ABSTRACT: A Novel Application of Nuclear Magnetic Resonance and Formation Tester data for the determination of gas saturation in Pretty Hill Sandstone Reservoirs, Onshore Otway Basin

Ramamoorthy, Raghu¹, Peter J. Boult², and Thomas J. Neville³
(1) Schlumberger (Malaysia) Sdn. Bhd, Kuala Lumpur, Malaysia
(2) Boral Energy Resources Limited, Adelaide, Australia 
(3) Schlumberger Oilfield Australia Pty. Ltd, South Melbourne, Australia

Resistivity based log interpretation of gas bearing reservoirs in the Lower Cretaceous Pretty Hill Sandstone, onshore Otway Basin, has recently been superceded by the application of the nuclear magnetic resonance (NMR) log, calibrated to core capillary pressure data. The borehole NMR log also assists in facies differentiation of the reservoir section and in the selection of wireline pressure and formation fluid sampling points. Formation tester data are used to initialize the capillary pressures. The use of NMR to refine the selection of formation test depths significantly improved the efficiency of the survey.

NMR interpretation parameters were refined by measuring the whole core at the wellsite directly using the borehole NMR tool in a novel application of the device. This provided both a validation of the downhole measurement and a means of differentiating the capillary bound and free fluid components in the NMR signal. This is the first time results of core measurements using the borehole NMR tool were actually applied in the interpretation of the downhole log.

This paper describes a novel approach to calibrate the borehole NMR data to capillary pressure information. The method facilitates the calibration when several core data points exist in a continuously logged interval. The results are validated through application both in the specified well and also in a subsequent well in a nearby field.

The combined application of nuclear magnetic resonance technology and formation tester information, calibrated and validated by core data, resulted in a 30% increase in average gas saturation in the reservoir at the Redman field. Saturations computed were further validated by formation fluid samples obtained with the formation tester. The method described herein has been successfully implemented in 4 wells in the Otway Basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia