Acoustical coupling is proposed as a new measurement to complement the attenuation rate measurement now available with multispacing cement bond logging tools. Coupling responds principally to the mechanical impedance and distribution of material outside the casing, but is largely insensitive to bonding. Estimates of cement strength may be obtained even in the presence of microannulus.
Acoustic techniques have been used for many years in cased wells for the purpose of determining cement quality. The principle consisted of interpreting the amplitude of the acoustic signal received at some axial spacing from a transmitter in the borehole fluid.' Improvements on this technique included using an array of transducers to determine the actual spatial attenuation rate of the signal. This attenuation rate is primarily a function of the mechanical properties of the cement that is bonded to the casing.
It must be recognized, however, that amplitude reduction between transmitter and receiver depends not only on attenuation from axial propagation along the casing but also on the efficiency of acoustical coupling between transducers and the casing wave. For single-spacing measurement systems it is not possible to separate effectively these independent contributions to amplitude decrement. Additionally, amplitude itself depends on borehole fluid properties and the temperature or pressure characteristics of the measuring apparatus. Gollwitzer and Masson have disclosed a means of computing a compensated spatial attenuation rate from multispacing data that is relatively independent of these environmental factors.
Still, a shortcoming of standard cement bond logging systems arises from microannuli, which frequently appear between the casing and the solid cement column after cementing and drilling operations. Often, these microseparations are small enough that the hydraulic seal offered by the cement column to fluids is not impaired. Microannuli do, however, severely reduce the acoustic attenuation rate, increasing the received amplitudes. Those measurements are thus not reliable measurements for use in estimating the hydraulic seal.
If, however, in interpretation the amplitude change between transmitter and receiver is separated into a change dependent on coupling, as well as a change related to attenuation rate, it is observed that coupling is an important amplitude-controlling mechanism that depends on properties of the material outside of the casing in functionally different ways than the attenuation. In particular, coupling is not dominated by mechanical bonding between the casing and the cement sheath.
Measurement of both coupling and attenuation rate allows determination of the presence of microannulus and estimation of average mechanical properties of material behind casing at each depth.