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Effect of Calcium in Pore Scale Oil Trapping by Low Salinity Water and Surfactant EOR at Strongly Water Wet conditions: In Situ Imaging by X-Ray Microtomography Hamid Hosseinzade Khanamiri, Ole Torsæter, and Jan Åge Stensen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01236 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016
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Energy & Fuels
Effect of Calcium in Pore Scale Oil Trapping by Low Salinity Water and Surfactant EOR at Strongly Water Wet conditions: In Situ Imaging by X-Ray Microtomography Hamid Hosseinzade Khanamiri1 *, Ole Torsæter1, Jan Åge Stensen2 1
Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and
Technology (NTNU), 7491 Trondheim, Norway; 2
Exploration and Reservoir Technology, SINTEF Petroleum Research, N-7491 Trondheim, Norway;
*
Corresponding author, Email address:
[email protected] ABSTRACT Interfacial tension (IFT) between crude oil and surfactant solution at low salinity is normally higher than IFT at optimal ionic strength. However it is possible to further reduce IFT by 1-2 orders of magnitude by changing the molar ratio of calcium to sodium cations (Ca2+/Na+) at a constant low ionic strength in the low salinity surfactant (LSS) solution. IFTs of 0.03mN/m and 0.1mN/m were measured at Ca2+/Na+ ratio of 0.005 and in absence of calcium in LSS solutions, respectively. Two flooding experiments using these fluid systems were performed on a strongly water wet Berea sandstone. The in situ core flooding in micro-CT showed that more oil was trapped in presence of calcium and the residual oil clusters were larger after LSS injection. The results showed that there was a small wettability shift from being strongly water wet toward being (weakly) water wet by low salinity surfactant (LSS) flooding both with and without calcium. The results also showed that this wettability shift was stronger in absence of calcium in the injected LSS, causing a reduction in the oil trapping. However, in
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the secondary low salinity water flooding, the residual oil saturation was smaller in presence of calcium. The cause of these behaviors is discussed in the relation of oil/surfactant and oil/water interfacial elastic modulus with initial rock wettability.
1. INTRODUCTION Enhanced oil recovery (EOR) by surfactants is a well-established method in the petroleum industry. In a surfactant flooding, oil is mobilized and recovered by reduction of the interfacial tension (IFT) between oil and water phases or changes in the rock wettability. Based on mainly the laboratory experiences, surfactant injection can lead to low residual oil saturation at ultralow IFT (
