Methods and apparatus for enhanced recovery of viscous deposits by thermal stimulation

Method and apparatus for enhanced recovery of subterranean deposits. A heating fluid circulates in a concentric tubing assembly which attaches to a downhole heat exchanger. A convertible fluid descends to the downhole heat exchanger in the concentric tubing assembly where it converts to vapor by transfer of heat from the heating fluid. The vapor can then be used to liquefy viscous subterranean deposits. A feed control valve controls the rate at which convertible fluid enters the downhole heat exchanger. Scale produced by the vaporization of the convertible fluid is purged by a purging valve into the well sump.

BACKGROUND OF THE INVENTION 
This invention relates to methods and apparatus for recovery of viscous oil 
deposits and in particular to the method disclosed by Klinger, U.S. Pat. 
No. 4,641,710 which is hereby incorporated by reference herein. 
Klinger, U.S. Pat. No. 4,641,710, describes a downhole heat exchanger which 
generates vapor to liquefy viscous oil deposits. A surface heater located 
at the wellhead heats a heating fluid which is then pumped down a closed 
tubing to the oil-bearing strata where the tubing ends in a "u-turn" 
before ascending back to the surface heater. A convertible fluid such as 
water is flashed on the hot tubing just above the "u-turn" to generate 
vapor. The vapor continues to absorb heat along the lower portion of the 
"u-turn" before entering the oil-bearing strata. 
This prolonged heating of the vapor ensures that the vapor, as it enters 
the oil-bearing strata, is of very high quality or even superheated 
depending on the relative rates of the heating and convertible fluids. 
Gondouin, U.S. Pat. No. 5,085,275, describes twin horizontal drainholes 
which operate in a cyclic "huff and puff" mode through the use of a 
three-way steam valve section. A surface-mounted steam boiler generates 
steam which is injected down a tubing in the well to the three-way valve 
section. The valve section directs steam to one of the horizontal 
drainholes which then functions in the "puff" mode creating a hot mobile 
oil zone around the drainhole as a result of the injected steam. The valve 
then switches so that the drainhole functions in the "huff" mode, 
withdrawing the hot mobile oil. At the same time, the opposite drainhole 
operates in the "puff" mode. 
Gondouin also describes tubing arrangements within the borehole which 
reduce heat loss from the steam injection tubing into the cold rocks which 
surround the well casing. In one embodiment, both the steam injection line 
and the production line carrying the heated oil are suspended within the 
gas-filled well casing. Because the production line contains the heated 
oil resulting from the steam injection, it warms the gas within the casing 
and reduces the temperature gradient across the steam injection tubing. In 
another embodiment, the production tubing is concentric with the steam 
injection tubing, the steam tubing being inside the production tubing. 
This concentric tubing arrangement is suspended within the gas-filled well 
casing. 
TERMINOLOGY 
The following terms are used in this disclosure and claims: 
Subterranean Deposits: Underground viscous deposits which can be liquefied 
by thermal stimulation from a heated vapor. 
Surficial Layer: That layer of earth between the surface and the 
subterranean deposits. 
Borehole: The hole resulting from conventional drilling for underground 
deposits. 
Well casing: Tubing which fills and seals the wall of the borehole. 
Heating Fluid: A suitable fluid for supplying heat to create vapor which 
can liquefy the subterranean deposits. 
Convertible Fluid: A suitable fluid which is converted to vapor by heat 
exchange from the heating fluid in order to liquefy the subterranean 
deposits. 
Concentric Tubing Assembly: Concentrically arranged tubing which carries 
the heating fluid and the convertible fluid to a downhole heat exchanger. 
Downhole Heat Exchanger: Apparatus located in the borehole within or 
adjacent to the subterranean deposits wherein the convertible fluid is 
converted to vapor by heat exchange from the heating fluid. 
SUMMARY OF THE INVENTION 
This invention features a downhole heat exchanger which generates vapor to 
liquefy viscous deposits. A heating fluid is heated by a surface-mounted 
surface heater to a temperature sufficient for downhole conversion at the 
heat exchanger of a convertible liquid to vapor. The heating fluid 
descends to the heat exchanger and ascends back to the surface heater in a 
concentric tubing. 
In one embodiment, the heating fluid, typically molten sodium chloride, 
descends to the heat exchanger in an insulated inlet tubing. The molten 
salt ascends from the heat exchanger to the surface in an outlet tubing 
concentric with and containing the inlet tubing. Other heating fluids 
which are acceptable include oil, Dow Therm, or water. 
The convertible fluid, preferably water, descends to the heat exchanger for 
vaporization in an feed tubing concentric with and containing the outlet 
tubing. Other suitable convertible fluids include diesel oil or gas oil. 
The entire concentric assembly is suspended in the low-pressure gas-filled 
well casing. This suspension reduces heat loss from the feed tubing to the 
cold rocks surrounding the well casing. The concentric assembly offers 
several other advantages as well. 
