Smart Water”as a wettability modifier in chalk：the effect of salinity and ionic composition.docx

Published on Web 03/26/2010“Smart Water” as a Wettability Modifier in Chalk: The Effect of Salinity and Ionic CompositionS. Jafar Fathi,* T. Austad, and S. StrandDepartment of Petroleum Engineering, Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, NorwayReceived November 6, 2009. Revised Manuscript Received March 7, 2010Seawater can improve the water wetness of chalk at high temperatures, which improves the oil displacementbyspontaneousimbibitionofwater.Itisexperimentallyverifiedthattheinteractionbetween Ca2t, Mg2t, and SO42- at the chalk surface will displace adsorbed carboxylic acids and increase the water wetness.Inthiswork,theeffectofsalinityandioniccompositionofsmartwateronoilrecoverywasstudied at different temperatures, 100, 110, and 120 C. The ultimate oil recovery was compared using seawater as thebasefluid.WhenNaClwasremovedfromseawater,boththeimbibitionrateandoilrecoveryincreased in comparison to seawater at the temperatures tested. At 110 and 120 C, the oil recovery from seawater depletedinNaClincreasedbyabout10%oforiginaloilinplace(OOIP)comparedtoseawater.Adecrease inoilrecoveryofabout5%ofOOIPwasobservedwhenincreasingtheamountofNaClinseawater4times. A systematic decrease in oil recovery was observed when using seawater diluted with distilled water as imbibing fluid. Imbibition tests at 110 C showed that the water-wet fraction increased 29% for seawater depleted in NaCl compared to 11% for ordinary seawater. Diluted seawater to 10000 ppm did not change wetting conditions at 110 C. The results confirmed that not only is the concentration of the active ions Ca2t, Mg2t, and SO42- important for wettability alteration to take place but also the amount of nonactivesalt,suchasNaCl,hasanimpactonthewettabilityalterationprocess,whichisdiscussedasadoublelayer effect at the chalk surface. No significant improvement in the ultimate oil recovery was observed during forced displacement by modified seawater.IntroductionIt is well-documented tha