In the Brunei deltaic environment, prediction of present fluid contacts from simulation models is difficult. Presence of sub-seismic faults as well as cross flows between reservoirs have been established in some cases. It is thus important to explore other cost-effective methods to confirm or locate by-passed/undrained reserves before embarking on any new perforations or in-fill well drilling. One such method is inside tubing pulsed neutron spectrometry (IT-PNS) logging.

IT-PNS logging entails logging slim PNS tools inside the long/single string with the well closed in more than two days before the logging. One pass in the SIGMA mode for gas detection and multiple passes in inelastic mode for oil "saturation" are logged.

The first (trial) IT-PNS well was logged in August 1998 and since then, as many as thirteen Brunei Darussalam wells have been logged using this technique. Results show that: (i) the technique can be successfully used to determine present formation fluid types, even in low porosity/permeability reservoirs. (ii) there is no need to use a rig for fluid contact measurement. (iii) in static conditions, the annular fluid type will have little or no impact on the final interpretation. (iv) three to four inelastic passes are adequate for interpretation of the liquid phase. (v) by-passed reserves have been located using the technique, thus reducing the risk of drilling dry holes.

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