During the completion of well drilling, cement is pumped into the annulus behind the casing to provide zonal isolation and a pressure barrier. The cement integrity inside the annulus is crucial for environmental and safety concerns. This paper describes a subsurface evaluation technique to evaluate the cement integrity behind the casing using an active gamma-gamma technique.
The target was a layered structure consisting of single or multiple layers of casing, cement in the annulus, and formation. The tool in use includes a radioactive gamma source and a gamma ray detector, which is designed to be sensitive only to the annular space away from the inner casing. Monte Carlo N-particle (MCNP) simulation based on high-energy photons transport was conducted to simulate the energy spectrum that would be measured by the gamma ray detector. Cement channels were represented by the void located in the annulus, which is characterized in volumetric terms and the localized area coverage of the voids. The detector responses change with different cement integrities. There are two attributes describing the gamma ray spectrum: count rates (CR) ratio and dipping point value. Through extensive computer modeling, the correlation between cement integrity and spectrum information was established, which are in terms used when generating equal CR-ratio curve and equal-dipping point curves. When investigating cement of unknown integrity, its gamma ray response is recorded by the detector and processed by algorithms. The dipping point value (Edip) and count rates ratio (CRratio) can be computed from the as-recorded gamma ray spectrum, and a volume-position chart can be generated in which the equal CR-ratio curve and equal-dipping point curve are plotted. The cement integrity information, the amount and the position of the void, are found simultaneously as the intersection of those two curves.
As compared to the conventional acoustic methods used in cement evaluation, this paper proposes an evaluation method using nuclear technology. The new method provides a data processing methodology to determine the void amount and position inside the cement, which could be used to image the void channel inside the cement.