Determination of Hydrocarbon Type From the Dual-Spaced Neutron Tool Responses.

Abstract

A new interpretation method that allows for the determination of the fluid type present in a formation of interest was developed. The new method is referred to as the "Three Neutron Porosity Log". The method is based on an algorithm that determines three apparent neutron porosities. These porosities, derived from the raw counts of the dual-spaced neutron tool, have three different radii of investigation. All three neutron porosities, near, far, and ratio, when properly calibrated read the true porosity in water-bearing zones. In oil or gas-bearing zones the mud filtrate invasion profile will cause the three apparent neutron porosities to be different. The resulting neutron porosity curves exhibit a pattern that allows for the identification of the fluid type. Pattern recognition is supplemented by cluster analysis of the same data. The new method works in environments where current methods are not applicable. Such is the case of cased holes where the density tool cannot be run and therefore the neutron-density crossplot is not available. The algorithm was validated with Monte Carlo simulations, and with field data. The method was tested using wireline and MWD data, and in open and cased holes. The algorithm presented in this paper relies on a proper calibration of the individual detector readings to the compensated neutron porosity in water zones. In order for the calibration to be applicable, the water zones used in the calibration must display a wide range of porosity. If this condition does not exist, the order of magnitude of the calculated low porosities will be skewed to a lower limit. This limitation is clearly present in the last two field examples near the top of the sand sections. The Monte Carlo simulations indicate the location of gas and oil clouds in the cluster analysis for open hole scenarios. An intuitive correction must be applied to the resulting points of a cased hole example.