AAPG Annual Convention and Exhibition

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Wettability Imaging of Unconventional Mudrock Reservoirs


Understanding wettability is key to optimizing oil recovery. Conventional methods of wettability characterization via core analysis are not feasible in mudrock (shale) reservoirs, owing to the severe difficulties in establishing well-defined initial states and performing fluid displacements in such matrix pore systems. An alternative method has been developed that utilizes high-resolution SEM imaging of fresh surfaces of preserved samples that have been mildly solvent cleaned to remove bulk fluids. In conventional reservoirs, asphaltene residues cling to oil-wet pore walls and can be distinguished (in secondary electron images at low voltage) by their distinctive nodular, nano-particulate film texture, while water-wet surfaces appear clean with no residues. In shale samples, the same characteristic textures are often seen, as well as thicker organic coatings interpreted to be bitumen. Examples from multiple shale reservoir formations will be shown, from a range of maturities and mineralogic compositions. Wettability is commonly seen to vary at the pore scale in shale formations, and assumptions that mineral pores remain water-wet can be misleading. The SEM technique also allows the same subarea of a shale sample to be imaged and re-imaged during a cleaning sequence to directly visualize the local removal of organics by harsher solvents. SEM surface imaging of some shale samples after cleaning will be compared to micro-CT imaging of their preserved state prior to cleaning, in which X-ray contrast enhancement techniques are employed to highlight the spatial distribution of in-place fluids at micron-millimeter scales and relations to wettability at these and finer scales.