The Attaka oil and gas field has been in production since 1972 and has produced nearly 600 MMBO and 1.3 TCFG.

In order to maintain production, wells are worked over regularly to shut out water production and to open new reservoir intervals. Determining the amount of gas, oil and water before working a well over is essential. Traditional methods using thermal neutron decay do not work for oil because formation waters are relatively fresh and the contrast between oil and water is too small for quantitative analysis.

Carbon/oxygen logging overcomes the low-salinity limitation in all but low-porosity and very shaly sands. It has been carried out routinely in selected intervals in the Attaka field since 1999 and has become an integral part of reservoir monitoring. This paper presents examples from several wells in the Attaka field, where hydrocarbon saturation from carbon/oxygen (C/O) logging was critical to the successful recompletion of the well. In particular, the drilling of the UA-7RD1HZ intermediate radius oil well was dependent upon accurate assessment of the oil leg location and thickness.

UA-7HZ was logged in the Main Deltaic 44-5A reservoir with the Schlumberger RST tool (combined thermal neutron and C/O log) in March 2000. The resulting log clearly showed an oil leg with enough remaining volume to warrant drilling an intermediate radius horizontal oil well. That well was successfully drilled later in March and has produced enough oil to date to pay out the re-drill. Decline curve analysis indicates that the total volume of oil to be recovered will be nearly 100 MBO. Because this reservoir was scheduled for gas-cap blow-down, this 100 MBO would not have been produced without accurate measurement of the remaining oil leg.

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