Modification of heavy-oil rheology via alkaline solutions

Abstract

Hamaca (Orinoco Belt, Venezuela) is considered here as a typical viscous and heavy oil whose rheology and recovery behavior is potentially modified by diluents and aqueous solutions of alkali. Viscosity of dry Hamaca crude oil, mixtures of Hamaca and n-decane, as well as emulsified crude oil was measured ex-situ and in-situ (sand pack) as a function of temperature and shear rate. Hamaca crude oil exhibits temperature-dependent rheological behavior. Between 30 and 50° C, it is slightly shear thinning, whereas from 55 to 80° C, properties are slightly shear-thickening. Shear stress sensitivity becomes greater as shear rate decreases. Crude oil rheological behavior in a sand pack (single-phase flow) has similar trends as bulk crude-oil viscometer measurements. The viscosity of the crude oil decreases most significantly as the concentration of diluent (decane) increases from 0 to 6 wt% but the magnitude of viscosity reduction is less as diluent concentration increases from 10% to 33%. After mixing crude oil with aqueous alkaline silicate and carbonate solutions, viscosity decreases markedly due to formation of oil in water (o/w) emulsions. For sodium hydroxide, however, oil viscosity increases because of formation of water in oil (w/o) emulsions. Given the encouraging changes in bulk oil properties, core flood recovery tests at different temperatures and flow rates were conducted. Alkaline flooding showed recovery of about 43% after 0.5 PV of injection and more than 60% with 5.6 PV of 1 wt% Na2CO3 solution. A significant fraction of the oil recovery occurred as o/w emulsion. The apparent relative permeability of the oil phase deviates markedly from the measurements for unemulsified oil.