A New Compensated Thermal Neutron Tool For Porosity Measurement

Thermal neutron instruments have been used to measure formation porosity for over 40 years. Combined with density instruments, they provide basic and important parameters for log analysis. However, the measurement made by compensated neutron instruments is both statistical and sensitive to a variety of borehole and formation conditions. A new compensated thermal neutron tool has been developed that features improved statistical precision and reduced environmental effects. An effort has been made to increase the count rate of the detectors while maintaining a high porosity sensitivity and minimizing environmental effects. Extensive computer modelling was used to change source-detector spacings, detector sizes, detector pressures, and shielding materials to obtain the best design possible. For a logging speed of 30ft/min and a sampling rate of 4 samples/ft, the statistical uncertainty in the porosity measurement of the tool is less than 1 p. u. up to 25 p. u. and less than 2p. u. up to 40p. u. The borehole salinity correction for a 7"/a-in. borehole filled with saturated saltwater is reduced to less than 2 pu. up to 35 p. u. Other environmental corrections are well within accepted limits for this class of instruments. To fully characterize the tool, a large set of data was produced using both computer modelling and measurements made in API and Atlas test formations. The effects of lithology, borehole size and salinity standoff temperature, pressure, formation salinity, formation capture cross section, mad weight, mudcake, casing, and cement were included. Computer modelling was also used to determine the tool response for a large set of randomly varying logging conditions in order to assess the accuracy of the correction algorithms. The corrected porosity obtained from applying the appropriate corrections is in good agreement with the true porosity used in the computer simulations.