--> The Importance of Detailed Flow Unit Assessment for Modeling Carbonate Reservoir Performance — Insight from the Bitumen-Saturated Grosmont Formation

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The Importance of Detailed Flow Unit Assessment for Modeling Carbonate Reservoir Performance — Insight from the Bitumen-Saturated Grosmont Formation

Abstract

The Upper Devonian Grosmont Formation (northern Alberta) contains the largest carbonate-hosted bitumen deposit in the world with an estimated 406 billion bbl in place. Reservoir quality is directly tied to meteoric alteration of the Devonian dolostones in the Mesozoic, when the formation was exposed for at least 30 million years, resulting in a mature karst landscape. Fracturing is intense and permeability in the reservoir units varies over four orders of magnitude. A steam injection pilot in the Grosmont Formation was in operation from 2010 to September 2015. Ten horizontal wells penetrated two reservoir units in the Grosmont; vuggy dolostones in the Grosmont C and stratiform breccia in the overlying Grosmont D unit. Seven vertical observation wells continually monitored pressure and temperature. Repeated high-resolution 3D seismic surveys, tied to core and petrophysical log data, provide a detailed characterization of steam injection into the Grosmont C reservoir unit and steam chamber development. Injection of steam and the nature of steam chamber development were controlled by the interplay between operational procedures, temperature, pressure, and reservoir parameters. There is a significant amount of vertical heterogeneity in reservoir units that influences performance and, to varying degrees, fractures enhance permeability and connectivity. Detailed core description has been integrated with FMI data, petrophysics, and outcrop analogues to define nine flow units in each of the Grosmont C and Grosmont D reservoirs and the properties of these flow units. In addition, a method of fracture characterization and a protocol for identifying thin but potentially significant networks of cavernous porosity have been developed. It has been found that controls on flow in the reservoir units exist at four levels: matrix properties that are linked to dolomitized primary depositional facies, vuggy porosity that enhances matrix properties, fractures and solution enhanced bedding planes, and high flow zones that represent networks of cavernous porosity. The high resolution framework of flow units with its variability in porosity, porosity-type distribution, and permeability captures the major and finer elements of heterogeneity in the two reservoirs. Realistic geologic modeling of the Grosmont needs to capture this framework and the different elements that control flow if plausible simulation and history matching results are to be created.