A Diffusion-Corrected Sigma Algorithm for a Four-Detector Pulsed-Neutron Logging Tool

Abstract

Sigma logging is a proven method for determining oil/water saturation behind casing. In this paper, we present a new, diffusion-corrected, sigma algorithm for a four-detector pulsed-neutron logging tool. Since the diffusion effect is inversely related to the distance between source and detector, there is an advantage in having multiple detectors at varying distances from the source. We will show how we use this advantage to design a diffusion-corrected sigma algorithm, and will verify the results using lab data, MCNP simulation, and field logs. Furthermore, we will discuss our use of a dual-exponential fit to cleanly separate the borehole decay from the formation decay. We will show that this approach works well, and can provide a certain advantage relative to the "moments method".