In-place detection of internal and external corrosion for underground casing pipes

Corrosion occurs frequently at the internal or external of steel casing pipes used in underground gas storage or oil fields. Corrosion monitoring and early detection of pits and wall thinning for casing pipes are considerably important to the gas and petroleum industry. A new AC-DC electromagnetic induction system has been developed which is capable of monitoring and detecting the corrosion internal and external of the underground casing pipes.;With the AC approach, changes on casing weight, casing wear, and gradual wall thinning can be detected, as well as their locations outside casing pipes; while small, isolated corrosion pits and holes on the casing wall can be detected by the DC approach.;Due to the rigorous working conditions in underground casing pipes, circuits in the system are specially designed so that it can continuously work under high temperature of up to 175;Multiple transmitter frequencies and multiple transmitter-receiver spacings have been adopted in the AC induction system design so that the corrosion detection can be effective in the multi-string well case. A complete logging of the casing pipes can be accomplished with a single scan, achieving maximum flexibility and the most extensive information in quantitatively determining the total wall thickness in single, dual, and three-string casing configuration.;The detecting system also detects changes of the average circumferential internal diameter of the interior string, so that the internal corrosion and external corrosion on the casing pipes of oil or gas wells can be discriminated.;Since the dynamic range of the signal to be received is very broad and the "road noise" is very severe, the high-stable high-accurate band pass filter and hard limiters are adopted in the receiver circuit and phase detect circuit.;The information represented by enormous data acquired by the downhole DC instrument needs to be transmitted to the logging system on surface completely and speedily. A wavelet compression technique is adopted which allows us to zoom in a specific area of the signal with various scales based on the multi-resolution analysis. Compressed and then reconstructed data provide full signature logging response which allows a easier interpretation of corrosion on casing pipes.;To achieve the real time mode during logging operations, a wavelet IC chip has been designed with the VLSI technology. Past algorithms have been incorporated, and CAD tools have been utilized. Simulation results show that the design is advantageous and the circuit performance is satisfactory.