Pulsed neutron logs have been used effectively for monitoring reservoir changes and, in many cases, predicting current water saturation. Monitoring becomes much more complicated when three phases (water, oil, and CO2) are considered simultaneously for analysis. Complex lithology and the resultant differences in the neutron capture cross section (sigma) of the rock matrix further complicate the determination of water and CO2 saturation.
The proposed technique uses open hole digital log analysis and a base run pulsed neutron log (PNC) to compute triangles whose vertices represent sigma and ratio (net near to far detector count rate) of 100% oil, water, and CO2 saturations for the given porosities. The triangles are computed and stored on a foot-by-foot basis. Subsequent PNC runs are analyzed by locating the latest sigma/ratio data point within its previously determined triangle and solving graphically for oil, water and CO2 saturations. This technique eliminates, for practical purposes, uncertainties caused by complex lithology.
The results from applying this technique in the Little Knife Field CO2 Minitest in North Dakota have demonstrated its utility. These results and an in-depth explanation of the technique are presented.