Publications

In Mahakam delta, density and neutron logs are routinely acquired in openhole (OH) condition over main interest zone for porosity computation, lithology and gas identification. However, there is no openhole logging for density-neutron (radioactive source) in shallow zone due to the operational constraints. In order to evaluate shallow gas potential in Mahakam and to differentiate between lithology (coal or carbonate) and sand gas bearing reservoir, the neutron behind casing (NBC) is logged systematically in front zone of interest. Neutron data can be used to identify gas bearing zone by comparing the count rates from near and far detectors. The ratio of neutron count rates from near and far detector is sensitive to the gas content in the reservoir which can be used for gas identification. The sensitivity of near and far count rates ratio is independent of water salinity that make this technique applicable in both fresh and saline water environment. Misinterpretation of fluid might be occurred over fresh water zone by using resistivity based standard technique due to lack of resistivity contrast between gas and formation water. This fresh water zone can be found at a certain interval in entire offshore Mahakam Delta. Therefore, this technique is very helpful to identify gas especially in the interval of fresh water zone where resistivity tool cannot help much. This technique reduces uncertainty of fluid interpretation that will optimize fluid analysis (FA) logging operation. This paper details the method of this technique and its application in Mahakam fields, East Kalimantan. This technique, referred to as neutron count rates ratio, provides a new alternative technique that might be applied in gas fields in other areas outside Mahakam.