SPWLA FOURTEENTH ANNUAL LOGGING SYMPOSIUM, MAY 6-9, 1973 PAPER F

THE ALUMINUM ACTIVATION LOG

H, D. Scott and M. P. Smith Texaco Inc., Bellaire Research Laboratories, Bellaire, Texas

ABSTRACT More reliable methods are needed for determining the shale content of reservoir formations, particularly in cased wells. In some areas, the natural gamma-ray log is unsuitable for this purpose due to mineralization of the formations or because of accumulations of radioactive deposits on the casing. The spontaneous potential log is unsatisfactory in some uncased wells where the log cannot delineate bed structure. This paper discusses the possibility of using a measurement of the aluminum in the formation to provide the shale content, since aluminum is a relatively constant component of shale.

Using a californium-252 neutron source emitting 10^9 neutrons/second, a continuous aluminum activation log has been developed which can be operated at logging speeds of 20 feet per minute. The log has been run in one uncased and four cased wells. Results indicate that the log has good potential for determining the shale content of formations and should overcome the limitations of the natural gamma and S.P. logs mentioned previously. Neutron absorbers in the formations such as salt water and boron can affect the log but variations in these quantities can be corrected for if a log of sigma, the formation thermal neutron capture cross section, is available.

Gamma rays in the range 1 to 2 Mev from the decay of 28Al are used for the measurement. The use of californium-252 as a neutron source instead of PuBe or AmBe is preferred in order to reduce the competing effect of silicon activation to a negligible level. Analyses of well data showed that aluminum activation is by far the dominant product with small contributions also occurring from casing activation, calcium activation, and oxygen activation.