On wettability of shale rocks

• Increase in pressure and temperature increases the contact angle of brine/shale system.
• Bivalent ions exhibit higher contact angle compared to monovalent ions.
• Effect of pressure and temperature on contact angle is more pronounced at high ionic concentrations.
• A physical model was developed to get insight into variation of contact angle.

The low recovery of hydraulic fracturing fluid in unconventional shale reservoirs has been in the centre of attention from both technical and environmental perspectives in the last decade. One explanation for the loss of hydraulic fracturing fluid is fluid uptake by the shale matrix; where capillarity is the dominant process controlling this uptake. Detailed understanding of the rock wettability is thus an essential step in analysis of loss of the hydraulic fracturing fluid in shale reservoirs, especially at reservoir conditions. We therefore performed a suit of contact angle measurements on a shale sample with oil and aqueous ionic solutions, and tested the influence of different ion types (NaCl, KCl, MgCl2, CaCl2), concentrations (0.1, 0.5 and 1 M), pressures (0.1, 10 and 20 MPa) and temperatures (35 and 70 °C). Furthermore, a physical model was developed based on the diffuse double layer theory to provide a framework for the observed experimental data.Our results show that the water contact angle for bivalent ions is larger than for monovalent ions; and that the contact angle (of both oil and different aqueous ionic solutions) increases with increase in pressure and/or temperature; these increases are more pronounced at higher ionic concentrations. Finally, the developed model correctly predicted the influence of each tested variable on contact angle. Knowing contact angle and therefore wettability, the contribution of the capillary process in terms of water uptake into shale rocks and the possible impairment of hydrocarbon production due to such uptake can be quantified.