Well Logging Tools & Techniques (Casing Inspection Logs)

Casing Inspection Logs

Casing Inspection Logs

Inspection of the mechanical state of the completion string is an important aspect of production logging. Many production (or injection) problems can be traced back to mechanical damage to, or corrosion of, the completion string. A number of inspection methods are avail-able, including

multifingered caliper logs

electrical potential logs

electromagnetic devices

borehole televiewers or borehole TV

The majority of these devices measure the extent to which corrosion has taken place. Only the electrical potential logs indicate where corrosion is currently taking place. With the exception of the caliper logs, all the devices require that the tubing be pulled before running the survey, since most methods are designed to inspect casing rather than tubing, and most employ large-diameter tools.

Caliper Logs

Various arrangements of caliper mechanisms are available to gauge the internal shape of a casing or tubing string. Figure 1 illustrates three such tools.

Tubing profile calipers determine the extent of wear and corrosion and detect holes in the tubing string--all in a single run into the well. The large number of feelers on each size of caliper ensures detection of even very small irregularities in the tubing wall.

In pumping wells, the tubing caliper log may be run by one person, not a whole pulling unit crew. A "pull sheet" showing the maximum percentage of wall loss of every joint of tubing in the well may be prepared. Before the well is pulled, a program of rearranging the tubing string can be provided. Moving partially worn joints nearer the surface and discarding thin-wall joints substantially prolongs the effective life of tubing strings and reduces pulling costs in pumping wells. In flowing or gas lift wells, the tubing profile caliper provides an economical method of periodically checking for corrosion damage, monitoring the effectiveness of a corrosion inhibitor program, or detecting and removing damaged tubing joints when "working over" a well.

One accessory tool that may be run in combination with the tubing profile caliper is a split detector. This tool, functioning much like a magnetic collar locator, is designed to detect and log vertical splits or hairline cracks in the tubing that might be difficult to locate with the profile caliper. In practice, the split detector is used to log down the tubing, and the profile caliper to log up the tubing. This gives a complete inspection for wall thickness and splits in one run of the cable in the well.

Casing profile calipers, which log 4 1/2-in. through 20-in. OD casing, are especially valuable where drilling operations have been carried on for an extended period of time through a string of casing. The determination of casing wear is of great importance when deciding if a liner can be safely hung, or if a full production string is required. In producing wells, the casing profile caliper will locate holes or areas of corrosion that may require remedial work. The tool is also valuable when abandoning wells because it permits grading of casing to be salvaged before it is pulled.

Electrical Potential Lags

An electrical potential log determines the galvanic current flow entering or leaving the casing.

This indicates not only where corrosion is taking place and the amount of iron being lost, but also where cathodic protection will be effective. The magnitude and direction of the current inside and outside the casing is derived mathematically from electrical potential measurements made at fixed intervals throughout the casing string. In order to achieve reliable results from this kind of survey, the borehole fluid must be an electrical insulator; i.e., the hole must either be empty or filled with oil or gas. Mud or other aqueous solutions cause a "short" that invalidates the measurements. The log itself is a recording versus depth of the small galvanic voltages detected.
Figure 1 illustrates such a log, showing three runs, for each of which a different cathodic protection voltage was applied to the casing string.

 

Figure 2 and Figure 3 show an interpretation of casing potential profile logs run both with and without cathodic protection. Note that in Figure 3 the metal loss has been reduced to practically zero by application of an appropriate cathodic protection.

 

Electromagnetic Devices

The most commonly used casing corrosion inspection tools are of the electromagnetic type. They come in two versions: those that attempt to measure the remaining metal thickness in a casing string, and those that try to detect defects in the inner or outer wall of the casing.

They operate in a manner similar to openhole-induction tools. Each consists of a transmitter coil and a receiver coil. An alternating current is sent through the transmitter coil. This sets up an alternating magnetic field that interacts both with the casing and the receiver coil (
Figure 1 ). The coils are spaced about three casing diameters apart to ensure that the flux lines sensed by the receiver coil are those that have passed through the casing.

 

The signal induced in the receiver coil will be out of phase with the transmitted signal. In general, the phase difference is controlled by the thickness of the casing wall. Thus, the raw log measurement is one of phase lag in degrees and the log is scaled in degrees. Figure 2 illustrates an ETT log in severely corroded casing. Note that an increasing thickness corresponds to an increase in the phase shift angle. Some presentations of this log show a rescaling in terms of actual pipe thickness. This requires that the operator make some calibration readings in the type of casing present in the well. It is common to see large differences in thickness between adjacent stands due to a number of variables, such as the drift diameter of the pipe, the weight/foot, and the magnetic relative permeability of the steel used.

