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We conduct spontaneous imbibition experiments using different fluids (deionized, DI, water and brines) and different media (unwashed and washed shale powder) to study the wetting behavior of the shale samples from the Horn River Basin (HRB), a massive unconventional gas play in the Western Canadian Sedimentary Basin. As expected, unwashed shale powder imbibes DI water faster than brine. Surprisingly, washing the powders results in faster imbibition of DI water. The imbibition of DI water into washed shale powders, which have a reduced soluble/leachable ion content, cannot be fully explained by osmotic effects. We explain the observed imbibition profiles using the electrostatic interaction theory. We measure the ion concentration of the brines by ICP-MS analysis and determine the ionic strength, I, of the in situ formed brine. We also calculate the characteristic thickness of electrostatic double layer, κ–1, formed around the surface of charged shale powders. The results indicate that the imbibition rate depends on the κ–1value of the in situ formed brine. Electrostatic interaction is part of the disjoining pressure which is not considered in the Young–Laplace equation. A higher κ–1 value enlarges the electrostatic interaction range, which results in formation of a thicker hydration shell around the surface of the shale powder and increases the imbibition rate.

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