Page 133 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 119

(ES-SAGD) in which a hydrocarbon solvent (to facilitate complete or partial dissolution of the
bitumen and help reduce its viscosity) is mixed with the steam.
Alternative processes that are being reviewed for bitumen recovery (or atleast a deriva-
tive of the bitumen) from tar sand deposits include vapor-assisted extraction (VAPEX),
which is a new process in which the physics of the process are essentially the same as for
SAGD and the configuration of wells is generally similar. The process involves the injec-
tion of vaporized solvents such as ethane or propane to create a vapor-chamber through
which the oil flows due to gravity drainage. The process can be applied in paired horizontal
wells, single horizontal wells, or a combination of vertical and horizontal wells. The key
benefits are significantly lower energy costs, potential for in situ upgrading and application
to thin reservoirs, with bottom water or reactive mineralogy.
Because of the slow diffusion of gases and liquids into viscous oils, this approach,
used alone, perhaps will be suited only for less viscous oils although preliminary tests
indicate that there are micromechanisms that act so that the VAPEX dilution process is
not diffusion rate limited and the process may be suitable for the highly viscous tar sand
bitumen.
Nevertheless, VAPEX can undoubtedly be used in conjunction with SAGD methods
(qv). A key factor is the generation of a three-phase system with a continuous gas phase
so that as much of the oil as possible can be contacted by the gaseous phases, generating
the thin oil-film drainage mechanism. Vertical permeability barriers are a problem, and
must be overcome through hydraulic fracturing to create vertical permeable channels, or
undercut by the lateral growth of the chamber beyond the lateral extent of the limited bar-
rier, or baffle.
However, as with all solvent-based processes, there is the potential for solvent losses in
the reservoir. These can arise, for example, due to unknown fissures in the reservoir rock
as well as clay lenses to which the solvent will adhere.
In situ combustion in heavy oil reservoirs and bitumen deposits has been notoriously
difficult to control but make a comeback with a new concept. The THAI (toe-to-heel air
injection) process is based on the geometry of horizontal wells that may solve the problems
that have plagued conventional in situ combustion. The well geometry enforces a short flow
path so that any instability issues associated with conventional combustion are reduced or
even eliminated.
In situ conversion, or underground refining, is a promising new technology to tap the exten-
sive reservoirs of heavy oil and deposits of bitumen. The new technology (United States Patent
6016867; United States Patent 6016868) features the injection of high-temperature, high-quality
steam and hot hydrogen ore into a formation containing heavy hydrocarbons to initiate conver-
sion of the heavy hydrocarbons into lighter hydrocarbons. In effect, the heavy hydrocarbons
undergo partial underground refining that converts them into a synthetic crude oil (or syncrude).
The heavier portion of the syncrude is treated to provide the fuel and hydrogen required by the
process, and the lighter portion is marketed as a conventional crude oil.
Thus, below ground, superheated steam and hot hydrogen are injected into a heavy
oil or bitumen formation, which simultaneously produces the heavy oil or bitumen and
converts it in situ (i.e., within the formation) into syncrude. Above ground, the heavier
fraction of the syncrude is separated and treated on-site to produce the fuel and hydrogen
required by the process, while the lighter fraction is sent to a conventional refinery to
be made into petroleum products (United States Patent 6016867; United States Patent
6016868).
The potential advantages of an in situ process for bitumen and heavy oil include
(a) leaving the carbon-forming precursors in the ground, (b) leaving the heavy metals
in the ground, (c) reducing sand-handling, and (d) bringing a partially upgraded product
to the surface. The extent of the upgrading can, hopefully, be adjusted by adjusting
the exposure of the bitumen of heavy oil to the underground thermal effects.