Cement integrity preservation during completion, stimulation, production, and even well abandonment is of critical importance for an operator from long-term economic, productivity, and safety perspectives. Traditionally, compressive strengths have been considered indicators of cement integrity. However, numerous squeeze cementing jobs regularly performed on completed wells are testament to the poor correlation between compressive strengths and cement integrity. Additional mechanical properties such as tensile and flexural strengths, elastic modulus, and Poisson's ratio are being taken into account with increasing frequency for maximizing the cement sheath performance during the life of the well. Unfortunately, all such measurements are performed on samples that have been cured, either under wellbore conditions (for example, pressure and temperature) or laboratory conditions (for example, atmospheric pressure), but tested at atmospheric pressure and temperature. Such properties may at best be useful for comparing different formulations in the selection process, but do not provide information about cement properties under downhole conditions.
Using a combination of ultrasonic shear wave and compression wave measurements, dynamic mechanical properties such as elastic modulus, bulk modulus, Poisson's ratio, and compressive strength are measured under pressure and temperature. These measurements are compared with mechanical properties obtained from load vs. displacement tests under static conditions and acoustic compression and shear wave measurements at atmospheric pressure and temperature. Correlations are calculated for several slurries, and the results are presented. These results include cases where the measurements made using this method demonstrated unique advantages over the conventional load vs. displacement techniques.
Cement slurry design and testing is used to provide a cement system that can withstand well operations. The laboratory-measured values provide input data for the engineering analysis needed to evaluate cement-sheath integrity. It is a common practice to cure cement formulations under downhole conditions, particularly at downhole temperatures, either under pressure or at atmospheric pressure, and at the end of the cure period, allow the samples to come to ambient conditions prior to testing for mechanical properties. However, the mechanical properties measured on such samples do not reflect the properties of the cement formulations at wellbore conditions. Moreover, the depressurization and cooling to ambient conditions before performing cement property measurements may have the following unavoidable consequences:
introduction of microdefects into the system, and
elimination of any effects (which are likely to be significant) of curing conditions (namely elevated pressure and temperature) on the measured mechanical properties. As a result, the engineering analysis based on mechanical properties measured at ambient conditions will not be a true representation of cement performance in a wellbore.
The challenge of measuring mechanical properties under downhole conditions is not trivial due to the lack of suitable instrumentation that can cure and maintain the cement under downhole conditions while testing for mechanical properties. The ultrasonic cement analyzer (UCA) is the only commercially available instrument that measures compressive strengths at least at downhole temperatures and pressures that are prevalent in a wellbore (Rao et al. 1982). This method is based on correlating the transit time of compression waves through cement and correlating the wave velocity with compressive strengths by crushing identically cured samples at ambient conditions using a mechanical load. Ultrasonic shear waves have been used for many years to measure dynamic mechanical properties, as well as to detect voids and cracks in concrete and rock samples under nondestructive conditions (Krautkramer and Krautkramer 1977; Leslie and Cheesman 1949). They have also been used for studying early-stage cement paste properties (Voigt and Shah 2004; Fam and Santamarina 1996). In oilfield applications, they have been used as a component of acoustic logging tools for many years. The relationships between the velocities of compressive and shear waves and the material properties of a homogeneous, isotropic, elastic solid are shown in Eqs. 1 through 4. Shear waves do not propagate in liquids and gases, and therefore shear wave velocities in a fluid medium are zero.