Experimental investigation of nonthermal enhanced oil recovery techniques for improving oil recovery on Alaska North Slope

Abstract

Exploitation of viscous and heavy oils on Alaska North Slope (ANS) requires nonthermal enhanced oil recovery (EOR) techniques. Currently, three nonthermal EOR methods, including solvent injection, low salinity water (LSW) flooding, and low salinity polymer (LSP) injection, have been proved to be useful on ANS. ANS viscous and heavy oils can be developed effectively by combining those three nonthermal EOR techniques. In this dissertation, lab experiments have been conducted to investigate the potential of the proposed hybrid nonthermal EOR techniques, including HSW (high salinity water)-LSW-softened LSW flooding, HSW-LSW-LSP flooding, CO₂-enriched LHS (light hydrocarbon solvent)-alternating-LSW flooding, LHS-alternating-LSW flooding, CO₂-enriched LHS (light hydrocarbon solvent)-alternating-LSP flooding, and LHS-alternating-LSP flooding, to improve ANS viscous oil recovery. Besides, the effect of essential clay minerals, including sodium-based montmorillonite (Na-Mt), calcium-based montmorillonite (Ca-Mt), illite, and kaolinite, on LSW flooding has been examined. In addition, the CO₂ influence on solvent-alternating-LSP flooding in enhancing ANS viscous oil recovery has been investigated. Furthermore, the blockage issue during CO₂-enriched LHS-alternating-LSP flooding has been investigated, and its solution has been proposed and analyzed. The EOR potential of the proposed hybrid EOR techniques has been evaluated by conducting coreflooding experiments. Additionally, relative permeability, swelling property, zeta potential, interfacial tension (IFT), and pressure-volume-temperature (PVT) tests have been conducted to reveal the EOR mechanisms of the proposed hybrid EOR techniques. Moreover, water ion analysis of DI-water/natural-sand and DI-water/natural-sand/CO₂ systems has been carried out to reveal the complex reaction between CO₂, sand, and LSP solution. It was found that, compared to conventional waterflooding, all the proposed hybrid EOR techniques could result in better oil recovery potential. It was noticed that the presence of CO₂ in LHS could be more beneficial to the solvent-alternating-LSW/LSP flooding processes during the 1st cycle due to the greater effectiveness of oil viscosity reduction. In particular, severe blockage issue occurred when conducting CO₂-enriched LHS-alternating-LSP flooding using sand pack due to the polymer precipitation. Additionally, the calculated water relative permeabilities are much lower than the typical values, implying more complex interactions between the reservoir rock, heavy oil, and injected water. Moreover, comparing to HSW, LSW could further swell Na-Mt significantly, which may benefit LSW flooding by improving sweep efficiency since in-situ swelling of Na-Mt has the potential to block the higher permeable water-flooded zone and divert the injected brine to lower permeable and unswept area. Comparing to Na-Mt, LSW couldn't swell Ca-Mt and illite further, whereas kaolinite was incapable of swelling in both HSW and LSW. Furthermore, about 60 mole% of solvent could be dissolved into the ANS viscous oil at target reservoir condition, resulting in oil swelling and viscosity reduction effects, which provided better microscopic displacement efficiency. Although the presence of CO₂ in LHS had a negative impact on the oil swelling effect, the influence on the oil viscosity reduction was positive. In addition, reducing the salinity of water could generate more negative zeta potential values on the surface of clay minerals and sand, making it more water wet. Besides, IFT of oil/LSW system is higher than that of oil/HSW system, indicating that IFT reduction is not an EOR mechanism of LSW flooding in our proposed hybrid EOR techniques. Additionally, after introducing CO₂ to the DIwater/natural-sand system, the concentration of multivalent cations was increased, which may be responsible for the polymer precipitation. The blockage issue could be solved by injecting LSW as a spacer between CO₂-enriched LHS injection and LSP injection.