Downhole steam apparatus

A downhole steam apparatus for location within the casing of a well borehole to facilitate oil recovery. A housing adapted to be lowered into the borehole includes a combustor for mixing and burning fuel and air, and a heat exchanger having an array of water tubes exposed to the heated gases from the combustor for converting water into steam. The steam is injected downwardly into the borehole, and the spent gases pass into the annulus between the casing and the housing. An expansible packer seals off the annulus between the steam injection area and the spent gas injection area. Compressed air for combustion is supplied at the lower spent gas pressure. Various arrangements are disclosed for the water tube array in the heat exchanger.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a downhole steam apparatus for generating 
steam in situ to facilitate oil recovery from relatively deep wells. 
2. Description of the Prior Art 
Initial production from an oil well utilizes the pressure of gases in the 
oil formation. This is followed by pumping when the gas pressure 
diminishes. Eventually, even pumping is inadequate to produce acceptable 
quantities of oil and resort must be had to secondary recovery methods. 
These include thermal stimulation of the well by raising the temperature 
of the oil formation to lower the oil viscosity and enhance its flow. 
Various types of thermal stimulation have been utilized, including electric 
or hot water heaters, gas burners, in-situ combustion, and hot water or 
steam injection. Of these, steam injection has many advantages. 
Present systems for injecting steam are not effective in deep wells. In 
most such systems the steam is generated on the surface and piped down 
through the casing to the base of the borehole. In a deep well a 
considerable amount of heat is lost through the casing, and the 
temperature and quality of the steam is generally inadequate to 
effectively thermally stimulate formations at the base of the borehole. 
Prior art attempts to generate steam in-situ or downhole have been 
ineffective since combustion requires that the fuel and air be provided at 
the pressure of the steam discharged from the combustor. The size and 
complexity of air compressors required to provide such high pressure 
become economically prohibitive. 
An effective system of generating steam of high quality and temperature 
in-situ is desirable because flooding the formation with such steam has 
been found to significantly lower the flow resistance of the oil in the 
vicinity of the borehole, thereby enabling extraction of the displaced 
oil. The steam penetrates and heats the formation over a considerable 
distance, and consequently oil production is greatly improved in viscous 
oil-bearing sands from which pumping is impractical. 
SUMMARY OF THE INVENTION 
According to the present invention, a downhole steam apparatus is provided 
which includes a combustion section to which conduits are connected for 
providing fuel and an oxidizing fluid for mixing and burning. The 
apparatus includes a heat exchanger connected to the combustion section to 
receive the heated gases and convert water fed to a separate portion of 
the heat exchanger into steam. 
The spent gases from the heat exchanger are discharged into the annulus 
between the heat exchanger and the borehole casing, and thereafter pass to 
the surface. The steam generated in the heat exchanger is discharged 
downwardly into the base of the borehole for heating the adjacent oil 
formation. 
The apparatus includes a packer expansible against the casing to isolate 
the areas of steam injection and spent gases discharge so that the high 
pressures of the steam injection zone do not exist in the heated gas 
portion of the heat exchanger. Consequently, the compressed air or other 
oxidizing fluid can be supplied at the lower pressures existing in the 
combustor, rather than at the higher pressures of the injected steam. 
The heat exchanger includes an array of water tubes which may be 
longitudinally oriented to parallel the flow of heated gases, or spirally 
oriented about the heated gas chamber. Suitable baffle means are 
preferably incorporated in the heated gas chamber of the heat exchanger 
and in the water tubes to induce turbulent flow and improved heat 
exchange. 
Other objects and features of the present invention will become apparent 
from consideration of the following detailed description taken in 
connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 through 5, there is illustrated a downhole steam 
generator or apparatus 10 adapted to be inserted within the tubular casing 
12 of a well borehole. Steam is generated by combustion of fuel and an 
oxidizing fluid, such as diesel fuel and compressed air. Combustion takes 
place in a water cooled combustion chamber from which heated gases pass to 
a tubular heat exchanger. Water vaporization produces steam which is 
injected downwardly into the borehole to enhance oil recovery, as by 
decreasing the viscosity of oil in the borehole formation. 
The inner diameter of the casing 12 is typically 61/2 inches. Accordingly, 
the apparatus 10 is preferably made with an outside diameter of 
approximately 51/2 inches to define a space or annulus 14 between the 
apparatus 10 and the casing 12. 
