Article pubs.acs.org/EF
Low-Salinity-Surfactant Enhanced Oil Recovery (EOR) with a New Surfactant Blend: Effect of Calcium Cations Hamid Hosseinzade Khanamiri,*,† Meysam Nourani,‡ Thomas Tichelkamp,‡ Jan Åge Stensen,§ Gisle Øye,‡ and Ole Torsæter† †
Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway ‡ Ugelstad Laboratory, Department of Chemical Engineering, NTNU, N-7491 Trondheim, Norway § Exploration and Reservoir Technology, SINTEF Petroleum Research, N-7491 Trondheim, Norway ABSTRACT: Coreflooding experiments on aged Berea sandstone cores were performed to study the effect of divalent cations on the low-salinity-surfactant enhanced oil recovery (EOR). In the experiments where the core samples were aged for 4 weeks, replacement of a small amount of sodium with calcium in the injected low-salinity surfactant did not lead to higher tertiary recovery. However, the effects on wettability alteration and relative permeabilities were substantial. For the experiments with longer aging duration of 7 weeks, addition of calcium to the injected low salinity surfactant led to better oil recovery and the impact on wettability alteration was strong. Further addition of calcium led to lower oil recovery. Results of the injection experiments were discussed based on interfacial tension (IFT), surfactant adsorption, end-point relative permeabilities, and contact angles. A part of discussion was also dedicated to the effect of calcium on the secondary low-salinity water (LSW) injection. Although the oil recoveries by LSW injection in the absence and presence of calcium are similar, calcium causes late oil mobilization during low-rate LSW injection.
1. INTRODUCTION Surfactant flooding is one of the major chemical enhanced oil recovery (EOR) methods. Surfactants can either reduce the oil− water interfacial tension (IFT) or modify the wettability and increase oil recovery.1 The residual oil saturation (Sor) after surfactant flooding can be correlated to IFT using the dimensionless capillary number, Nc =
Recently, there have been attempts to study the performance of surfactant flooding at which the tertiary surfactant injection is a surfactant solution with low salinity; and it is injected after a secondary LSW. In 2010, Alagic and Skauge reported high tertiary oil recovery by surfactant injection after establishing a low saline environment by injection of low salinity water in core flooding experiments. They also observed lower tertiary oil recovery by surfactant without a low salinity preflush.10 They also reproduced similar results in another work.11 In 2014, Johannessen and Spildo observed that reduction in residual oil saturation by injection of low-salinity surfactant (LSS) is greater than in regular surfactant injection experiments.12 Johannessen and Spildo (2013)13 and Khanamiri et al. (2015)14 recently reported similar recoveries for LSS and optimal salinity surfactant flooding while the capillary number in optimal salinity was roughly 2 orders of magnitude higher than that in LSS flooding. These results suggest that the rock−fluid interaction in LSS is stronger than in optimal salinity surfactant flooding, such that it compensates for the higher IFT of LSS. The complexity of the LSS process has also driven studies of fundamental measurements with the objective of obtaining a better understanding of fluid−fluid and rock-fluid interactions. In 2014, Spildo et al. showed that, although ultralow IFTs (
