken off production, the formation pressure was sufficient to keep the hole full while the test was run. A conventional Electrolog sonde with an OD of 2 in. was used. This tool could pass through the 2.875-in. EUE tubing readily, as the ID of this 6.4-lb tubing was 2.441 in. No special safety precautions were taken in logging this well, as the oil was quite dead. Fig. 1 shows a section of the SP log obtained in July, 1963. The potential measured is the difference between the surface electrode, buried in damp earth about 50 ft from the well and the No. 2 electrode of the Electrolog sonde. Potential differences of as high as 120 mv were observed in it 5-ft interval. Distinct positive peaks were observed at 25-ft intervals. The tubing length was Range 2. A Dia-Log survey was run to determine the interior condition of the tubing and to locate the tubing collars. The position of the tubing couplings, shown as circles in Fig. 1, was not responsible for the positive peaks observed. The Sucker rods were 25-ft long and the pump had a 54-in.
JPT
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