The apparatus 10 comprises an assembly or housing 16 which includes a 
combustion section or combustor 18 and a heat exchanger section or heat 
exchanger 20 having a downward extension 22. The terms "upper" and "lower" 
refer to the orientation of the apparatus 10 in the borehole. 
In one suitable embodiment the combustor 18 is approximately six feet long. 
It is cylindrical and includes a plurality of water passages 24 which are 
closed at their upper ends except for a radially inwardly directed passage 
26 which connects the passages 24 to a water feed line or conduit 28 
extending to surface equipment (not shown). 
In addition to the water conduit 28, an oxygen conduit 30, a fuel conduit 
32, and an oxidizing fluid conduit 34 are also connected to the upper end 
of the combustor 18, the conduits 30, 32 and 34 extending into 
communication with an internal chamber 36 of the combustor 18. 
Diesel oil and compressed air are preferred combustion materials, but it 
will be apparent that other materials may be utilized if desired. 
The lower end of the combustor 18 includes a threaded, reduced diameter 
nozzle section 38, which mounts a suitable ignitor schematically indicated 
at 40. 
On start up of the apparatus 10, oxygen and fuel are fed into the chamber 
36 and ignited by operation of the ignitor 40. The particular form of 
ignitor 40 is not illustrated in detail because it does not form a part of 
the present invention. A suitable ignitor could be a spark plug or the 
like actuated by an electrical charge derived from electrical leads (not 
shown) extending to the surface. 
Once the apparatus 10 is started, oxygen flow is terminated and compressed 
air is fed to the system for combustion. The burning fuel and air pass 
through the central opening or nozzle of the section 38 and form a 
downwardly extending flame generally indicated at 42. 
The nozzle section 38 is threaded in fluid tight relation into a 
complemental central opening or inlet in the heat exchanger 20. The inlet 
opens into an elongated internal first portion or gas chamber 44 of the 
heat exchanger 20. In the embodiment illustrated, the heat exchanger 20 is 
approximately 36 feet long and includes a plurality of parallel, 
longitudinal water tubes 46 extending from the bottom end to approximately 
four feet from the upper end. The tubes 46 are approximately 0.5 inches in 
outside diameter, and have a wall thickness of approximately 0.065 inches. 
The upper ends of the tubes 46 are received within suitable openings in an 
annularly configured cylindrical header 48 which is mounted within the 
chamber 44. The opposite or lower ends of the tubes 46 are similarly 
received within a plurality of openings in a cylindrical header 50 which 
closes the lower end of the gas chamber 44. 
As generally indicated in FIGS. 2 through 4, the heat exchanger 20 includes 
a plurality of parallel, circumferentially arranged and longitudinally 
oriented water passages 52 in communication with the water passages 24 of 
the combustor 18. The lower ends of the passages 52 are reversely directed 
to admit water to the lower ends of every other one of the heat exchanger 
tubes 46. The upper ends of the tubes 46 are connected by passages 54, as 
seen in FIG. 4, to adjacent tubes 46. Thus, the water makes an upward pass 
through half the tubes 46, turns in the passages 54, and makes a second, 
downward pass through the other half of the tubes 46, from which the water 
passes to a plurality of steam discharge passages 56 formed in the header 
50. 
The circumferential arrangement of the tubes 46 about the cylindrical 
chamber 44 places them in thermal exchange relation with heated gases 
flowing downwardly through the chamber 44. The base or lower end of the 
chamber 44 is made conical to direct the spent gases radially outwardly 
into four spent gas passages 58 which, as seen in FIG. 5, extend radially 
outwardly and upwardly. The spent gases are thus discharged into the 
annulus 14 and pass upwardly to the surface. 
The heat exchanger 20 preferably includes baffles spaced along its length 
to cause the heated gases to follow circuitous flow paths which bring the 
gases into repeated, more prolonged contact with the peripheries of the 
tubes 46 for improved heat exchange. The baffles may include, for example, 
a plurality of circular plates or elements 60 having arcuate cut outs in 
their peripheries for welded connection to the radially inwardly oriented 
portions of the tubes 46. Alternating with the elements 60 are a plurality 
of doughnut or annularly shaped plates or elements 62 which are each 
characterized by a plurality of circumferential openings to receive the 
tubes 46, and a central opening to permit passage of the heated gases 
through the element 62. The elements 60 and 62 are longitudinally spaced 
apart along the length of the chamber 44 adjacent the tubes 46 and direct 
the flow of heated gases in a generally undulating, circuitous pattern. 
