Cement is typically provided between a casing and a formation to occupy the annular space between the casing and the formation of a wellbore. The cement has a height that effectively provides a plug to prevent fluid flow, which isolates a lower zone from an upper zone with respect to the plug. Successful primary cementing operations result in a cement sheath to bond and support casing and provide zonal isolation. Inter-zonal flow may occur as fluid migrates due to differential pressure between two zones. A continuous void in the cement may allow migration of fluid, which is undesirable. Good zonal isolation helps prevent the loss of production, control inter-zonal flow and/or flow to the surface, reduce water production, and improve confinement of stimulation treatments. Stimulation treatments are applied to a formation does not easily produce hydrocarbons. An example stimulation treatment includes using acids to increase channels that make paths to enhance flow of hydrocarbons into a wellbore. Another example stimulation treatment includes using hydraulic pressure to create factures to enhance flow of hydrocarbons into a wellbore. The physio-mechanical properties of the cement need to be optimized for the specific attributes of each well. For example, weak formations with low fracture gradients use low-density cement systems to reduce the hydrostatic pressure of the fluid column during cement placement.
A cement evaluation log was introduced around the 1960s with sonic attenuation measurements. Through more than 50 years of evolution, the acoustic solutions now consist of sonic attenuation measurement with borehole compensation and ultrasonic impedance imaging with azimuthal sensitivity. These evaluation technologies are being challenged to meet new environmental conditions (such as deep water) and new regulatory requirements. The acoustic impedance contrasts between heavy drilling mud and light cement for example may be close to or beyond the measurement sensitivity limitations of existing sensors, while heavy casings present an additional technical barrier for the traditional acoustic solutions.