Reliable evaluation of the cement-bonding quality and identification of isolation zones of a cased-hole well are challenging problems, particularly for a plugged and abandoned (P&A) well. Ultrasonic tools have been developed to conduct pitch-catch and/or pulse-echo measurements for cement evaluation at high spatial resolutions. Recently, extended data processing of pitch-catch measurements has been developed to identify third-interface echoes (TIE) from flexural mode waveforms. The derived information of TIEs can be integrated with flexural attenuation rates of casing and acoustic impedances of annulus materials to enhance the accuracy and confidence of evaluations of cement quality and zone isolation.
However, there are limitations in conventional pitch-catch measurements. The conventual pitch-catch measurements are longitude measurements. Their vertical resolution is limited by the spacing between transducers. A utilized piezoelectric transducer used by such measurements needs a liquid couplant. The received signals of this kind of sensor are sensitive to the mud density. The heavy mud may cause strong attenuations of intensities of received flexural mode waveforms. Additionally, a piezoelectric sensor is sensitive to the direction of wave propagation. Therefore, a TIE can be missed if two walls of the annulus of a well are not parallel, such as a deviated well.
This paper introduces a new compensated pitch-catch measurement method for reliably detecting the eccentricity of the inner pipe and annulus material in a cased-hole environment. The electromagnetic acoustic transducers (EMATs) are utilized to excite and acquire Lamb and shear horizontal waves, respectively, which propagate circumferentially. The operation parameters of this new measurement method are optimized to excite and acquire waves for more reliably extracting TIEs from received waveforms. Compared with piezoelectric sensors, EMAT sensors do not require couplants and are not sensitive to the wave propagation angle, the mud density, and the rugosity of the pipe surface. The vertical resolution of the Lamb wave measurements is controlled by the vertical sampling rate of the tool and the sensor size.
This new measurement method has been validated with Lab measurements. The test fixtures with varied annulus spacings were designed, constructed, and cemented. Multiple tests were designed and conducted to verify the modes of Lamb and shear horizontal waves, existences of TIEs with different operation parameters of measurements, and the relations between arrival times of TIE and annulus spacing, as well as filled in materials of annulus. The visibility of TIE for a deviated inner pipe has also been confirmed. The tests results confirmed the optimal operation parameters of this new measurement method. The detected arrival times of TIEs are consistent with their predicted values.
This new measurement method has some key technical advantages. The tool measurements do not require a liquid-filled inner casing for acoustic coupling. The arrangement of the transducers in the tool enables fully compensated measurements. Furthermore, the vertical resolution of detected tubing eccentricity is governed by the vertical sampling rate of the tool rather than the physical transmitter-receiver spacing. The long length of received waveforms can provide the time window to exposure the trainlet of TIE for revealing the types of filled-in materials of annulus and acoustic impedance contrast of filled materials and well barriers.