L11577 Development of Techniques for Improving Downhole Inspection of Steel Pipe Casing

Steel pipe casing used in underground gas storage is subject to corrosion primarily on the outside pipe surface. Corrosion in the form of pitting and wall thinning can cause serious maintenance and operational problems. Early detection and characterization of corrosion in downhole pipe casing used in underground gas storage can provide considerable savings for the gas industry. Current magnetic and alternating current methods available from inspection service companies do not provide the desired defect definition and size characterization for making decisions regarding repairing, replacing, or abandoning the string. The purpose of this program was to investigate techniques of improving downhole inspection tools using the latest state-of-the-art eddy current technology. Major areas of investigation centered on (1) use of multiple frequencies rather than a single frequency for coil excitation and signal output, (2) improvements and additions to signal processing, and (3) incorporation of a side coil in the downhole inspection tool. Results of this study indicate that MFEC data can be processed to remove unwanted pipe variables (e.g., magnetic variations) to provide increased sensitivity compared to raw single frequency data. A comparison of results obtained with the side receiver coil with those obtained with an annular receiver coil show that the side coil is more sensitive topitting. Recommendations are made to well logging companies and or technology companies interested in designing and developing a downhole inspection tool incorporating these improvements having the highest potential into an existing well logging tool for field evaluation and further development of the most promising approaches for developing an improved field inspection device.