Page 131 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 117

weight of bitumen, and about 50 percent by weight of water. After bitumen extraction, the
tailings are pumped to a settling basin. Coarse tailings settle rapidly and can be restored to
a dry surface for reclamation. Fine tailings, consisting of slow-settling clay particles and
water, are more problematic.
The U.S. tar sands have received considerably less attention than the Canadian deposits.
Nevertheless, approaches to recover the bitumen from U.S. tar sands have been made. An
attempt has been made to develop the hot water process for the Utah sands. The process dif-
fers significantly from that used for the Canadian sands due to the oil-wet Utah sands contrast-
ing to the water-wet Canadian sands. This necessitates disengagement by hot water digestion
in a high shear force field under appropriate conditions of pulp density and alkalinity. The
dispersed bitumen droplets can also be recovered by aeration and froth flotation.

4.5.2 In Situ Processes
The other aboveground method of separating bitumen from tar sand after the mining opera-
tion involves direct heating of the tar sand without previous separation of the bitumen.
Thus, the bitumen is not recovered as such but is an upgraded product. Although several
processes have been proposed to accomplish, the common theme is to heat the tar sand to
thermally decompose the bitumen to produce a volatile product with the coke remaining
on the sand.
In general, the viscous nature of the bitumen and its immobility in the deposits has precluded
other forms of recovery. However, bitumen recovery from deep deposits is not economical by a
mining method. Therefore the bitumen viscosity must be reduced in situ to increase the mobility
of bitumen to flow to wellbores that bring the bitumen to the surface.
Bitumen viscosity can be reduced in situ by increasing reservoir temperature or by
injecting solvents. Steam-based thermal recovery is the primary recovery method for heavy
oil in the Cold Lake and Peace River areas. Various steam-based methods have been shown
to be inefficient for bitumen but more recently a method known as steam-assisted gravity
drainage (SAGD) has been applied to the Athabasca tar sand with success.
In the process, a pair of horizontal wells, separated vertically by about 15 to 20 ft is
drilled at the bottom of a thick unconsolidated sandstone reservoir. Steam is injected into
the upper well. The heat reduces the oil viscosity to values as low as 1 to 10 cP (depending
on temperature and initial conditions) and develops a steam chamber that grows vertically
and laterally. The steam and gases rise because of their low density, and the oil and con-
densed water are removed through the lower well. The gases produced during SAGD tend
to be methane with some carbon dioxide and traces of hydrogen sulfide.
To a small degree, the noncondensable gases tend to remain high in the structure, filling
the void space, and even acting as a partial insulating blanket that helps to reduce vertical
heat losses as the chamber grows laterally. At the pore scales, and at larger scales as well,
flow is through counter-current, gravity-driven flow, and a thin and continuous oil film is
sustained, giving high recoveries.
Operating the production and injection wells at approximately the same pressure as
the reservoir eliminates viscous fingering and coning processes, and also suppresses water
influx or oil loss through permeable streaks. This keeps the steam chamber interface rela-
tively sharp, and reduces heat losses considerably. Injection pressures are much lower than
the fracture gradient, which means that the chances of breaking into a thief zone, an insta-
bility problem which plagues all high-pressure steam injection processes, such as cyclic
steam soak, are essentially zero.
Thus, the SAGD process, as for all gravity-driven processes, is extremely stable because
the process zone grows only by gravity segregation, and there are no pressure-driven instabili-
ties such as channeling, coning, and fracturing. It is vital in the SAGD process to maintain a