Chemical Methods for Ugnu Viscous Oils

• Main Author: Mohanty, Kishore
• Other Authors: United States. Department of Energy.
• Format: Report
• Language: English
• Published: University of Houston System 2012
• Subjects: 02 Petroleum; Milne Point; Psi; Beaufort Sea; north slope; permafrost; Alaska
• Online Access: https://doi.org/10.2172/1048103; https://digital.library.unt.edu/ark:/67531/metadc838566/

The North Slope of Alaska has large (about 20 billion barrels) deposits of viscous oil in Ugnu, West Sak and Shraeder Bluff reservoirs. These shallow reservoirs overlie existing productive reservoirs such as Kuparuk and Milne Point. The viscosity of the Ugnu reservoir on top of Milne Point varies from 200 cp to 10,000 cp and the depth is about 3300 ft. The same reservoir extends to the west on the top of the Kuparuk River Unit and onto the Beaufort Sea. The depth of the reservoir decreases and the viscosity increases towards the west. Currently, the operators are testing cold heavy oil production with sand (CHOPS) in Ugnu, but oil recovery is expected to be low (< 10%). Improved oil recovery techniques must be developed for these reservoirs. The proximity to the permafrost is an issue for thermal methods; thus nonthermal methods must be considered. The objective of this project is to develop chemical methods for the Ugnu reservoir on the top of Milne Point. An alkaline-surfactant-polymer (ASP) formulation was developed for a viscous oil (330 cp) where as an alkaline-surfactant formulation was developed for a heavy oil (10,000 cp). These formulations were tested in one-dimensional and quarter five-spot Ugnu sand packs. Micromodel studies were conducted to determine the mechanisms of high viscosity ratio displacements. Laboratory displacements were modeled and transport parameters (such as relative permeability) were determined that can be used in reservoir simulations. Ugnu oil is suitable for chemical flooding because it is biodegraded and contains some organic acids. The acids react with injected alkali to produce soap. This soap helps in lowering interfacial tension between water and oil which in turn helps in the formation of macro and micro emulsions. A lower amount of synthetic surfactant is needed because of the presence of organic acids in the oil. Tertiary ASP flooding is very effective for the 330 cp viscous oil in 1D sand pack. This chemical formulation includes 1.5% of an alkali, 0.4% of a nonionic surfactant, and 0.48% of a polymer. The secondary waterflood in a 1D sand pack had a cumulative recovery of 0.61 PV in about 3 PV injection. The residual oil saturation to waterflood was 0.26. Injection of tertiary alkaline-surfactant-polymer slug followed by tapered polymer slugs could recover almost 100% of the remaining oil. The tertiary alkali-surfactant-polymer flood of the 330 cp oil is stable in three-dimensions; it was verified by a flood in a transparent 5-spot model. A secondary polymer flood is also effective for the 330 cp viscous oil in 1D sand pack. The secondary polymer flood recovered about 0.78 PV of oil in about 1 PV injection. The remaining oil saturation was 0.09. The pressure drops were reasonable (<2 psi/ft) and depended mainly on the viscosity of the polymer slug injected. For the heavy crude oil (of viscosity 10,000 cp), low viscosity (10-100 cp) oil-in-water emulsions can be obtained at salinity up to 20,000 ppm by using a hydrophilic surfactant along with an alkali at a high water-to-oil ratio of 9:1. Very dilute surfactant concentrations (~0.1 wt%) of the synthetic surfactant are required to generate the emulsions. It is much easier to flow the low viscosity emulsion than the original oil of viscosity 10,000 cp. Decreasing the WOR reverses the type of emulsion to water-in-oil type. For a low salinity of 0 ppm NaCl, the emulsion remained O/W even when the WOR was decreased. Hence a low salinity injection water is preferred if an oil-in-water emulsion is to be formed. Secondary waterflood of the 10,000 cp heavy oil followed by tertiary injection of alkaline-surfactants is very effective. Waterflood has early water breakthrough, but recovers a substantial amount of oil beyond breakthrough. Waterflood recovers 20-37% PV of the oil in 1D sand pack in about 3 PV injection. Tertiary alkali-surfactant injection increases the heavy oil recovery to 50-70% PV in 1D sand packs. As the salinity increased, the oil recovery due to alkaline surfactant flood increased, but water-in-oil emulsion was produced and pressure drop increased. With low salinity (deionized) water, the oil recovery was lower, but so was the pressure drop because only oil-in-water emulsion was produced. Secondary waterflood of the 10,000 cp heavy oil in 5-spot sand packs recovers 30-35% OOIP of the oil in about 2.5 PV injection. Tertiary injection of the alkaline-surfactant solution increases the cumulative oil recovery from 51 to 57% OOIP in 5-spot sand packs. As water displaces the heavy oil, it fingers through the oil with a fractal structure (fractal dimension = 1.6), as seen in the micromodel experiments. Alkaline-surfactant solution emulsifies the oil around the brine fingers and flows them to the production well. A fractional flow model incorporating the effect of viscous fingering was able to match the laboaratory experiments and can be used in reservoir simulators. The chemical techniques look promising in the laboratory and should be tested in the fields.