Physical properties of oils
Bulk physical properties of oils are often obtained for engineering purposes (e.g. fluid viscosity under reservoir conditions). Stabilised oil is required for most analyses.
Among the possible analyses are:
- Topping – loss of extreme volatiles determined by evaporation
- Density/gravity – major value determinant
- Pour point – lowest flow temperature before gelling/solidification
- Viscosity – pipeline flow characteristics
- Wax content – assessment of propensity for wax precipitation in flow lines
The volume of stabilised oil required for each analysis differs – preferably 100 mL for viscosity/pour point, 20 mL for topping and 5 mL for density.
A rotary evaporator is used and ~1 mL of oil is weighted accurately into a small round bottom flask. The oil is evaporated for 15 min at 90°C. The oil is weighed again to determine mass lost. An aliquot of NSO-1 is run as a reference sample together with the topping series.
An Anton Parr instrument is used, calibrated using air and distilled water. All measurements are done at 15°C, with NSO-1 included in each batch as a reference. If the viscosity of the oil is very high at 15°C, a gravimetric method is used.
API gravity is calculated from the density (r in g/cm3) by:
API Gravity (°) = 141.5/r – 131.5
This analysis is performed at Intertek West Lab AS (Stavanger) using a modified ASTM D-97 method.
Viscosity of low volume crude oil is determined according to ASTM D1092. Prior to viscosity measurement, thermal beneficiation is performed. The stabilised crude oil is heated to about 60°C and rapidly (to minimise exposure to air and light, which may cause oxidation processes to form surface active species) transferred to a 1 L gas-tight stainless steel pre-treatment cell. The crude oil is topped with N2 and heated to 80°C for 2 h in order to dissolve wax (erasing the ‘memory’ of previous wax formation). Finally, the crude oil is cooled at a rate of 12.5°C/h to the test temperature. Apparent viscosity is then measured with a Physica MCR 501 rheometer. Viscosity at various shear rates may be determined at 40, 50 and 60°C.
Dynamic viscosity is determined according to ASTM D445 using a Cambridge Electromagnetic viscometer, calibrated with standards from Cannon Instruments over a range of 0.2 – 100 cP. Viscosity is determined by a cylindrical piston cycling in the viscometer tube (volume ~2 cm3) inclined at an angle of 45°.
This analysis is performed at Intertek West Lab AS (Stavanger).