The abundant hydrocarbon resources in low-permeability formations are now technically accessible due to advances in drilling and completion of multi-lateral/multi-fractured horizontal wells. However, measurement and modeling of petrophysical properties, required for reserve estimation and reservoir-engineering calculations are the remaining challenges for the development of tight formations. In particular, characterizing wettability (wetting affinity) of tight rocks is challenging due to their complex pore structure, which can be either in hydrophobic organic materials or in hydrophilic inorganic materials.

We conduct comparative and systematic imbibition experiments on ten binary core plugs from the Montney tight gas formation, which is an enormous tight gas fairway in the Western Canadian Sedimentary Basin. Both contact angle and imbibition data indicate that the formation has a stronger affinity to oil than to water. However, the ratio between oil and water uptake of these samples is usually higher than what capillary-driven imbibition models predict. This discrepancy can be explained by the strong adsorption of oil on the surface of a well-connected organic pore-network that is partly composed of degraded bitumen. We also define a wettability index based on the equilibrium oil and water uptake of the binary samples. Oil wettability index is positively correlated with Total Organic Carbon (TOC) and clay content of the rocks which increase with depth from the Upper Montney to the Lower Montney.

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