Abstract
Mixed wettability develops naturally on a pore scale in oil reservoirs after primary drainage. The invading oil fills pore interiors that become oil-wet by asphaltene deposition, while the residual water retreats into the pore corners, masking them and retaining their water wetness. This wettability alteration hinders oil mobilization during secondary waterflood. Therefore, a proper understanding of the conditions controlling pore-scale imbibition into mixed-wet pores may lead to a substantial increase in oil recovery from the portions of reservoir rocks bypassed during the original waterflood. We use selective silane coating to fabricate reservoir-representative mixed-wet capillaries with angular cross-sections. We validate our procedure on silica and glass substrates and characterize the mixed-wet surfaces by atomic force microscopy, scanning electron microscopy, and contact angle measurements. Subsequently, we investigate experimentally the invasion of water against air in mixed-wet, water-wet, and oil-wet square capillaries and compare our findings with the theoretical predictions of dynamic (Washburn, Szekely, and Bosanquet) and quasi-static [Mayer–Stowe–Princen (MSP)] meniscus-invasion models. None of the dynamic models for ducts of uniform wettability can fully describe our experimental data in mixed-wet capillaries. However, the experimental results agree with the predictions of MSP theory. We discuss the similarities and differences between experiment and theory and the reasons for the failure of the dynamic models. To our knowledge, this is the first direct experimental validation of MSP theory under mixed-wet conditions in such a controlled manner. We confirm the possibility of spontaneous piston-type imbibition with high (>90°) advancing contact angles into mixed-wet pores, given that the contact angle is lowered below a critical value that is a function of the pore geometry and water saturation. In oil reservoirs, injection of custom-designed brines would be required to change the contact angle to values below the imbibition threshold.