Enhancing Through-Tubing Formation Evaluation Capabilities with the Dual-Burst Thermal Decay Tool

Abstract

ABSTRACT The determination of hydrocarbon saturation through tubing and/or casing requires an accurate knowledge of formation capture cross section and porosity. In many cases an openhole porosity log is not available and the presence of tubing prevents running a compensated neutron porosity tool into the borehole. The Dual-Burst* TDT service has recently been enhanced to provide a real-time wellsite value of the formation porosity. This neutron porosity is based on the ratio of capture gamma rays accumulated in selected gates late after the long burst in the near and far detectors. Corrections are performed for borehole status (open or cased); borehole size; and for borehole and formation fluid salinities, obtained internally from the borehole and formation thermal neutron decay rates. The environmentally corrected ratio is then transformed into porosity according to one of three lithology-dependent transforms. A laboratory data base consisting of more than 750 different cases covering a wide range of porosities, borehole geometries, lithologies, formation and borehole fluid salinities has been analyzed to establish the porosity algorithm. Field examples are presented to compare this new porosity to openhole neutron porosity data over a wide range of borehole conditions. Previous publications have demonstrated that the Dual-Burst TDT formation capture cross-section measurement is accurate and independent of standard borehole conditions. In this paper, we demonstrate through the use of examples that the appropriate choice of the neutron pulsing and gating scheme and the use of the diffusion model extend the validity of the formation capture cross-section measurement to areas where it has been questionable in the past. More precisely, examples are shown of logs recorded before and after gravel packing operations, through drillpipes and in concentrical casings. The effect of a large cement annulus, as well as the absence of cement, is studied in known laboratory test formations.