Organization, flow impact and modeling of natural fracture networks in a karstified carbonate bitumen reservoir: an example in the Grosmont Formation of the Athabasca Saleski leases, Alberta, Canada

Abstract The Grosmont Formation in Alberta contains one of the largest hydrocarbon accumulations of a carbonate reservoir in the world. It is also a highly fractured reservoir, where natural fracture networks have a key bearing on production and final recovery. The present fracture study focused on the Grosmont C and D units of the Saleski leases, where a steam injection pilot for the Grosmont was initiated in 2010 by Laricina Energy Ltd. and its partner, Osum Oil Sands Corp. This study aimed at characterizing the types, scales and organization of fractures in the reservoir units in order to build a representative fracture model and derive corresponding fracture properties to be used in dual porosity dynamic simulations. Detailed core and borehole image analyses revealed that fracturing in the Grosmont C and D units is organized into four sets of metre-scale joints and isotropic, centimetre-scale dissolution-related cracks. The joint density is controlled by facies. Dynamic data analysis revealed a link between mud losses, rock dissolution and facies types, especially in the Middle D unit. It also emphasized the fracture contribution to overall reservoir permeability. All of these findings were integrated to build a multiscale fracture model which, once dynamically calibrated, allowed computing fracture porosity, permeability tensor and matrix block sizes. These outputs are critical for thermal dual porosity dynamic simulations.