D. K. Larue* and J. Hovadik**

*San Joaquin Valley Business Unit, Chevron Corporation, 9525 Camino Media, Bakersfield, CA 93311, USA.

Author for correspondence email: dkla@chevron.com

**Chevron Energy Technology Company, 6001 Bollinger Canyon Rd, San Ramon, CA 94583, USA.

To address uncertainty, models created for appraisal and development studies try to capture the full range of possible rock and fluid properties, and reservoir and structural architecture. Using experimental designs, each parameter can be varied to indicate how uncertainty for the parameter affects the outcome (for example, recovery after 30 years). In numerous models in appraisal and development studies, stratigraphic or reservoir architecture is not recognized as a significant uncertainty. This is a concern for two reasons. First, reservoir architecture is known to be an important parameter influencing oil production. It is puzzling that experimental design studies indicate it is not a significant uncertainty. Second, building geologically realistic earth models can involve months of time and can impact timelines for development projects. If reservoir architecture is in fact a non-significant uncertainty, perhaps detailed modelling could be postponed. The current study addresses in what situations reservoir architecture is important in appraisal and development studies. Reservoir architecture characteristics considered are net to gross, channel orientation, channel stacking pattern, channel sinuosity, channel width/thickness, type of architectural element, influence of shale units within channel fills, influence of channel drapes at the base of channels, presence of other facies, influence of valley fills, and sheet geometries. Two field characterizations were studied using the same workflow, and results are compared with results of conceptual modelling studies.

For studies of recovery uncertainty, it is shown here that models that look to be very different may have similar dynamic performances because of similar reservoir connectivities and path lengths. Moreover, it is shown that the elements of reservoir architecture investigated (channel width/thickness ratio, sinuosity, orientation, thickness, element type) do not control recovery efficiency to a large degree unless reservoir connectivity is impacted. Reservoir architecture may not be an important uncertainty in appraisal and development studies unless reservoir connectivity or tortuosity are affected. Although reservoir architecture may not represent a key uncertainty in appraisal and development studies, it may well be a significant uncertainty for well planning in mature fields.

Key words: reservoir architecture, connectivity, waterflooding, permeability heterogeneity, channel architecture, deep-water channel deposits, fluvial channel deposits, dynamic uncertainty.

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