Uniform and mixed wetting in square capillaries

Square capillaries provide a better model of the pore space in rocks than do capillary tubes of circular cross-section because they have corners where wetting fluid can be retained. We report experimental observations of imbibition of an aqueous phase into square capillary tubes with uniform and mixed wetting. Each capillary had an oil-wet end to ensure that water imbibed in one direction only. Swelling of arc menisci in the corners of the square tubes and formation and movement of main terminal menisci were observed during water imbibition. Clean, strongly water-wet capillaries were studied with water displacing decane or air. Wetting was altered by exposing clean capillaries to crude oil or to decane with polar additives. Uniform changes in wetting were effected by filling the capillary with a single phase. Capillaries exposed first to brine, then drained with crude oil, became mixed-wet. The rates and amounts of oil produced are reported. For crude oil-treated capillaries, a consistent trend of decreased rate and extent of oil production with increased aging time emerged. Wetting alteration by polar and acid/base interactions was consistent with previously proposed interaction mechanisms based on contact angle observations.