Publications

Factors Affecting Hydrocarbon and Water Mobility in Shales

Bryan, Charles R.; Brady, Patrick V.

In hydrofractured shales, elevated formation pore pressures drive movement of fluids toward hydrofractures, but other forces are equally important. Connate waters are saline to hypersaline, and have much higher osmotic potentials than the dilute hydrofracturing/treatment waters. Moreover, very low water saturations and small matrix pore sizes produce high in situ capillary potentials. Strong capillary and osmotic potentials drive matrix imbibition of dilute fracking fluids, and the resulting counterflow of hydrocarbons may account for a significant fraction of initial hydrocarbon yields. However, strong capillary and osmotic pressures can result in formation damage through water blocking or sanding, adversely affecting hydrocarbon production. In oil shales, imbibition of aqueous fracking fluids provides access to matrix mineral surfaces, allowing chemical amendments in process fluids to modify oil adhesion and improve oil mobility. While gas sorption in shales is physisorption, oil adhesion is controlled by surface complexation and is affected by the surface chemistry of the oil and of the minerals, and the chemistry of the intervening water layer. Shale solid surfaces are likely dominated by illite and kerogen; oil‐illite adhesion is controlled by electrostatics. Oil adhesion to the mineral matrix is favored by lower salinities and low pH.