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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Through Tubing Pulsed Neutron Log in a Heavy Oil Carbonate Reservoir for Time Lapse Monitoring

Afzal Iqbal1; Mansoor A. Rampurwala2; Barry Reik3; Talal Al-Anezi4; Falah M. Al-Yami1; Azhar Hussain2; Christy Kuriakose2

(1) Saudi Arabia Chevron, Kuwait, Kuwait.

(2) Schlumberger, Kuwait, Kuwait.

(3) Chevron Energy Technology Company, SAN RAMON, CA.

(4) Kuwait Gulf Oil Company, Kuwait, Kuwait.

The most common log type used for cased-hole saturation analysis is Pulsed Neutron Capture log. In high salinity reservoirs the Sigma log is proved useful for cased-hole saturation monitoring. In low and mixed salinity reservoirs the C/O log is used as the primary cased-hole saturation monitoring tool. However the C/O application is limited due to the statistical nature and shallow depth of investigation of the measurement and the interpretation limitation requiring tool characterization. In this paper we present a case where C/O logs provided meaningful results in a shallow, high porosity, carbonate, heavy-oil reservoir undergoing steam injection. The time lapse C/O logs were acquired through tubing in four observation wells.

The RST* tool, used in this study, measures C/O ratio and uses characterization database to derive saturation. The measurement is sensitive to borehole and completion type besides saturation. In this case, although the tool was run through tubing, it was found that the data required very minor offset to fit with the points representing non tubing characterized tool response. The water filled borehole and zone of known saturation served as the controlling factors to determine the amount of C/O offset.

The tool also measures the neutron capture spectral data, which was used to derive lithology. The open-hole suite of logs in the studied wells included ECS* (Elemental Capture Spectroscopy) data providing lithology. The good match between the cased-hole, through tubing and the open-hole lithology provided direct indication that the data was comprised of formation signal and increased confidence.

The tool was run through 3 inch tubing in four temperature observation wells completed with 7 inch casing. The field is subject to steam flood and the logs were run as baseline logs for future steam flood monitoring. The three passes of C/O log provided data with good statistics in this reservoir. Although the data was acquired with the objective of time lapse analysis, it provided water saturation within acceptable uncertainty. The variation between open-hole versus cased-hole saturation was believed to be due to varying oil gravity not accounted for in case of C/O interpretation and the varying rock texture, wettability and formation water salinity not accounted for in the resistivity based interpretation. Due to very low formation water salinity, we believe the possibility that C/O based results could be more accurate in some zones.