Page 128 - Synthetic Fuels Handbook

114 CHAPTER FOUR

Thus, changes in the fractional composition can occur due to oxidation of the bitumen
during separation, handling, or storage. Similar chemical reactions (oxidation) will also
occur in heavy oil giving rise to the perception that the sample is bitumen.

4.3.2 Properties
The specific gravity of bitumen shows a fairly wide range of variation. The largest degree of
variation is usually due to local conditions that affect material lying close to the faces, or expo-
sures, occurring in surface tar sand deposits. There are also variations in the specific gravity of
the bitumen found in beds that have not been exposed to weathering or other external factors.
A very important property of the Athabasca bitumen (which also accounts for the suc-
cess of the hot water separation process) is the variation of bitumen density (specific gravity)
of the bitumen with temperature. Over the temperature range 30 to 130°C (86–266°F) the
bitumen is lighter than water; hence (with aeration) floating of the bitumen on the water is
facilitated and the logic of the hot water process is applied.
The API gravity of known U.S. tar sand bitumen ranges downward from about 14° API
(0.973 specific gravity) to approximately 2°API (1.093 specific gravity). Although only a
general relationship exists between density (gravity) and viscosity, very low gravity bitu-
men generally has very high viscosity. For instance, bitumen with a gravity of 5° or 6° API
can have viscosity up to 5 million centipoises. Elements related to API gravity are viscosity,
thermal characteristics, pour point, hydrogen content, and hydrogen-carbon ratio.
It is also evident that not only are there variations in bitumen viscosity between
the major Alberta deposits, but there is also considerable variation of bitumen viscos-
ity within the Athabasca deposit and even within one location. These observations are,
of course, in keeping with the relatively high proportions of asphaltenes in the denser,
highly viscous samples, a trait that appears to vary not only horizontally but also verti-
cally within a deposit.
The most significant property of bitumen is its immobility under the conditions of tem-
peratures and pressure in the deposit. While viscosity may present an indication of the immo-
bility of bitumen, the most pertinent representation of this property is the pour point. (ASTM
D-97) which is the lowest temperature at which oil will pour or flow when it is chilled without
disturbance under definite conditions. When used in consideration with reservoir temperature,
the pour point gives an indication of the liquidity of the heavy oil or bitumen and, therefore,
the ability of the heavy oil or bitumen to flow under reservoir conditions.
Thus, Athabasca bitumen with a pour point of 50 to 100°C (122–212°F) and a deposit
temperature of 4 to 10°C (39–50°F) is a solid or near solid in the deposit and will exhibit
little or no mobility under deposit conditions. Similar rationale can be applied to the
Utah bitumen where pour points of 35 to 60°C (95–140°F) have been recorded for the
bitumen with formation temperatures on the order of 10°C (50°F) also indicate a solid
bitumen within the deposit and therefore immobility in the deposit. On the other hand,
the California oils exhibit pour points on the order of 2 to 10°C (35–50°F) at a reservoir
temperature of 35 to 38°C (95–100°F) indicates that the oil is in the liquid state in the
reservoir and therefore mobile.
Irrespective of the differences between the various tar sand bitumen the factor that they
all have in common is the near-solid or solid nature and therefore immobility of the bitumen
in the deposit. Conversely, heavy oil in different reservoirs has the commonality of being
in the liquid state and therefore mobility in the reservoir.
In the more localised context of the Athabasca deposit, inconsistencies arise because
of the lack of mobility of the bitumen at formation temperature. For example, the propor-
tion of bitumen in the tar sand increases with depth within the formation. Furthermore, the
proportion of asphaltenes in the bitumen or asphaltic fraction (asphaltenes plus resins) also