Drastic Modification of Carbonate Reservoir Permeability Architecture from Differential Bitumen Plugging - Effects on Field Development Options

Wagner, Paul D.¹
(1)Shell, Calgary, AB, Canada.

Pilot options are currently being planned for an ultra-large bitumen accumulation in Canada. Hydrocarbon resource is a 4o to 9o API bitumen. Reservoir rock is a highly fractured dolomite. Fractures range continuously from thin-bed centimetre-sized elements to 100m+ high fracture corridors and small faults.

Reservoir conditions are shallow and cold - about 300 to 400 meters depth and 10o C. Under ambient reservoir conditions, the bitumen has extremely high viscosity. The viscous bitumen is observed in core to be differentially plugging the matrix and fracture system.

In particular, vertical fracture permeability architecture is drastically degraded. Available core, log and dynamic information indicates the plugging pattern follows geomechanical stratigraphy - even in geographic areas of the field with large cross-cutting structural features. Fracture partitioning exists where fewer, large-aperture fractures are associated with brittle carbonate intervals, and abundant, small-aperture fractures are associated with more ductile carbonate and clastic beds. Clusters of fractures with smaller apertures appear to be much more prone to bitumen plugging than their larger cousins. Other factors at play such as matrix petrophysical properties appear to be less important in defining the overall bitumen plugging pattern in three dimensions.

Presence of extensive fracturing has a strong positive effect on expanding available development options. However, vertical permeability will drastically change during the life cycle of production as plugging bitumen is progressively produced from the fracture system. Modeling the changing permeability architecture through time is a difficult challenge. Effects of differential bitumen on a range of development options are discussed.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.