Experimental study of Crude oil/Brine/Rock interactions during low salinity Waterflooding

Several laboratory and oil field experiments demonstrated that brine salinity can have an influence on oil recovery. The first extensive study on the ability of low salinity waterflooding to become an EOR method started in the 90' [Jadhunandan and Morrow, 1991] and has been followed by several experimental campaigns to understand the mechanisms leading to additional recovery of oil. The objective of our study was, on one hand, to quantify the additional oil recovery by low salinity waterflooding using a well characterized system (Core Oil and Brine). On the other hand, from the different physicochemical measured parameters, to propose some explanations of the microscopic displacement improvement. Our experiments were held on a COBR system composed of a consolidated outcrop sandstone (DU3) with an average permeability (400-800 mdy) containing 10% of clays without kaolinite, a filtered crude oil (Oil A) containing 2.3 %w of asphaltenes, a connate high salinity brine of 50 g/l (90 %w NaCl, 10 %w CaCl₂) and different low salinity brines containing, or not, divalent ions. The results of secondary and tertiary experiments put in evidence additional oil recoveries ranging between 9 and 11% (OOIP) when changing the brine salinity from 50 g/l to 1 g/l and between 4 and 6% (OOIP) when changing from 1 g/l to 0,1 g/l. This oil recovery improvement, that is still accompanied by pH increase and permeability reduction, appears only below a salinity value threshold ranging between 2.5 and 1 g/l for our COBR system. Additional oil recovery was also observed whereas no divalent ion was present in the injection brine. The production increment is observed after injection of 2 to 3 PV of brine. This is due to a dispersive effect which is in coherence with the salinity value threshold. SEM observations of the cores show that pore walls are partially preserved by the residual oil, therefore clay particles release and production are prevented. pH increase appears to be a consequence rather a cause of low salinity mechanism. Permeability reduction was also observed without additional oil recovery. This is due to interparticle expansion as explained in DLVO theory. Clay particles release during one phase dispersion experiments and partial divalent ions retention when reducing salinity show the importance of ionic exchanges in the microscopic displacement of hydrocarbons. (author)