Cyclic steam stimulation (CSS) and steamflood performances are strongly influenced by shaly barriers and cemented baffles in the heavy oil reservoir. Marine shales do act as a permanent barrier in most conditions, while non-marine shales and diagenetically altered cemented siltstone baffles may not act as barrier under different operating and reservoir settings. In this paper an investigation was made to understand the impact of barriers and baffles on performance of a reservoir operating under CSS and steamflooding.

Out of several hundred wells drilled, seven isolated wells were selected for CSS on a trial basis. Two of them were completed in the Upper-A layer and three in the Upper-B layer. Marine cap shale as barrier exists on top of Upper-A while a cemented sandy siltstone baffle separates Upper-A and Upper-B layers. Wells completed in Upper-A performed better than those completed in Upper-B in the first cycle though sand thickness of Upper-A is much less than that of Upper-B.

Production performance of the CSS wells was analysed considering the geological nature of baffles and shale barrier, and from results obtained during steamflood experiments conducted on plugs from baffles, and it was concluded that the main reason for the better performance of Upper-A layer compared to Upper-B layer was the presence of impregnable cap shale barrier. Cap shale does not allow the steam to migrate upward, thus forcing it to remain in Upper-A layer while when injecting steam in the Upper-B layer, some of the steam passes through the baffle above it, resulting in poor production performance and higher steam oil ratio. This analysis suggests that the aspect of competent shale being barrier to steam and baffle not so effective barrier should be given due consideration during commercial application of the CSS and steamflooding processes in the field.

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