Early Casing Diagnosis with Proper Tools, Prevents Costly Casing Failures
	por Gary Frisch - Principal Scientist

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Introduction

Casing conditions if not properly monitored can create catastrophic results with high environmental, economical, and social costs when the casing fails. These casing failures could be the result of chemical corrosion and/or mechanical erosion. With the proper tool(s), the casing condition can be monitored and the casing failure can be prevented if the initial problem is diagnosed early in the failure cycle.

The rotating ultrasonic tools were initially designed for use in openhole imaging applications. The Circumferential Acoustic Scanning Tool Visualization TM (CAST-V) is a device that acoustically scans the circumference of the casing to provide casing inspection and cement evaluation capabilities. The combination of CAST-V and the standard Cement Bond Log (CBL) provide the basic cement evaluation solution for the industry. When these tools are combined with the Advanced Cement Evaluation (ACE), the art of cement evaluation now becomes a science. ACE provides the detailed answers to complex cementing issues that the industry needs and requires. (See additional article in newsletter).

In image mode, the scanner evaluates only the inner surface of the casing. With the high vertical and horizontal resolution minor casing, damage can be located, evaluated and imaged. These images evaluate casing integrity by revealing distortion, wear, holes, parting, and other anomalies on the inner wall of the casing. In the cased hole mode, the CAST-V, besides providing cement evaluation, provides an accurate measurement of the internal diameter or radius of the casing, along with pipe thickness. These measurements provide an excellent method of monitoring casing wear.

A new software package CASE consists of three programs: HoleShape, CastCase, and CastJoint, which uses the information from the CAST-V to provide the solution for casing inspection. The software package will use the CAST-V data which was acquired in either the image or cased hole mode. Outputs from these programs include an easy to understand log, text files, spreadsheets, and additional data for three-dimensional imaging.

HoleShape
With ultrasonic data, slight tool eccentering problems will lead to an inaccurate analysis. Spiral or patterns similar to a barbershop pole are indications of eccentricity problems, not necessarily casing wear. The program HoleShape corrects the travel time data for tool eccentering. The correction permits the travel time image to be as detailed as the amplitude image, thus leading to an improved visual interpretation of casing defects. The complete theory for this correction can be found in the technical paper "Advanced Ultrasonic Scanning Tool and Evaluation Methods Improve and Standardize Casing Inspection" by Gary Frisch and Batakrishna Mandal, presented at the 41st Annual SPWLA Logging Symposium, Houston, 2001.

Portion of new outputs from CASE showing perforated zones, casing damage and grading based on total wear. (Click image to enlarge)

CastCase
This program uses the corrected travel time, the pipe amplitude, and fluid travel time to evaluate the casing condition. The major change with respect to the previous program CastCali is the use of normalized and percentage with respect to both casing radius and thickness. In CastCali, the measurements of both casing thickness and radius measurements were presented in inches or mm. This created confusion and frustration when multiple sizes and weights of casing were present in the same wellbore. With each casing size change, a new plot was required to represent the current casing size.

With CastCase, we are normalizing the data for each size of casing present, and determining the percent of wear of the casing. We can determine the expected or known radius and pipe thickness based on the casing ID and OD. The CAST-V provides the current or measured casing radius and thickness. CastCase uses both the expected and measured pipe thickness and radius to determine the "normalized data". If we had perfect pipe (no damage in radius or thickness) then the normalized radius and thickness measurements will be zero. If the casing has internal corrosion, the radius measurement will be larger than the known, and the thickness will be smaller. Therefore, the normalized data will show the loss of casing wall as an increase in the pipe radius (PRADN) and a decrease in pipe thickness (THIKN). The next step is to use both the measured and expected casing radius and thickness to determine the percent of pipe remaining and/or loss.

Assuming that the casing thickness measurement is measurement of the total pipe damage, and the radius measurement determines the internal damage, the external damage can be resolved. We now have all the ingredients to grade pipe based on the total loss of metal, and can determine if the damage is inside/outside or a combination of both for the casing in question.

Casing damage has historically been classified by grades (i.e., grade 1 (or A) < 20% damage; grade 2 (B) 20<40% damage, grade 3 (C) 40<60%, grade 4 (D) 60<80%, grade 4+ > 80%). Other casing inspection tools grade in much the same way, however they cannot separate the damage into both inside and outside damage at the same depth. The measured damage reported is either internal or external but not both. CastCase allows this grading scheme automatically.

CastJoint
Both CastCase and CastJoint generate both spreadsheets and text files that list minimum, maximum, and average values of both the internal radius and the thickness of the casing. These files will allow continuous monitoring of the casing wear.

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CASE Log Display
The log presentation shown is the result of running all three models described previously. The depth track contains a joint counter and collar marker. The pipe amplitude image (PAMP) is in track 1, which provides a detailed representation of the internal wear/corrosion/damage of the interior wall of the casing. The total casing damage (TOTDAMG) is broken into internal damage (INTDAMG) (blue) and external damage (magenta) measured in percent as shown in track 2. The internal and external damage correlate to the DAMAGJ image in track 3. DAMAGJ consists of the percent of pipe remaining, which is color-coded into internal and external damage, grades based on total damage, and collar response.

This image is very similar to a cross-sectional view of the casing, going from inside to outside (left to right), with the internal damage (blue) being on the left, external damage (magenta). The color of the between indicates the pipe grade based on total damage which is the summation of the interior and exterior damage. The following table explains the color-coding.

Amplitude images may reveal damage and/or casing conditions that are not present on other logs as shown with the pipe weld or possible scar, which is visible on PAMP on the top joint. PAMP shows two perforated zones at X048-X067 and X080-X090. DAMAGJ really highlights these two perforated zones while ranging from almost 100% white (showing no damage) to 100% blue indicating 100% pipe wear from the inside. The color between the interior and exterior damage indicates the pipe grade. By examining this image, it is apparent the grade of the casing, and if the damage is internal, external or both.

CASE, along with the imaging package in PetroSite, provides 3-dimensional images for both image and cased hole mode data. The image shown is from X048-X053 of the log example except that the data was recorded in the CAST-V image mode. It should be evident from the image the condition of the casing is less than optimum.

3-dimensional image using CASE and PetroSite shows damage from X048-X053. (Click image to enlarge)