First, unlike the method disclosed by Klinger, U.S. Pat. No. 4,641,710, 
only the inlet tubing need be insulated. Because the insulated tubing is 
at least five times more expensive than bare tubing, this represents a 
major cost savings over that design. 
Second, the arrangement of the feed tubing concentrically containing the 
uninsulated outlet tubing allows the convertible fluid to be efficiently 
pre-heated before entering the downhole heat exchanger. This pre-heating 
of the convertible fluid occurs using the surface of the outlet tubing 
alone with the convertible fluid and the heating fluid in an efficient 
counter-current flow. 
Third, because this concentric tubing assembly provides for efficient 
pre-heating of the convertible fluid, the design of the heat exchanger is 
simplified. The heat exchanger now needs only provide the latent heat of 
vaporization, the necessary sensible heat having been acquired as the 
convertible fluid descends the length of tubing towards the downhole heat 
exchanger. The necessary heat exchange surfaces in downhole heat exchanger 
are smaller than in the previous method disclosed by Klinger, U.S. Pat. 
No. 4,641,710, which again lowers the manufacturing costs. 
Other features and advantages of the invention will be apparent from the 
following description of the preferred embodiment thereof, and from the 
claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The earth formation 5 shown in FIG. 1 includes a subterranean deposit 10 
below a surficial layer 12 topped by a surface 15 which typically is the 
surface of the earth. 
Extending through the surficial layer 12 into the subterranean deposit 10 
is a borehole 18 which can be formed by conventional oil exploration 
drilling techniques. In usual operation, borehole 18 is filled or encased 
by a tubular well casing 20. 
Within borehole 18, a concentric tubing assembly 19 is suspended from a 
well head 22. Concentric tubing assembly 19 then descends to a downhole 
heating apparatus 25 wherein vapor 30 is generated by transfer of heat 
from a heating fluid 32, which preferably is a molten salt, to a 
convertible fluid 35, preferably water. 
Heating fluid 32 enters an inlet tubing 40 at the well head 22 and descends 
to downhole heating apparatus 25. Inlet tubing 40 is insulated by 
insulation 42. At downhole heating apparatus 25, inlet tubing 40 connects 
to a heat exchanger tubing 60 within a steam collector portion 65 of the 
downhole heating apparatus 25. Heat from heat exchanger tubing 60 
vaporizes convertible fluid 35 within steam collector portion 65. Vapor 30 
enters the steam collector tubing 70 near a shell 75 so that the steam is 
maintained at high quality or even superheated by heat from the 
downward-extending heat exchanger tubing 60. Vapor 30 can then be used to 
liquefy a subterranean deposit 10 by a conventional steam flood method or 
by the huff and puff technique. 
After passing through downhole heating apparatus 25 in heat exchanger 
tubing 60, return heating fluid 45 ascends borehole 18 in the an outlet 
tubing 50 which contains insulated inlet tubing 40. At surface 15, return 
heating fluid 45 is reheated in a surface heater (not shown) and pumped 
back down insulated inlet tubing 40 as heating fluid 32. 
The same surface heater can be used to preheat convertible fluid 35 within 
a conventional economizer tubing (not shown) before pumping down a feed 
tubing 80 to downhole heating apparatus 25. Feed tubing 80 contains outlet 
tubing 50. Unlike inlet tubing 40, outlet tubing 50 is not insulated. In 
this way, convertible fluid 35 is continually and efficiently heated 
within feed tubing 80 by the still-hot return heating fluid 45 using as 
the heat exchange surfaces the wall of outlet- tubing 50 alone. Because 
this heat exchange continues until convertible fluid 35 enters downhole 
heating apparatus 25 , downhole heating apparatus 25 need only provide the 
latent heat of vaporization, the necessary sensible heat being provided by 
concentric tubing assembly 19. In turn, downhole heating apparatus 25 
design is simplified and production costs lowered because heat exchanger 
tubing 60 can be shorter as it need only provide the latent heat of 
vaporization. 
Feed tubing 80 requires no insulation because its heat loss through the 
well casing 20 is reduced by suspension the within low-pressure gas-filled 
borehole 18. Thus, the only insulation required is on inlet tubing 40. 
A feed valve 31 controls the rate of convertible fluid 35 into downhole 
heating apparatus 25. Feed valve 31 responds to the pressure differences 
between the convertible fluid 35 at the base of feed tubing 80 and the 
vapor pressure within the steam collector 65 portion of downhole heating 
apparatus 25 so that vapor quality is maintained at a high value. 
Scale buildup on downward extension tubing 60 is reduced because of the 
narrow diameter of this tubing which causes the scale to periodically 
slough off. This sloughed-off scale then builds up at the base of heating 
apparatus 25. A purging valve 85 is periodically opened to drain this 
accumulated scale into an oil sump 90 of the well. In addition, 
conventional scale removing chemicals can be added to the hot water 50 at 
the surface before pumping to the heating apparatus 25. 
The foregoing description illustrates specific applications of the 
invention. Other useful applications of the invention which may be a 
departure from the specific description will be apparent to those skilled 
in art. Accordingly, the present invention is not limited to those 
examples described above.