Dual-burst thermal decay time logging overview and examples

The Dual-Burst TDT (TDT-P) tools is a 1 11/16-in. diameter pulsed neutron, dual detector tool system that uses a new method of analyzing the decay of a burst of fast neutrons in the downhole environment. The method provides an accurate measurement of the neutron capture cross section of the formation by employing a physically realistic model of the decay, which accounts for neutron diffusion effects, rather than displaying the decay as a sum of exponentials from the formation and borehole. Intrinsic sigma is computed directly from the decay curve instead of by using correction charts. The model is evaluated using over 2,500 data obtained in well characterized laboratory test formations. The benefit of the method is that the formation cross section is independent of the fluid in the borehole as well as independent of the neutron diffusion characteristics of the formation, characteristics which vary with lithology. Selected results over the data base are discussed and compared to the results of a two-exponential processing over the same data