Page 124 - Synthetic Fuels Handbook

110 CHAPTER FOUR

attributed to the relative lack of mineral cement (chemically precipitated material that binds
adjacent particles together and gives strength to the sand, which in most sandstone occupies
a considerable amount of what was void space in the original sediment).
Permeability is a measure of the ability of a sediment or rock to transmit fluids. It is, to a
major extent, controlled by the size and shapes of the pores as well as the channels (throats)
between the pores; the smaller the channel, the more difficult it is to transmit the reservoir
fluid (water, bitumen). Fine-grained sediments invariably have a lower permeability than
coarse-grained sediments, even if the porosity is equivalent. It is not surprising that the
permeability of the bitumen-free sand from the Alberta deposits is quite high. On the other
hand, the bitumen in the deposits, immobile at formation temperatures [approximately 4°C
(39°F)] and pressures, actually precludes any significant movement of fluids through the
sands under unaltered formation conditions.
For the Canadian tar sands, bitumen contents from 8 to 14 percent by weight may be
considered as normal (or average). Bitumen contents above or below this range have been
ascribed to factors that influence impregnation of the sand with the bitumen (or the bitumen
precursor). There are also instances where bitumen contents in excess of 12 percent by weight
have been ascribed to gravity settling during the formative stages of the bitumen. Bitumen
immobility then prevents further migration of the bitumen itself or its constituents.
The bitumen content of the tar sand of the United States varies from 0 to as much as 22 percent
by weight. There are, however, noted relationships between the bitumen, water, fines, and mineral
contents for the Canadian tar sands. Similar relationships may also exist for the United States tar
sands but an overall lack of study has prevented the uncovering of such data.
While conventional crude oil and heavy oil either flows naturally or is pumped from
the ground, tar sand must be mined and the bitumen removed from the sand and water. Tar
sand currently represents 40 percent of Alberta’s total oil production and about one-third
of all the oil produced by Canada. By 2005, oil sands production is expected to represent
50 percent of Canada’s total crude oil output and 10 percent of North American produc-
tion. Although tar sands occur in more than 70 countries, the two largest are Canada and
Venezuela, with the bulk being found in four different regions of Alberta, Canada: areas
of Athabasca, Wabasha, Cold Lake, and Peace River. The sum of these covers an area of
2
nearly 77,000 km . In fact, the reserve that is deemed to be technologically retrievable
today is estimated at 280 to 300 billion barrels, which is larger than the Saudi Arabia crude
oil reserves. The total reserves for Alberta, including oil not recoverable using current
technology, are estimated at 1700 to 2500 billion barrels.

4.3 CHEMICAL AND PHYSICAL PROPERTIES
OF TAR SAND BITUMEN

Tar sand bitumen is a naturally occurring material that is frequently found filling pores and
crevices of sandstone, limestone, or argillaceous sediments or deposits where the perme-
ability is low. Bitumen is reddish brown to black in color and occurs as a semisolid or solid
that can exist in nature with no mineral impurity or with mineral matter contents that exceed
50 percent by weight.
Tar sand bitumen is extremely susceptible to oxidation by aerial oxygen. The oxidation
process can change the properties (such as viscosity) to such an extent that precautions need
to be taken not only in the separation of the bitumen from the sand but also during storage
(Wallace, 1988; Wallace et al., 1988a, 1988b).
Bitumen is a high-boiling resource with little, if any, of the constituents boiling below 350°C
(662°F). In fact, the boiling range may be approximately equivalent to the boiling range of an
atmospheric residuum that is produced as a refinery product (Tables 4.2 and 4.3).