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The Electromagnetic Thickness Tool (ETT) is a slimline device for the measurement of wall thickness in downhole tubulars and is sensitive to both internal and external metal loss across a range of pipe sizes. Information derived from the electromagnetic thickness measurement made by the ETT can identify and quantify corrosion, metal loss from drilling wear and variations in tolerance from manufacture.
The ETT has a body diameter of 1.7 inches and has an array of 12 sensors individually deployed against the pipe ID, each making a continuous localised electromagnetic measurement of metal thickness. The ETT may be run in Surface Read-Out (SRO) or Memory configuration and is combinable with other casing inspection technologies (MFC and PTT) and any ProSTAR production logging sensors. The electromagnetic measurement from the ETT is not affected by well fluids, enabling operation in heavy drilling muds and gas environments. Typically, the ETT is run in combination with an MFC (multifinger caliper) as the derived thickness data from the ETT can supplement the high resolution measurement of the tubular ID from the MFC. The data set from the MFC-ETT combination thus gives an accurate burst and collapse rating for tubulars, and a good understanding of both internal and external condition. This information is processed and displayed with the use of the PVS™ pipe visualisation software giving a complete qualitative and visual representation of the casing or tubing.
The ETT operates by the transmission and sensing of a low frequency electromagnetic wave whose propagation is directly affected by the thickness of metal of the tubular in which it lies. The magnetic wave originates from a single transmitter element that lies inside the tool body. This wave travels radially through the well-fluid before permeating through the tubing wall to the area outside. The wave then travels a short distance along the length of the tubular before re-entering the pipe, at which point it is intercepted by an array of sensors deployed along the inner circumference. As the wave propagates through the metal wall of the tubular its velocity and signal strength are reduced, however, the wave is unaffected by well fluid or formation properties. The transmitter-sensor transit time and the amplitude of the electromagnetic wave are measured by the tool, which in turn are used to derive wall thickness. For example, If the tool is positioned within a thin pipe then the measured transit time will be low and the amplitude will be high, whereas if the tool was positioned within a thicker pipe then the transit time would be high and the amplitude would be low.