Another closely related measurement uses a slightly different technique and forms the basis of the pipe analysis log (PAL), also known as the vertilog. Two electromagnetic measurements are of interest in the context of the pipe analysis tool: magnetic flux leakage and eddy current distortion.

If the poles of a magnet are positioned near a sheet of steel, magnetic flux will flow through the sheet ( Figure 3 ). As long as the metal has no flaws the flux lines will be parallel to the surface. However, at the location of a cavity, either on the surface of the sheet or inside it, the uniform flux pattern will be distorted. The flux lines will move away from the surface of the steel at the location of the anomaly, an effect known as flux leakage. The amount of flux distortion will depend upon the size of the defect. If a coil is moved at a constant speed along the direction of magnetic flux parallel to the metal sheet, a voltage will be induced in the coil as it passes through the area of flux leakage.

 

The larger the anomaly, the greater the flux leakage, and therefore the greater the voltage. The magnetic flux is distorted on both faces of the sheet, regardless of the location of the defect, and therefore the coil only needs to be moved along one surface to survey the sheet completely. As the coil must be moved through a changing magnetic flux to produce a voltage, no signal is generated when it is moved parallel to the surface of an undamaged sheet of steel.

When a relatively high frequency alternating current is applied to a coil close to a sheet of steel, the resulting magnetic field induces eddy currents in the steel ( Figure 4 ). These eddy currents in turn produce a magnetic field that tends to cancel the original field, and the total magnetic field is the vector sum of the two fields. A measure voltage would be induced in a sensor coil situated in the magnetic field. The generation of eddy currents is, at relatively high frequencies, a near-surface effect, so if the surface of the steel adjacent to the coil is damaged, the magnitude of the eddy currents will be reduced and, consequently, the total magnetic field will be increased. This will result in a variation in the sensor coil voltage. A flaw in the sheet of metal on the surface away from the coils will not be detected and, depending upon its distance from the surface, a cavity within the sheet will not influence the eddy currents either.

The measuring sonde of the pipe analysis tool consists of an iron core with the pole pieces of an electromagnet at each end, and twelve sensor pads in two arrays between the pole pieces ( Figure 5 ). The two arrays are juxtaposed to ensure complete coverage of the inner surface of the casing. Each of the pads contains a transmitting coil for the eddy current measurement, and two sensor coils wound in opposite directions for both the flux leakage and eddy current measurements. The two sensor coils are wound in opposite directions so that for both measurements there is zero voltage so long as no anomaly exists, but a signal will be produced when the quality of the casing is different below the two coils. The same sensor coils can be used for both measurements, as two distinct frequencies are involved. A frequency of 2 kHz is used for the eddy cur-rent measurement, giving a depth of investigation of about 1 mm. The sensor pads are mounted on springs so that they are held in contact with the casing, facilitated through centralization of the sonde. Various sizes of magnet pole pieces are available and are selected according to the inside diameter of the casing (casing ID) to optimize the signal strength for the flux leakage measurement.

 

Six measurements of flux leakage and eddy current distortion are made on each array, and the maximum signal from each array is sent uphole to the surface instrumentation. Four signals are recorded, both eddy current and flux leakage data from the two arrays.

The flux leakage data correspond to anomalies located anywhere in the casing, while eddy current distortion only occurs at the inside wall of the casing. The standard presentation of the measurements is as shown in Figure 6 , with the data from the two arrays displayed in tracks 2 and 3. Enhanced data are displayed in track 1, making any anomalies more obvious. At any particular depth the larger of the two flux leakage readings is selected and held for about 0.3 seconds on the display; the same is done for the eddy current data. This enhancement only occurs if the signal amplitudes exceed a certain threshold, to ensure that only significant defects are made more apparent. The holding of the signal allows signal levels to be seen more clearly.

Borehole Televiewer

Tools with TV capability are available for borehole scanning.

The oldest is the borehole televiewer (BHTV), which uses a rotating ultrasonic transmitter and receiver to produce an image of the borehole or casing. There is also a borehole television camera that uses a TV camera and an intense light source to transmit a visual image of the borehole wall to the surface ( Figure 1 ). The borehole television camera records on videotape and can be viewed with conventional video playback equipment.