GB1321271A - Cement evaluation logging utilizing reflection co-efficients - Google Patents

Abstract

1321271 Acoustic inspection devices SCHLUMBERGER INLAND SERVICES Inc 9 July 1970 [9 July 1969] 33438/70 Heading H4D The integrity of the cement packing between a well casing and surrounding earth formations is investigated by transmitting acoustic energy so that a component propagates from the well bore into the casing and earth formations, receiving a reflected portion of the energy and separating it into acoustic frequency spectrum bands, and generating signals respectively representing the acoustic reflection coefficient in at least three selected spectrum bands. It is shown mathematically that, for a given well casing and borehole size, three frequency bands can be selected such that the reflection coefficients for two of the bands would be relatively low if there were good cementing, while at most one reflection coefficient would be relatively low for poor cementing. In the embodiment of Figs. 6 and 7 a conventional logging tool (Fig. 6) has several acoustic pulse transmitters and receivers T, R which can be selected in pairs by means of programmer 36, control signal generator 34, selector control 54 and selector unit 56. The tool is constructed substantially to prevent direct passage of acoustic energy between the transmitters and receivers. Receiver output signals are passed via channel 60 (which is blanked during the acoustic pulse transmission) and detector 42 (which is gated on when acoustic signals are expected to reach a receiver via the earth formations, cement and well easing) to acoustic travel time computer 44 and to cement evaluation computer 70, the computer outputs being logged at 52 and 71 as a function of tool depth in the well. In the cement evaluation computer (Fig. 7) the time-gated receiver signals are split into the three selected frequency bands (e.g. 10-15 kHz, 15-25 kHz and 25-40 kHz) by filters 81, 82, 83 whose amplified outputs are integrated (with respect to frequency) respectively at 87, 88, 89 to provide three signals of amplitude representing the reflection coefficients in those bands. Selectable gain amplifiers 84, 85, 86 compensate for the frequency response of the logging tool transducers. Circuit 90 (detailed in Fig. 9, not shown) selects the lowest two of the three reflection coefficient signals and averages them, this average being logged. The reflection coefficient signals are recorded individually at 91. The cement evaluation computer may be operated off-line, by recording on magnetic tape the logging tool acoustic waveforms or the outputs of integrators 87, 88, 89, and may be a digital filtering and processing system instead of the system shown (details given). The cement integrity log may be displayed on an oscilloscope or recorded by a digital printer.