Natural gas liquid (NGL), a mixture consisting primarily of ethane, propane, and butane, is an excellent enhanced oil recovery (EOR) solvent. However, NGL is typically about ten times less viscous than the crude oil within the carbonate or sandstone porous media, which causes the NGL to finger through the rock toward production wells resulting in low volumetric sweep efficiency in five-spot patterns or during a linear drive displacement. The viscosity of candidate polymeric NGL thickeners is measured with a windowed, close-clearance falling ball viscometer, and an expression for the average shear rate associated with this type of viscometer is derived. High molecular weight polydimethyl siloxane (PDMS, Mw 9.8 105) can thicken ethane, propane and butane, but the viscosity enhancement is very modest (e.g. a doubling of butane viscosity with 2% PDMS at 7MPa and 25°C), making field application of PDMS unlikely. A dilute concentration of a drag-reducing agent (DRA) poly-α-olefin that has an average molecular weight greater than 2.0 107 is more promising as a potential thickener for liquid butane, liquid propane and liquid or supercritical ethane. The DRA polymer, which is introduced as an extremely viscous 1% or 2% solution in hexane, is soluble in butane and propane at 25–60°C and concentrations up to at least 0.5wt% at pressures slightly above the vapor pressure of butane or propane. The DRA polymer is much more difficult to dissolve in ethane, however, requiring pressures of more than 20MPa. The DRA polymer is especially effective for thickening butane (e.g. a 4.8-fold viscosity increase at 25°C, 55.16MPa and 0.2wt% DRA). The DRA is less effective for increasing propane viscosity (e.g. a 2.3-fold viscosity increase at the same conditions), and even less effective for thickening ethane. In general, viscosity enhancement increases with decreasing temperature, increasing pressure, and an increase in the carbon number of the light alkane, which are reflective of increased NGL solvent strength at low temperature and high pressure. Practical application of DRA during EOR may be hindered, however, by the relatively high concentration (~5000ppm) of DRA polymer required for order-of-magnitude viscosity increases, very high pressure requirements for DRA dissolution if the ethane content of the NGL is high, and the large amount of hexane that would have to be introduced if the DRA polymer if it is introduced as a solution in hexane.
Journal of Petroleum Science and Engineering – Elsevier
Published: Sep 1, 2016
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