Page 129 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 115

increases with depth within the formation that leads to reduced yields of distillate from the
bitumen obtained from deeper parts of the formation. In keeping with the concept of higher
proportions of asphaltic fraction (asphaltenes plus resins), variations (horizontal and vertical) in
bitumen properties have been noted previously, as have variations in sulfur content, nitro-
gen content, and metals content.
Nondestructive distillation data (Table 4.2) show that tar sand bitumen is a high-boiling
material. There is usually little or no gasoline (naphtha) fraction in bitumen and the major-
ity of the distillate falls in the gas oil–lubrication distillate range [greater than 260°C
(500°F)]. Usually, in excess of 50 percent by weight of tar sand bitumen is nondistillable
under the conditions of the test. On the other hand, heavy oil has a considerable proportion
of its constituents that are volatile below 260°C (500°F).

4.4 MINING TECHNOLOGY

Proposed methods for recovery of bitumen from tar sand deposits are based either on in situ pro-
cesses or on mining combined with some further processing or operation on the tar sands in situ.
The typical in situ recovery methods are not applicable to bitumen recovery because bitumen,
in its immobile state, is extremely difficult to move to a production well. Extreme processes
are required, usually in the form of a degree of thermal conversion that produces free-flowing
product oil that will flow to the well and reduce the resistance of the bitumen to flow. Tar sand
deposits are not amenable to injection technologies such as steam soak and steam flooding.
In fact, the only successful commercial method of recovering bitumen from tar sand deposits
occurs at the two plants in Alberta (Canada) and involves use of a mining technique.
The equipment employed at a tar sand mine is a combination of mining equipment and
an on-site transportation system that may (currently) either be conveyor belts and/or large
trucks. The mining operation itself differs in detail depending upon the equipment; bucket-
wheel excavators sit on benches; the draglines sit on the surface.
The Suncor (formerly Great Canadian Oil Sands Ltd.) mining and processing plant,
located 20 miles north of Fort McMurray, Alberta, started production in 1967. The Syncrude
Canada mining and processing plant, located 5 miles (8 km) away from the Suncor plant,
started production in 1978. In both projects, about half of the terrain is covered with muskeg,
an organic soil resembling peat moss, which ranges from a few inches to 23 ft (7 m) in
depth. The total overburden varies from 23 to 130 ft (7–40 m) in thickness.
Mining the Athabasca tar sands presents two major issues: in-place tar sand requires
very large cutting forces and is extremely abrasive to cutting edges, and both the equipment
and pit layouts must be designed to operate during the long Canadian winters at tempera-
tures as low as −50°C (−58°F).
There are two approaches to open-pit mining of tar sand. The first uses a few bucket-
wheel excavators and large draglines in conjunction with belt conveyors. In the second
approach, a multiplicity of smaller mining units of conventional design is employed.
Over time, different techniques have been used for oil sands mining. Suncor started
operations using bucketwheel excavators that discharged their loads onto conveyor belts.
The initial Syncrude operation used large draglines to remove oil sands ore from the mine-
face and place it in windrows from which bucketwheel reclaimers loaded it onto conveyor
belts for transportation to the extraction plant. Suncor and Syncrude have now retired their
bucketwheel and dragline-based mining systems.
Large mining trucks and power shovels were introduced to replace these early mining sys-
tems. By the early 1990s, Syncrude was mining about one-third of its ore using trucks and
shovels, while Suncor totally converted to a truck and shovel operation in 1993. Truck and
shovel mining is considerably more flexible and less prone to interruption of service than the