Holistic and Mechanism-Focused Evaluation of Low Salinity Waterflood in Carbonate Reservoirs ?Fluid-Fluid Interaction Screening and Core Floods

Abstract

This paper demonstrates a holistic and mechanism-focused approach to select the reservoir with the highest potential for low salinity water (LSW) EOR applied to a carbonate oil field in the UAE. This approach combines fluid-fluid interaction tests and core floods to systematically and cost-effectively cover all potential combinations of high potential reservoir and optimum salinity of LSW. The subject carbonate field is considered to become the first offshore LSW application in UAE. A total of four stock tank oil (STO) samples were collected from three target sub-layers (L1 / L2 / L3) and one reference sub-layer (U1). Brine samples were prepared representing formation water (FW), sea water (SW), and LSW (diluted to 1-20% SW). As the first step, fluid-fluid interaction tests were conducted to select the highest potential target sub-layer for LSW EOR. Micro-dispersion ratios (MDR), which represent water content increment by the oil-water interfacial reaction, were used as the indicator of the reservoir fluid with the most positive EOR potential. During fluid-fluid tests, no micro-dispersion (MD) was formed for FW/SW. MDR increase was observed only for LSW. The MDR comparison revealed STO-L1, L2 and L3 as positive while reference STO-U1 as negative. A clear increase of MDR was found between 2-3% SW for STO-L1 and L2. This indicates that there is a certain threshold salinity triggering the radical oil-water interfacial reaction. The results showed 3-4% SW was the minimum effective SW dilution ratio followed by more effective ratio: 2% SW. Consequently, MDR method enabled us to cost-effectively select the most positive oil STO-L2 and the optimum LSW salinity 1% SW. After selecting the target sub-layer and optimum LSW salinity based on MDR comparison, two core floods (secondary & tertiary modes) were performed using reservoir core. An identical composite reservoir core aged to the original oil wet condition (i.e., core reused after solvent cleaning and re-aged to avoid uncertainties associated with difference in core samples) was used. The result revealed +3%-IOIP recovery by tertiary LSW injection to secondary SW. Re-use of identical core allowed for a quantitative apple-to-apple comparison. The geochemical analysis of effluent (ions: Na+, Ca2+, Mg2+, Cl-, and pH variations) revealed ion-diluting behavior (due to high salinity connate water mixed with LSW) as more pore volume was injected. All the behaviors reaching down to the cut-off concentrations were consistent with a trigger timing of oil recovery profile increase that represented MD formation as evidence of LSW effect. It should be highlighted that this approach was also effectively utilized to quickly screen and evaluate effective additives to LSW to boost the EOR effect. In our case, after confirming that the addition of diethyl ketone (DEK) to LSW doubled MDR compared with pure LSW, core flooding was conducted to reveal significant incremental tertiary recovery of +10%-IOIP after secondary SW flooding.