CASING and TUBING INSPECTION BASICS
Casing and tubing placed in well bores may suffer from damage due to corrosion or mechanical distress. Corrosion is caused by natural electrical currents that flow in the earth, and from man-made currents from power lines and surface equipment.

Corrosion can be reduced by cathodic protection systems but not always with  perfectly satisfaction. Corrosion inspection logs and cathodic protection evaluation logs are widely used in areas where corrosion is known to be a problem.

Holes in tubing or casing caused by corrosion can seriously affect production rates and fluid composition, resulting in economic loss.

Loss of metal may result in strength loss without holes in the pipe, allowing burst or collapse, again with serious economic consequences. Bent, squashed, or oval pipe can impede or prevent the passage of equipment or tools into and out of the wellbore. Moving parts, such as pump rods, an cause wear that eventually will cause problems if not repaired in time.

Although most tubular goods are inspected before running into the hole, bad pipe has been found in numerous cases after the well completion has proved unsuccessful. Damage may also occur during testing of the well, drilling of plugs, or well stimulation. Un-cemented or poorly cemented casing can be damaged easily with excessive pressure. The effect on production can be severe and remediation will be assisted by an accurate diagnosis of the problem by an adequate inspection program. So there are lots of potential problems to look for.

The logs offer precise depth correlation when recorded with gamma ray or casing collar locator logs. The following descriptions are condensed from the 1999 Schlumberger Services Catalog. Equivalent logging tools are available from all major ,and many smaller, service companies.

CASING INSPECTION with CEMENT MAPPING LOGS
Cement evaluation tools are traditionally used to assess cement quality, cement bond, channels, and cement fill-up using  azimuthal ultrasonic transceivers. These tools also have a corrosion inspection mode, measuring the thickness of the casing from its resonant frequency. Simultaneously. the internal dimensions of the casing are determined from the travel time of sound reflected off the casing walls. An azimuthal image of the casing inside diameter can be displayed.

Casing diameter, casing thickness, and ovality logs from CET tool are use for casing inspection.

Ultrasonic imaging tools  evaluate the quality of the cement and determines both internal and external casing corrosion. A single rotating sensor emits ultrasonic pulses and measures the resulting resonance. The USI echo strength and casing resonance are processed to produce detailed images of cement quality and distribution that can spot channels as narrow as 1.2 in. Alternate presentations provide images of casing thickness, internal radius, and internal corrosion. Casing ovality, kinks, and holes can be observed on the image logs. The size of the rotating head is chosen appropriately for the size of the tubular to be surveyed.

Typical USI composite presentation with casing cross section and internal radius measurements.

CASING and TUBING INSPECTION with ELECTROMAGNETIC LOGS
Electromagnetic corrosion and protection evaluation tools measure casing potential and resistance in cathodically protected wells to evaluate protection and determine the extent of any corrosion. Using four hydraulically activated sets of three measurement electrodes spaced at 2 foot intervals. This service provides axial and radial current density, corrosion rate, and casing thickness. In wells without protection, the tool determines the rate and location of external corrosion.

This log shows current flow in a casing string with no cathodic protection. Corrosion is occurring at the depths shaded red. 
 

Multi-frequency electromagnetic thickness tools are nondestructive and non-contact induction devices  to determine metal loss and changes in casing geometry, regardless of fluid type. Generally used to find large scale corrosion or casing splits, the tool also detects metal loss in the outer casing of multiple casing strings. A coil centered in the borehole generates an alternating magnetic field that interacts with the casing; a second coil measures phase shift.

These electromagnetic measurements. made at multiple frequencies. are related to casing wall thickness. inner diameter, and permeability / conductivity, Each of these parameters is averaged around the pipe circumference.

This composite corrosion log run across a lost circulation zone shows the axial and radial currents in the casing. The log shows anodic and cathodic sections along the casing, Anodes have developed in front of the poorly cemented section resulting in metal loss, pits, and holes.

Pipe analysis logs monitor casing quality and discriminate between internal and external defects. This corrosion-monitoring service is designed primarily for detecting small holes and defects. A high frequency eddy current test detects flaws on the inner casing. and a magnetic flux Ieakage test inspects the full casing thickness. With these measurements. small defects and corroded areas in pipe are detected. and defects on the inner and outer walls of a single casing string can he identified. Twenty-four sensor signals (12 flux and 12 eddy) are digitally recorded for complete surface processing. When combined with other services. severe corrosion and defects can be detected and identified on the inner or outer pipe in a double casing string.

