Low Salinity Waterflooding Effect: Crude Oil/Brine Interactions as a Recovery Mechanism in Carbonate Rocks

Abstract

The objective of this research was to study the formation of water-in-oil microdispersions due to crude oil-brine interactions as a novel mechanism that improves oil recovery with low salinity waterflooding in carbonate formations. This novel interpretation was studied by intergrating petrographic and spectroscopic observations, dynamic interfacial tension measurements, thermogravimetrical analyses and core flooding techniques. To address this interpretation, this study examines crude oil-brine interactions and the compositional changes that occur in the crude oil when it is in contact with brines of various concentration. Three different brine concentrations ranging from formation water salinity (FWS, 160,000ppm) to sea-water salinity (SWS, 32,895 ppm) and low salinity water (LSW, 2,000ppm) were contacted with a crude oil from the limestone formations of the Lansing-Kansas City group at reservoir temperature of 40 °C. Core flooding was performed on a properly aged Indiana limestone using the same crude oil and brine compositions as the reservoir with the effluent samples being taken for analysis. Another core flooding experiment conducted on a non-aged Indiana limestone rock hightlighted the potential of fluid-fluid interaction as a dominant recovery mechanism. Amott tests show the contribution of wettability alteration due to aging on improved oil recovery using low salinity waterflooding. A quantitative and qualitative analysis of the LKC crude oil-brine samples was conducted using Environmental Scanning Electron Microscopy (ESEM) imaging, Fourier Transform Infrared (FTIR) spectroscopy and Thermal Gravimetric Analysis (TGA) techniques This workflow helped with determination of the compositional changes that occurred when the crude oil came into contact with the various brine concentrations. They also provided a visual evidence to the fluid-fluid interaction phenomena experienced in low salinity waterflooding process. The data show that LSW brine caused a greater change in the crude oil composition when they were contacted as compared to SWS brine and FWS. FWS created almost no change to the crude oil composition indicating the limited effect of FWS on the crude oil. These compositional changes in crude oil when contacted with LSW were attributed to the formation of water-in-oil microdispersions within the crude oil phase. FTIR data also showed that at brine concentration levels above 6000 ppm, this phenomena was not experienced. Oil production data for non-aged limestone cores showed an improved recovery of about 5% and 3% for SWS and LSW brines, respectively. Although, wettability alteration effect was minimized by the use of non-aged cores, improved oil recovery was still evident. This was interpreted to represent the formation of water-in-oil micro-dispersions of about 50µm in diameter for LSW. The formation of the micro-dispersions is believed to increase the sweep efficiency of the waterflood by swelling and therefore blocking of the pore throats causing the low salinity brine sweeping the unswept pore spaces. SWS brine also experienced improved oil recovery when used as a displacing fluid. This observation was attributed to the change in dynaminc IFT measurement experienced using SWS brine as the continuous phase as compared to the use of LSW and FWS brines. This change caused a higher surface dilatational elasticity which leads to a suppression of the snap-off effect in coreflooding experiments hence causing improved oil recovery.