Uniform and mixed wetting in square capillaries

Square capillaries provide a better model of the pore space in rocks than d o capillary tubes of circular cross-section because they have corners where wetting fluid can be retained. We report experimental observations of imbi bition of an aqueous phase into square capillary tubes with uniform and mix ed wetting. Each capillary had an oil-wet end to ensure that water imbibed in one direction only. Swelling of are menisci in the corners of the square tubes and formation and movement of main terminal menisci were observed du ring water imbibition. Clean, strongly water-wet capillaries were studied w ith water displacing decane or air. Wetting was altered by exposing clean c apillaries to crude oil or to decane with polar additives. Uniform changes in wetting were effected by Filling the capillary with a single phase. Capi llaries exposed first to brine, then drained with crude oil, became mixed-w et. The rates and amounts of oil produced are reported. For crude oil-treat ed capillaries, a consistent trend of decreased rate and extent of oil prod uction with increased aging time emerged. Wetting alteration by polar and a cid/base interactions was consistent with previously proposed interaction m echanisms based on contact angle observations. (C) 1999 Elsevier Science B. V. All rights reserved.