A new logging tool provides a fast and accurate method of measurement in situ.

Obtaining a true picture of a well integrity problem is not easy. Multifinger caliper (MFC) systems traditionally have provided the technology to create that picture.
A new pipe thickness tool directly measures casing wall thickness. Used in conjunction with the MFC system, this tool provides a rapid measurement of casing or tubing condition in a single logging pass.
The MFC tool measures changes in diameter of a few thousandths of an inch, using an array of 24 to 80 low-contact, pressure-sensor fingers that allows logs to be run inside pipe diameters from 2 in. to 14 in. Individual finger measurements and tool orientation data are transmitted to the surface or stored in memory downhole, giving 1/4-in. depth resolution at logging speeds of 3,600 ft/hour (1,098 m/hour). Motorized roller centralizers and fingers can be opened or closed at any time, allowing multiple logging passes. The centralizers hold the tool on the well axis even in a horizontal position.
The equipment required to perform a survey is compact and heliportable. A standard system includes the downhole tools, a depth system, a personal computer and the telemetry interface. The compact tool string can be rigged up in less than 20 ft (6 m) and run on any electric line in surface readout mode.
Pipe thickness tool
The new pipe thickness tool (PTT) is an array of ultrasonic transducers, which can be run in combination with the MFC to give a complete picture of tubular geometry. The thickness of metal in the casing or tubing is measured by analyzing the travel time of an ultrasonic signal from each transducer. The transducers are held against the casing wall by bow springs, thus reducing environmental effects and optimizing signal quality.
The combination PTT/MFC yields the most accurate measure of casing condition available. The PTT also can measure changes of a few thousandths of an inch and logs continuously in casing diameters varying from 5 in. to 14 in. The PTT/MFC combination's slim diameter allows it to pass through typical tubing restrictions to record data in a casing string below the tubing - as well as the tubing itself - on a single run in the well. Logging speeds of 3,600 ft/hour (1,098 m/hour), at which the tools make a complete measurement every 1/4 in., can save precious rig time. The tool has performed more than 30 surveys successfully.
Data presentation and processing
At the well site, powerful visualization software is used to display the acquired data. The MFC Well Viewer software provides one of the most graphic tools available for evaluating and monitoring downhole well geometry. The inside of the wellbore is presented in multiple formats, such as a 3-D color image (Figure 1), "camera" view, color scale map or standard log curves. Basic calculations of percentage metal loss and burst pressure also can be presented at the well site.
Fractions of an inch can be critical when assessing the extent of casing wear, the condition of a seal bore profile or calculating the burst pressure of a completion string. Post-survey processing is necessary to deliver the precision required to give confidence in planning remedial action. Modern communication technology can transmit the raw data to a data processing center for detailed analysis and calculations.
Case study 1
During a workover on a North Sea well, operator Talisman needed casing wear measurement and analysis. Differentiation between internal metal loss and tubular deformation was achieved by running a PTT in combination with a 60-finger mechanical caliper. The 20-ft (6-m) string was run on standard monoconductor.
Accurate computation of casing burst rating, as well as identification of a suitable site for packer setting, was required as soon as possible after logging. Rapid overnight processing of caliper and ultrasonic data transmitted to Read's data processing center gave the required results back at the rig within 10 hours. The ultrasonic measurements were critical in calculating residual casing strength in areas of high internal metal loss.
Case study 2
A recent exploration well drilled offshore the Faeroes required a rush evaluation of casing wear in the 95/8-in. casing when ditch magnets indicated significant downhole metal loss. A casing patch solution was mobilized, but the MFC/PTT combination log showed that the burst rating of the casing remained within acceptable limits, thanks to the oversize outer diameter of the casing discovered by the pipe thickness tool.
The method is in the model
Modern software allows modeling of completions predictively and with field data.
Maximum allowable wear can be predicted using commercial software like Maurer's C-Wear, computations based on the highly conservative API 5C3 standard or a full-axial stress model like TDAS.
"Few engineers fully understand the limits and assumptions of the model used," said Keith Young, senior analyst at Read's data processing center. "As the limits of the design envelope tighten, the rigor of the well model must increase to ensure integrity is maintained at a minimum cost."
Whichever model is used, accuracy of input data is a critical factor. The mechanical caliper still provides the most accurate measurement of internal diameter available. Combined with ultrasonic measurement of wall thickness, casing integrity can be assessed with confidence. Precise computation requires an accurate knowledge of actual wall thickness, as API manufacturing tolerance leaves an uncertainty on the order of 12%, which can easily tilt the decision for or against costly remedial workovers.
Expandable casing tieback trial
Expandable tubular technology is at the forefront of well engineering design. Read has developed and tested a unique and patented technique in partnership with Oil States (OCMCS). One application achieves reconnection of two tubulars downhole, with no reduction in original inner diameter. Onshore trials have successfully tested a 95/8-in.connection under 800 tons of compression and 11,000 psi of pressure.
The solution was developed for a major European operator with a specific casing tieback problem. The system is also the subject of a program sponsored by Norsk Hydro and Statoil for development of a hydraulically expandable casing patch deployed on drillpipe or coiled tubing.
The downhole tool is based on OSMCS' Hydra Lok swaging tool. The method's ability to provide pressure-tight, metal-to-metal sealing, mechanical strength and minimal loss in inner diameter offers a major technical advance for remedial intervention (Figure 2).

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