During the completion of well drilling, cement is pumped into the annulus between the casing and formation to provide zonal isolation and a pressure barrier. The cement integrity inside the annulus is crucial for environmental and safety concerns. One aspect in the evaluation of cement integrity includes the determination of how much void exists inside the cement. This manuscript describes a sub-surface evaluation technique to estimate the cement void volume behind the casing using a gamma ray spectra indicator. The tool includes a radioactive gamma source, a gamma ray detector, and associated electronics. The target under investigation is a layered structure that consists of single or multiple layers of casing, cement in the annulus space, and the formation.

Monte Carlo N-Particle (MCNP) modeling, based on high energy photons transport, was conducted to simulate the energy spectrum deposited on the gamma ray detector. The various cement integrities were represented by a different void volume inside the cement. The detector responses change as the void volume within the cement changes. A spectra indicator was defined to describe the change of the spectra shape. The correspondence between the cement void volume and the spectra indicator, known as the quality curve, was generated through analysis of detector responses from a series of basic structures of known cement integrity, which were built through extensive computer modeling. When cement of unknown integrity is investigated, its spectra indicator can be computed from the recorded gamma ray detector spectra. The cement void volume behind the casing is determined by correlating the spectra indicator with the pre-generated quality curve.

You can access this article if you purchase or spend a download.