Electromagnetic-Based Tool Allows In-Situ Inspection of Multiple Metallic Tubulars
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A new pulsed-eddy-current (PEC) electromagnetic (EM)-based tool called an enhanced pipe-thickness-detection tool (ePDT) has been introduced for the corrosion inspection of multiple pipes. The tool can measure the metal wall thickness of five concentric pipes with a maximum outer diameter (OD) of up to 26 in. This capability, along with the tool’s unique configuration, provides an advanced downhole solution for tubular evaluations of production, injector, and storage wells.
As hydrocarbon production and storage wells age and new production and storage wells are exposed to elevated concentrations of corrosive fluids, well-integrity monitoring is gaining more attention. These factors have influenced well-performance, safety, and environmental concerns. Early, periodic monitoring of tubular corrosion and other defects can reduce the risks of serious leaks or well failures in a cost-effective way.
Historically, the continuous sinusoidal-signal-based far-field eddy-current (FFEC) technologies have been developed and deployed for multiple-concentric-pipe average-thickness measurements. However, FFEC is adversely influenced by the strong interference of direct excitation signal transmission coupling and the EM skin effect; this reduces the pipe response because of a decreasing signal-to-noise ratio (SNR) that does not allow quantitative evaluation of more than two concentric pipes.
In contrast, the PEC-excited EM signal contains wideband frequency components from kilo-Hertz to sub-Hertz range. The low-frequency EM signals penetrate concentric metal pipes effectively. The induced PEC on each tubular has different initial amplitude and decay rates because of a combination of OD, thickness, and EM parameters. After excitation, magnetic-field changes triggered by a combination of mutual inductive interactions among the pipes, eddy-current diffusion, and damping are picked up by the receiving coil during the acquisition window. Research and field applications have proved that the PEC method is more reliable for average thickness detection in multiple pipes. In previous studies by the authors, three pipes with up to 17-in. OD can be detected by magnetic tools. However, because of the combination of high signal dynamic range, inadequate SNR, extraneous tool motion, and troublesome interference of pipe magnetization, it has proved difficult for these tools to quantify thicknesses of outer pipes reliably in the presence of more than three concentric tubulars or at ODs greater than 17 in.
In this paper, the authors present a new generation of PEC tool, the ePDT. The tool features a segmented fractal array transducer in different spatial apertures to cover measurements for different pipes. The smallest such aperture (TX-S) accurately measures EM parameters from the smallest tubular for data correction; the medium (TX-M) provides an optimal balance between log resolution and SNR for each pipe measurement; and the largest (TX-L) generates high SNR for larger OD measurements. The synthetic aperture design of the coil array can help to reduce extraneous tool motion and tubing-remnant magnetization noise. By applying a hybrid fast-inversion method, the thickness of each pipe can be calculated. The design concepts, tool specifications, and measurement integrity of ePDT have been verified and validated through simulations, laboratory tests, and field loggings.
The tool contains two modules: a sensor section with TX (transmitting coil) arrays and RX (receiving coil) arrays, and an electronics section with various units for power supply, excitation drive, data acquisition, data preprocessing, data communication, and tool control. The two sections are connected through the field joint (Fig. 1). For logging operations, the complete system requires a surface panel, downhole telemetry, and centralizers.
During logging, ePDT excites the current signal through a TX coil array to charge the metal pipes magnetically. Once the magnetic field is stabilized, the tool halts the charging current rapidly to induce high initial eddy current (EC). Concurrently, the tool acquires the corresponding time-transient signal from the RX coil array. The time-variant signal is associated with pipe properties and geometries that indicate the pipe-thickness changes caused by corrosion or other defects.
The RX signal not only includes the EC corresponding signal but also unwanted interference related to extraneous tool movement, tubing remnant magnetization, and other noise. Consequently, the deteriorated SNR of the RX signal for the outer pipes under the condition of multiple or large pipes renders thickness estimations inaccurate or even impossible.
Sensor Design and Tool Configuration. Tradeoffs exist when considering high charging power for a high-RX-signal level vs. transducer core saturation, total number of pipes detected vs. proper logging resolution at acceptable logging speed, and extraneous tool motion vs. SNR. The ePDT tool incorporates three transducers in different configurations and geometries as well as the designed synthetic apertures. The theoretical apertures of each transmitter are displayed on the right-side curves. To detect the inner pipe, the TX-S transducer is operated to enable high SNR with high logging resolution. For the second and third pipes with an OD of less than 12 in., the TX-M transducer provides the optimally balanced measurements in resolution and SNR. The TX-L transducer delivers adequate SNR that is critical for producing high-integrity measurements and analysis in large- and multiple-tubular environments.
In addition, the variation of tubing-EM parameters, conductivity, and permeability from joint to joint affects RX signals for thickness estimations. Measurements of the first tubular’s EM properties are obtained from TX-S, which provides the input for data correction.
In simulation results of sensor synthetic apertures, the RX aperture is wider than the TX apertures. During logging acquisition, the RX signal is proportional to the convolution of the RX aperture and EC spatial-density distribution. The RX aperture is associated with the motion of the tool. The EC shape is initialized by the RX aperture and changes dynamically as a result of diffusion and damping in the pipes. The synthetic aperture design for TX and RX takes into account sensor convolution, logging speed, and tool motion. Better SNR is achieved by compensating for extraneous tool motion and facilitates faster logging speed.
Logging modes and speeds are optimized for any particular wellbore-tubular configuration. For instance, a small two-pipe configuration would only require TX-S and TX-M transducer activation with a corresponding logging speed of up to 25 ft/min. For a large-OD pipe with a three- or four-pipe design, TX-S, TX-M, and TX-L are activated and require logging speeds of 5 to 8 ft/min.
The fast-forward modeling that has been developed for simulating tool response is detailed in the complete paper.
Tool Laboratory Validation
Both stationary and motion tests (the latter of which are described in the complete paper) have been performed to validate tool performance.
Sensor Verification. Fig. 2a shows the stationary sensor testing in the laboratory with four pipes:
- The first pipe has an OD of 5.5 in. and a thickness of 0.275 in.
- The second pipe has an OD of 9⅝ in. and a thickness of 0.352 in.
- The third pipe has an OD of 13⅜ in. and a thickness of 0.33 in.
- The fourth pipe has an OD of 20 in. and a thickness of 0.375.
Fig. 2b shows the tool-measurement results that exhibit a good match compared with forward modeling. The slight difference in late channel response can be attributed to the unknown EM parameter of the pipes.
This paper discusses a fieldworthy 2-in.-OD ePDT instrument that provides a synthetic TX/RX aperture in different measurement configurations. This design permits high SNR for large-pipe evaluation, optimal vertical resolution for inner-pipe analysis, and increased logging speed for optimized acquisition efficiency.
Field tests demonstrated that high-integrity measurements and analysis can be produced for 20-in. OD pipe in the presence of 13⅜- and 10¾-in. pipes, and potential can be inferred for evaluating a 30-in. pipe present in the same tubular configuration. The tool can deliver a unique well-integrity-evaluation solution over a wide range of metallic tubular sizes and configurations in an efficient and effective way.
Electromagnetic-Based Tool Allows In-Situ Inspection of Multiple Metallic Tubulars
01 July 2019
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09 July 2019