Abstract

Conventional analysis on core plugs from unconsolidated heavy oil sands is challenging to perform. Sometimes doubts exist about whether the results are representative of the in-situ rock fabric. A digital rock pilot study was designed to provide unique data to understand the rock and allow independent verification of laboratory data, such as calculated petrophysical properties, characterization of damage associated with coring and preservation, direct imaging of oil distribution and pore-wall wettability, and characterization of pore types, clays, grains and grain contacts.

The pilot study presents successful cryogenic helical X-ray micro-computed tomography (microCT) of preserved plugs maintained in the frozen state throughout the imaging process. Those 3D images were processed to calculate porosity and absolute permeability. Direct imaging of oil distribution by 3D microCT, with X-ray contrasting agents to highlight the oil with complementary characterization of pore-wall wettability by secondary electron Field Emission Scanning Electron Microscopy (FESEM) is presented to confirm observations from the 3D microCT imaging. Finally, Back scattered SEM (BSEM) imaging and automated, quantified mineral mapping are presented.

The 3D tomograms from cryogenic microCT imaging were analysed based on morphological characteristics to identify damaged zones, representing >30% of the volume. The calculated absolute permeability of those 3D tomograms ranged from 8.50D to 24.8D across the plug. The undamaged region had distinctly lower kabs (average = 9.71D) than the most damaged region (average = 21.6D), compared to measurements by a traditional laboratory on adjacent screened and cleaned plugs reported as >20D. The oil saturation from direct microCT imaging was 76.3%, compared to conventional core analysis of oil saturation on a sister plug (62.1%). The 3D images show oil filling most of the open pores and the pore-filling and porous minerals. Furthermore, the oil appears to be in direct contact with the mineral grains, indicating that oil is the wetting phase. Wettability imaging by FESEM confirmed that the majority of the mineral surfaces are oil-wet but the layers of asphaltenes are thin. The mineral composition is dominated by quartz (81%) and feldspar (16%) with some minor clay (1.7%) and carbonate (0.3%). The mineralogy indicates that the sample is likely to be unreactive to steam and/or chemical stimulants.

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