APPLICATIONS OF THERMAL MULTIGATE DECAY PULSED NEUTRON LOGS IN UNUSUAL DOWNHOLE LOGGING ENVIRONMENTS

Abstract

The Thermal Multigate Decay (TMD) logging system utilizes a pulsed 14 Mev generator and two gamma ray detectors to obtain measurements of the capture cross sections of downhole formations. The composite decay curve from both formation and borehole capture gamma rays is detected, and is separated into the two individual components : sigma formation and sigma borehole. The resulting sigma formation measurement is only minimally affected by borehole conditions, especially in cased wells.

In addition to sigma formation and sigma borehole, the TMD system generates a dual-spaced porosity ratio and several other parameters which provide information on log quality, borehole parameters, and tool stability. These features can be used to assist the log analyst in understanding the down- hole borehole environment as well as formation characteristics. Under appropriate conditions, the TMD log can also provide production logging information relating to fluid movement in or near the wellbore.

The paper briefly reviews the TMD logging system, after which it concentrates on log examples in unusual borehole conditions : logs run across intervals where borehole conditions change; logs run in air filled boreholes; logs run in badly washed out boreholes; logs run through stuck drill pipe/ collars; logs run in flowing wells. The supplemental TMD curves in these situations identify the presence of gravel packs and packers, changes in borehole fluid salinity, possible communication between zones, and water and oil producing perforations. Gas in the borehole can also be clearly differentiated from gas in the formation.