The CAD, A Circumferential Acoustical Device For Well Logging (1981)

Abstract A new logging device has been developed for measuring acoustical transmission properties of subsurface formations along circumferential paths on a borehole wall. The device employs directional ultrasonic transducers mounted on movable arms which contact the wall to device four quadrant paths. Ultrasound is transmitted and received separately along each of the paths and amplitudes of the received waves are recorded. A separate curve is recorded for each quadrant and vertically continuous anomalous differences between the curves may be interpreted as indications of vertical fracturing in open hole logging; such differences are expected to indicate channeling when the device is used for cement bond logging. It is anticipated that the device will prove useful also for providing high resolution (an inch or so) correlations between wells and for detecting thin porous zones interbedded with denser material. To achieve optimum resolution of open vertical fractures and other features on the borehole wall, shear waves are employed. These are excited by compressional waves produced in the drilling mud and so directed as to be converted to shear waves at the wall. Open vertical fractures will attenuate the shear waves in a manner described by Koerperich in 1975. Introduction This paper will describe a new acoustical logging tool developed for use as a detector of vertical fractures. Past literature has described several acoustical devices which have been proposed or used for the purpose. In 1954 Goodman described an acoustical device which directed shear waves along the borehole wall and variations thereof were observed as an indication of the presence of open fractures. Goodman's device utilized continuous waves, which present special problems in acoustical logging. Subsequently, Vogel, Henry and Mickler separately described devices in which transient shear waves were employed to measure acoustical transmission properties of the borehole wall and the effect of fractures on the transmission. In 1975 Koerperich presented data obtained with the Mickler device and demonstrated thereby the validity of the concept of detecting open fractures by means of shear waves propagating along a borehole wall. In this paper we present a progress report of our development of an improved circumferential acoustic device, the CAD. We have successfully designed, constructed and field tested a system which generates and records circumferentially traveling waves. The full wavetrains have been successfully brought to the surface and recorded on tape and pen recorder traces have been produced which express the amplitudes of selected portions of the wavetrains. The present device was developed in response to the need for a readily available and reliable fracture logging device which would produce an easily interpreted log at the well site. In addition, this device should be relatively simple and inexpensive, and should provide unambiguous information about shear wave transmission properties of subsurface formations, or could be adapted for a variety of uses such as cement bond logging. The CAD has been constructed and tested in a well, but only meager data has been obtained to date to demonstrate its application to field problems. PRINCIPLE OF THE TOOL PRINCIPLE OF THE TOOL The CAD was designed to provide an acoustically simple tool. The transmitting and receiving transducers were selected and mounted so the paths of sound waves therein would be predictable and the various kinds of received waves would be relatively simple and distinguishable. The device employs ultrasonic transmitters and receivers mounted on wall contacting pads and positioned along the circumference of the borehole wall. positioned along the circumference of the borehole wall. Figure 1 is a cross-section view through the transmitters and receivers of the present device in a borehole. The figure is roughly to scale.