Page 123 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 109

in situ recovery techniques, whereby the bitumen is separated from the sand in situ and
produced to the surface through wells drilled into the oil sands reservoir.
In order to accomplish this, the tar sand properties are of extreme importance and are
outline below.

4.2.1 Mineralogy
The mineralogy of tar sand deposits is also worthy of note as it does affect the potential for
recovery of the bitumen. Usually, more than 99 percent by weight of the tar sand mineral
is composed of quartz sand and clays. In the remaining 1 percent, more than 30 minerals
have been identified, mostly calciferous or iron based. Particle size ranges from large grains
(99.9 percent is finer than 1000 μm) down to 44 μm (325 mesh), the smallest size that can
be determined by dry screening. The size between 44 and 2 μm is referred to as slit; sizes
below 2 μm (equivalent spherical diameter) are clay.
The Canadian deposits are largely unconsolidated sands with a porosity ranging up to
45 percent and have good intrinsic permeability. However, the deposits in the United States,
in Utah, range from predominantly low-porosity, low-permeability consolidated sand to, in
a few instances, unconsolidated sands. In addition, the bitumen properties are not condu-
cive to fluid flow under normal reservoir conditions in either the Canadian or United States
deposits. Nevertheless, where the general nature of the deposits prohibits the application of
a mining technique (as in many of the United States deposits), a nonmining technique may
be the only feasible bitumen recovery option.
By definition, tar sand is a mixture of sand, water, and bitumen with the sand component
occurring predominantly as quartz. The arrangement of the sand, water, and bitumen has been
assumed to be an arrangement whereby each particle of the sand is water-wet and a film of
bitumen surrounds the water-wetted grains. The balance of the void volume is filled with bitu-
men, connate water, or gas; fine material, such as clay, occurs within the water envelope.
One additional aspect of the character of Athabasca tar sands is that the sand grains are not
uniform in character. Grain-to-grain contact is variable and such a phenomenon influences
attempts to repack mined sand, as may be the case in studies involving bitumen removal from
the sand in laboratory-type in situ studies. This phenomenon also plays a major role in the
expansion of the sand during processing where the sand to be returned to the mine site might
occupy 120 to 150 percent of the volume of the original as-mined material.
The tar sand mass can be considered a four-phase system composed of solid phase
(siltstone and clay), liquid phase (from fresh to more saline water), gaseous phase (natural
gases), and viscous phase (black and dense bitumen, about 8° API).
In normal sandstone, sand grains are in grain-to-grain contact but tar sand is thought to
have no grain-to-grain contact due to the surrounding of individual grains by fines with a water
envelope and/or a bitumen film. The remaining void space might be filled with water, bitumen,
and gas in various proportions. The sand material in the formation is represented by quartz and
clays (99 percent by weight), where fines content is approximately 30 percent by weight; the
clay content and clay size are important factors which affect the bitumen content.

4.2.2 Properties
Tar sand properties that are of general interest are bulk density, porosity, and permeability.
Porosity is, by definition, the ratio of the aggregate volume of the interstices between the
particles to the total volume and is expressed as a percentage. High-grade tar sand usually
has porosity in the range from 30 to 35 percent that is somewhat higher than the porosity
(5–25 percent) of most reservoir sandstone. The higher porosity of the tar sand has been