Recognition Of Potentially Producible Oil And Gas Zones In Cased Holes With Neutron Lifetime Logs

Pulsed neutron logging service, currently available under the trade name "Neutron Lifetime Log" by Dresser Atlas, offers a means of calculating water saturation (Sw sigma) in the completed cased hole; or in the open hole itself, under favorable conditions. Drawing an analogy from the "moveable-oil-plotting" technique; a means has been devised to present from the recorded sigma curve, a value of apparent water-filled "porosity" (phi sigma), as seen by the pulsed neutron log. Overlaying this phi sigma onto an independently derived log of true porosity (phi t), recorded on compatible linear scale, yields an "eyeball estimate" or qualitative value of water saturation (Swa sigma). The overlaid curves of phi sigma and phi t will "stack" in zones of 100% water saturation and "separate" in zones of interest. A separate and detailed calculation incorporating appropriate values for Sigma w, Sigma h and Sigma ma within the zones of "separation" displayed on the phi sigma/phi t overlay permit analysis of water saturation (Sw sigma). Complementing the Neutron Lifetime Log, a unique combination acoustic-epithermal neutron cement bond tool has been made available by Dresser Atlas. This combination tool can simultaneously record on a second surveying run, both acoustic and neutron porosities through bonded casing, with the supplementary benefit of identification of minerals and secondary porosity. Both the existence and quality of cement bondcan be detected and evaluated, even in "hard rocks", employing the "Half Wave Acoustic Signature" presentation now available with the combination tool.