The Seismic AVO of Wet and Dry CBM Reservoirs

The Seismic AVO of Wet and Dry CBM Reservoirs

Jason McCrank*

University of Calgary, Calgary, AV, Canada

mjmccran@ucalgary.ca

and

Han-xing Lu, Kevin Hall and Don C. Lawton

University of Calgary, Calgary, AV, Canada

The seismic reflection characteristics of an Ardley Coal CBM reservoir are studied. The study reservoir has a high water saturation. Synthetic modelling is used to investigate the AVO response of the existing “wet” coals and that of hypothetical “dry” coals. Both the PP-reflection and the converted wave PS-reflection are investigated. The model results show that with sufficient bandwidth in the recorded signal, the AVO response of wet and dry coals is sufficiently distinct to discriminate between the different states of water saturation. In part to determine what bandwidth could be achieved in the field, the University of Calgary conducted a 1C-2D seismic survey at the study area in the summer of 2006. An ENVI mini-vibe source was used to generate a 10-200 Hz sweep. The signal to noise ratio diminished significantly above 100 Hz in the recorded data. However, although the signal bandwidth is limited and the targets are closely spaced thin beds, the coals are resolved as two distinct events in the migrated seismic section.

Introduction

This paper investigates the whether seismic AVO analysis can resolve the difference between wet and dry coals. Petrophysical log data from a well near Alder Flats, Alberta in the Pembina Field are used to model the AVO response of a prospective CBM reservoir. The coals belong to the Ardley Coals which possess high inherent water saturation. The AVO response of the wet coals and that of hypothetical dry coals are modelled in order to determine if the response is sufficiently distinct to discriminate the two saturation conditions in seismic data.

Petrophysical Logs

The well logs from a well in the study area show three coal seams here identified as Ardley 1, 2, and 3. Ardley 1 is at a depth of 414 metres and is approximately 9 m thick. Ardley 2 is at a depth of 404 m and is 4 metres thick. Ardley 3 is at a depth of 356m and is 9m thick. All are considered thin beds for seismic analysis. Additionally, Ardley 2 and 3 are closely spaced which will make them challenging to resolve as separate events in seismic.

et it Flow – 2007 CSPG CSEG Convention L607

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