A Neutron Logging Method for Locating the Top of Cement Behind Borehole Casing
Authors: Ahmed A.E. 
Published: December 2007

Introduction
A few methods are known in the oil industry for determining the top of cement in the annulus around a casing. Before the advent of electrologging surveys, inflow and circulation methods provided information on the quality of cementation jobs. Temperature logging is presently the technique commonly used for this purpose. The radioactive tracer survey and sonic log based on the principle of "cycle skipping" also have been used principle of "cycle skipping" also have been used advantageously in selected cases. These surveys indicate only the presence or absence of cement. The cement bond log, however, gives additional information with respect to the bonding of cement to the casing. Grosmangin et al. have given a comprehensive account for analyzing the quality of cementation using the cement bond log.
A new technique is proposed in this paper for locating the top of cement behind the casing using a neutron log. This method was used successfully in a North Kadi well in the Cambay basin, India, where temperature survey control was inadequate and the services of a cement bond tool were not available.

Discussion
The neutron curve responds primarily to the amount of hydrogen present in the borehole fluid and the fluid in the pores of the rocks. The neutron log is also sensitive to the pores of the rocks. The neutron log is also sensitive to the casing, cement sheath, and borehole diameter. However, the borehole effect is reduced considerably in the SNP log. Better results are achieved with the CNL log, where the effects of mud column, mud cake, rugosity, and even casing and cement are reduced considerably .
On a neutron curve, the top of the fluid in a well is indicated by a "right-hand shift" in the neutron diagram. This is known as a fluid shift in the neutron log. As the neutron device is continuously pulled up hole, the device records the hydrogen content of the rocks of the wellbore and borehole fluids. Of these two fluids, the borehole fluid is more effective in slowing down the fast-moving neutron. Consequently, the intensity of the secondary gamma ray is reduced. As the tool is pulled farther up, there is an abrupt increase in the secondary gamma ray radiation on leaving the borehole fluid. This gives rise to the fluid shift on the neutron curve. Based on this principle, an anomaly is expected on the neutron log at the principle, an anomaly is expected on the neutron log at the interface of set cement and the overlying fluid zone in a cemented casing. The geometry of the cemented casing comprises a cemented and an overlying cement-free fluid zone sandwiched between the casing and the wellbore. As the neutron device is pulled up in a borehole against the cemented zone, the borehole fluid has little influence on the recording of the log. The effect of the fluid inside the pipe is insignificant on the neutron log because it is the common fluid against both the cemented and cement-free columns in the annulus. As the tool moves up into the fluid column overlying the set-cement zone, the borehole fluid slows down the fast-moving neutrons and reduces the secondary gamma ray intensity, giving rise to a "left-hand shift" on the neutron curve.
The neutron log in Fig. 1 was recorded with an uncompensated neutron tool in a North Kadi exploratory well. In this well, the temperature survey did not indicate conclusively the quality of cementation against the pay zone. The cement bond log could not be recorded because the tool was not available. Under these circumstances, a neutron log was planned to check the quality of cementation. In this well, a shift is indicated in the neutron curve recorded after the production-casing cementation at 840 m. This shift can be explained by the increase in the hydrogen ion concentration in the fluid column as compared with that of the zone of set cement. This shift is interpreted to represent the top of cement in the well. To dispel the concern that any lithological change caused this anomaly, an analysis was performed with the SP curve, gamma ray, microlog, and formation density log. This analysis did not show any significant lithological break at this depth.
JPT
P. 1089