Neutron Lifetime Logging in the Gulf Coast Area

Abstract

Abstract A new type of nuclear log has been developed which responds to variations inthe lifetime of thermal neutrons in formation materials. The log is particularly sensitive to the presence of chlorine and can reliably distinguish between oil-bearing and salt-water-bearing formations. Many wells have been logged in the U.S. Gulf Coast area. Typical performance characteristics and interpretation techniques are discussed. Applicability of this new log in Canadian oil provinces is considered. Introduction A new nuclear logging method, recently introduced in the Gulf Coast area ofthe United States, can distinguish between brine and oil through casing. Itemploys a pulsed accelerator-type neutron source and a scintillation counter tomeasure the lifetime of thermal neutrons in formations by recording the intensity of neutron capture radiation during the interval between sourcepulses when neutrons in the borehole have died away (1). The log is called a "Neutron Lifetime Log,™" based on the fact that, in ordinary rocks, thermal neutrons have a measurable lifetime which varies with variations in reservoir fluid and rock characteristics. Although field experience is limited, actual results in commercial wells and test pits have demonstrated that the log has important advantages over other nuclear logs. It has been found to have much better sensitivity to formation fluid salinity than any other cased hole log. As experience is gained in interpretation techniques, the new log is expected to be applicable to many formation evaluation problems where other logs are inadequate. It has been found that, in general, there is a quantitative relationship between the log response and the fluid and rock properties. This makes it possible, infavourable cases, to measure fluid saturation quantitatively. Moreover, it isfeasible to differentiate between oil and water zones even when borehole conditions are such that other logging methods would be inaccurate or inoperative.