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Effective processing of nonrepeatable 4-D seismic data to monitor heavy oil SAGD steam flood at East Senlac, Saskatchewan, Canada

PanCanadian Petroleum Limited, Calgary, Alberta, Canada

Steven Weber and Rodney Couzens

Sensor Geophysical Limited, Calgary, Alberta, Canada

Corresponding author: G. Li, guoping_li@ pcp.ca

The first 20% of the full text of this article appears below.

The concept of time-lapse (or 4-D) seismic monitoring is straightforward: A baseline 3-D survey describing initial reservoir conditions and a subsequent monitor survey (or surveys) recorded after conditions have changed are calibrated, compared, and the intersurvey differences analyzed and interpreted in terms of net variations in pore fluid saturation, pressure, or temperature inside a hydrocarbon reservoir.

Seismic monitoring thus depends largely upon whether "repeatability" of surveys is achievable. Ideally, the difference between surveys should contain nothing but the changes in reservoir properties. However, it is almost impossible to obtain perfectly repeatable (or identical) time-lapse seismic surveys in the real world. Ross and Altan (1997) report that even a "zero-time repeatability test" (in which two offshore 3-D surveys were shot only one day apart by the same crew using identical equipment and survey geometry in an area of no production-related dynamic influences) could yield visually coherent energy on difference sections, if the processing sequence was not optimal.

Therefore, uniform processing of time-lapse surveys is crucial for successful 4-D reservoir interpretation, especially when there are large differences in acquisition or in ambient recording conditions. An important component of uniform 4-D data processing is cross-equalization to remove artificial effects caused by nonreservoir factors, such as changes in acquisition and/or processing parameters, so that time-lapse 3-D data will be identical everywhere except within the reservoir zone (where dynamic changes should be expected).

This article presents a case study of effective 4-D seismic data processing. Time-lapse seismic surveys monitoring a heavy oil thermal EOR site in west Saskatchewan, Canada, were uniformly processed by a sequence including: spatial realignment, amplitude balancing, and cross-equalization. This case involved three 3-D seismic surveys with significantly different parameters—different types of source (Vibroseis, weight-drop, and dynamite), varying orientations in geometry, and varied spatial parameters such as line spacing and bin size. . . . [Full Text of this Article]