Each of the water tubes 46 also preferably includes baffles or internal 
flow directors in the form of spiral directors 64 which induce a 
turbulent, swirling water flow for heat transfer. 
FIG. 6 illustrates an alternative embodiment in which the water tube array 
takes the form of a helical coil 66 connected at its downstream or lower 
end to the water passages 52 by a circular passage 68 in a header 50a 
similar to the header 50 of the first embodiment. The opposite end of the 
coil 66 is reversely formed and extends downwardly through the center of 
the coil for connection to an opening 70 formed in the header 50a. The 
header 50a also includes radially outwardly directed passages 58a 
corresponding to the spent gas passages 58 of the first embodiment. 
Other forms of heat exchanger will suggest themselves to those skilled in 
the art, although the embodiment of FIG. 1 has been found to be 
particularly effective. 
The downwardly extending cylindrical extension 22 of the heat exchanger 20 
mounts a packer diagrammatically indicated at 74. The packer 74 is carried 
by the apparatus 10 for sealing engagement with the casing 12. Many 
suitable types of packers are known to those skilled in the art which are 
operative to expand against the casing and provide the desired fluid tight 
seal. These may include a fluid expansible type requiring a connection 
(not shown) to a fluid source such as the fluid conduit 34; or a thermally 
responsive type; or a type adapted to seat by an upward pulling upon the 
drill string; or a type which seats upon twisting of the drill string. The 
latter type is that which is diagrammatically indicated. 
In operation of the apparatus 10, after combustion has been initiated, as 
previously indicated, and the packer 74 is seated, heated gases are 
developed at a temperature of approximately 3200 degrees Farenheit. In 
passing through the four foot space between the nozzle section 38 and the 
header 48, the temperature drops to approximately 1650 degrees Farenheit 
by virtue of heat transfer, particularly by hot gas radiation, to the 
water passages 52 which surround the zone of the flame 42. This preheats 
the water before it reaches the tubes 46 and also cools the walls of the 
apparatus 10 to avoid undesirable overheating. 
On passing through the remainder of the chamber 44, the heated gases give 
up further heat to the preheated water in the tubes 46. Water passing 
upwardly through the tubes 46 is raised in temperature by the heated gas 
and begins to boil at the upper ends of these tubes. As the water reverses 
its path and flows downwardly through the other tubes 46, it vaporizes and 
is discharged as steam through the passages 56 and out of the discharge 
outlet 76 of the extension 22. The steam in this injection zone is at a 
pressure of approximately 2000 lbs. per square inch absolute. It is 
estimated that close to 90% of the heat released in the combustion process 
is recovered in the steam for a steam outlet quality of approximately 70%. 
The spent gases at the lower end of the heat exchanger 20 leave the 
passages 58 at a temperature of approximately 700 degrees Farenheit. This 
is low enough to avoid high temperature damage to the adjacent walls of 
the casing 12. Further heat transfer occurs as the spent gases pass 
upwardly through the annulus 14. Heat passes to the adjacent heat 
exchanger portions defining the water passages 52, and also then to the 
surrounding earth formation. The temperature of the spent gases at the 
upper end of the apparatus 10 is thereby reduced to approximately 432 
degrees Farenheit, which is an acceptable level of temperature exposure 
for electrical and other connections in that area. 
The relatively high pressure steam injection zone is isolated by the packer 
74 from the relatively low pressure spent gases injection zone in the 
annulus adjacent the passages 58. Consequently, compressed air for the 
combustor 18 need only be supplied at a pressure sufficient to overcome 
the back pressure existing in the spent gases injection zone, which is 
approximately 250 to 300 psia. Consequently, much less elaborate and 
expensive air compressor equipment is needed, compared to the air 
compressor equipment necessary if air had to be supplied at the 2000 psia 
which exists in the steam pressure injection zone adjacent the discharge 
outlet 76. 
The in-situ generation of steam by the present apparatus 10 completely 
eliminates the heat losses which characterize those systems utilizing 
surface steam generators. Moreover, the described arrangement of heated 
gas and water passages minimizes thermal gradients, and consequently 
structural stresses, which significantly prolongs service life and reduces 
maintenance costs. 
Various modifications and changes may be made with regard to the foregoing 
detailed description without departing from the spirit of the invention.