PAL log recorded to evaluate casing corrosion. Perforations are at 1145 to 1160 ft, and perforations that have been squeeze cemented are at 1165 to 1167  and  to 1190 fl.

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CASING and TUBING INSPECTION with MULTI-FINGER CALIPERS
Tubing geometry logs are multi-finger calipers, used to precisely measure variations in the internal diameter of the tubing and to provide data for collar corrosion analysis. The spherical lips of the caliper~ exert only 2 kg of contact force, thereby avoiding damage to pipe coatings. Interchangeable fingers permit corrosion monitoring over a wide range of tubing sizes. Each set of feelers can be opened and closed downhole for repeat passes over the zone of interest. Each of the 16 sensors generates an independent signal recorded versus depth and both well site and computed products are available.

Multi-finger tubing caliper with grey scale caliper map - white is 4.0 inches, black is 4.5 inches.

Multi-finger casing caliper logs are mechanical casing inspection devices. Using from 36 to 72 fingers. depending on the diameter to be measured. The tool gives very high resolution, radially and vertically, to identify casing corrosion ranging from small pits and scale to axial splits. The tool can be opened and closed downhole. Radii measurements include three minimum and three maximum (one each per 120-degree section) or six maximum (one each per 60-degree section). All raw data and computations are available at the well site.

CASE HISTORY: CASING INSPECTION
This example combines both cement integrity and casing integrity issues. The problem is to determine where a water flow behind casing is coming from. The first thing you need is a a well diagram to see what plumbing is in place, as shown at the right. The water flow is coming up between the 9-5/8 and 13-3/8 inch casing strings. The only water can get there is through poor quality cement behind the 9-5/8" casing OR through a leak in the 13-3/8" casing combined with poor cement behind that casing string.

Well diagram showing casing sizes and depths

The cheapest logs to check for water flow are noise logs and temperature logs. In this well a temperature log was run. Water inflow might show a slight low temperature at the point of entry, caused by gas released from solution, with a small high temperature anomaly above geothermal gradient, as the warm water moves upward through cooler horizons. This water flow is 100 bbl/day, so it is significant enough to cause such an anomaly.

Compressed vertical scale temperature log. Dark diagonal line is a constant gradient. Coolest temperature is at large arrow, with a warm spot above it (circled) reverting to geothermal gradient above suspected entry point.

 

The illustration below is a casing inspection log showing good casing at this depth. The casing ID and OD are normal and the map shows no breaks or holes. Since water is not entering this casing string, the log merely confirms our understanding of the situation.

Below that is an illustration of the cement integrity log, showing a large cement channel with only about 50% of the annulus filled with cement. While the cement is well bonded, it does not provide isolation, and this problem persists throughout the well bore.

A cement squeeze to fill this channel is needed. The interval should be re-logged after the squeeze to verify isolation and fill up of the annulus.

Casing inspection log over suspect area shows good casing. Ignore the cement quality indicator on the right edge - it is just plain WRONG - see below.

Cement integrity log over suspect interval shows a bad cement job. White on cement map is a channel and black is good cement., giving a Bond Index of only 50%. Water can easily flow up through such a channel. The cement quality flag at the extreme right is WRONG (black is supposed to represent good cement but the trigger level is not set correctly - you need at least 80 to 90% bond for isolation to water.

This may not be the end of the story. The integrity of the intermediate casing also needs to be checked. Fortunately, casing and cement logs were run before the well was drilled to total depth. Samples are shown below. Lots of corrosion both inside and outside the casing is indicated, possibly because this was re-used casing. Kinks and dents are also evident on the casing ID map. The cement fill up is also poor and a remedial squeeze job should have been performed before the well was deepened. There is one anomaly on the casing that might indicate a split (see below.. There is no easy way to squeeze behind this casing now and the temperature log is too insensitive to confirm a water leak at this depth. The salinity of the water flowing in the annulus might provide a clue as to whether the water is coming from a deep or shallow source.

Casing profile shows corrosion an inside and outside of pipe. Map of inside diameter shows casing is oval - dark red is large diameter, pale colour is small diameter. Casing thickness shows possible splits (white diagonal areas with dark blobs- white is thin, black is thick). "Good" cement flag at far right is not correct - see image below.

Cement bond and cement map show poor bond with a medium size channel and no bond to the formation If the casing is split as indicated in the previous image, this cement could allow water inflow between this casing string and the liner. Note that the apparent "casing splits" on the casing log are at the same place as the missing cement. The interpretation of a casing split here is not certain - an electromagnetic pipe evaluation tool would have been needed to confirm.