Cement is typically relied upon as the main zonal isolation barrier in the annulus. In addition, cement is relied upon for structural support, and both factors are important considerations for the industry. In-situ cement properties are affected by many environmental factors, the main one being contamination of the cement during and after placement. When contamination occurs, cement properties and their changes can differ greatly from the standard simulated lab sample measurements. In severe cases, the cement may not set and develop the required strength, resulting in no structural support or zonal isolation.
Typical methods for evaluating cement use attenuation of compressional acoustic waves to derive compressive strength or to compute apparent cement compressional impedance. These measures offer information about the cement, but they have limitations and they may not fully characterize the cement mechanical properties. Additional uncertainties arise when trying to differentiate between impedance due to mechanical modulus versus fluid attenuation resulting from dense fluids, formation collapse, or where the barite mud weighting material that has settled ("barite sag"). Recent developments in using shear horizontal (SH) guided acoustic waves in the casing have enabled clear insight to the shear rigidity of the material coupled to pipe. These SH waves respond only to the materials with shear rigidity, irrespective of the density or bulk modulus of the material behind the casing. The response enables characterization of the solid material on the backside of the casing. This information can be used standalone or integrated with the conventional methods to provide an enhanced understanding of the material behind the casing. For cements, these measurements can differentiate between unset (green cement) and set cement, as well as provide the shear modulus and shear strength of the set cement.
This paper will discuss the theoretical background, numerical modeling, and data evaluation from detailed lab measurements to build the foundation. This understanding will then be applied to demonstrate how the additional SH attenuation measurement extends the understanding of cement properties behind casing.
