The analysis of earth media using steady state neutron techniques

The feasibility of using steady state neutron methods to follow the passage of fluid artificially injected into a geological formation is investigated. These measurements give information on formation characteristics such as rock permeability, which are useful in some geophysical applications (eg enhanced oil recovery). It has been found that both neutron and #betta#-ray flux measurements can be used to classify the size and shape of a region occupied by the injected fluid, in terms of ellipses and possibly higher order shapes. This classification scheme is insensitive to the borehole environment, and can be calibrated to remove the effects of the formation properties. The variation found in the calibration of a prompt gamma-ray interaction analysis probe, when the probe is used to analyse different formations, is studied. The aim is to find a method of reducing the effect of the formation on the calibration of the probe to increase the reliability of the probe measurements. It is shown that when the probe responds mainly to thermal neutron reactions, neutron flux measurements can be used to reduce these formation effects. Careful probe design is needed to reduce the effects of the borehole. Methods of estimating the effects of epithermal neutron reactions are discussed. (author)