Abstract:
A method for producing viscous hydrocarbon formations involves the use of a downhole burner. The well undergoes a mild hydraulic fracturing process that limits the fractured zone to a relatively small dimension so as to avoid intersecting any drainage zones of adjacent wells. The operator pumps fuel, steam and oxygen to the burner, which bums the fuel, causing the flow of hot, gaseous fluids into the fractured zone. The steam delivered from the surface cools the burner and becomes superheated as it enters the fractured zone. The operator allows the fractured zone to soak, then produces the oil. After the production declines, the operator may repeat the fracturing to incrementally increase the fractured zone, then repeat the injection and soak cycles.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of provisional application Ser. No. 60/646,790 filed Jan. 25, 2005.  
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates in general to methods for producing highly viscous hydrocarbons, and in particular to injecting steam from a downhole burner into a fractured zone.  
       BACKGROUND OF THE INVENTION  
       [0003]     There are extensive viscous hydrocarbon reservoirs throughout the world. These reservoirs contain a very viscous hydrocarbon, often called “tar”, “heavy oil”, or “ultraheavy oil”, which typically has viscosities in the range from 3,000 to 1,000,000 centipoise when measured at 100 degrees F. The high viscosity makes is difficult and expensive to recover the hydrocarbon. Strip mining is employed for shallow tar sands. For deeper reservoirs, heating the heavy oil in situ to lower the viscosity has been employed.  
         [0004]     In one technique, partially saturated steam is injected into a well from a steam generator at the surface. The heavy oil can be produced from the same well that the steam is injected by allowing the reservoir to soak a selected time after the steam injection, then producing the well. The heavy oil can also be produced by means of a second well spaced apart from the injector well. When production declines, the operator repeats the process. A downhole pump may be required to pump the heated heavy oil to the surface. If so, the pump has to be pulled from the well each time before the steam is injected, then re-run after the injection.  
         [0005]     Another techniques uses two horizontal wells, one a few feet above and parallel to the other. Each well has a slotted liner. Steam is injected continuously into the upper well bore to heat the heavy oil and cause it to flow into the lower well bore. Other proposals involve injecting steam continuously into vertical injection wells surrounded by vertical producing wells.  
         [0006]     U.S. Pat. No. 6,016,867 discloses the use of one or more injection and production boreholes. A mixture of reducing gases, oxidizing gases, and steam is fed to downhole combustion devices located in the injection boreholes. Combustion of the reducing gas oxidizing gas mixture is carried out to produce superheated steam and hot gases for injection into the formation to convert and upgrade the heavy crude or bitumen into lighter hydrocarbons. The temperature of the superheated steam is sufficiently high to cause pyrolysis and/or hydrovisbreaking, which increases the gravity and lowers the viscosity of the hydrocarbon in situ. The &#39;867 patent also discloses fracturing the formation prior to injection of the steam. The &#39;867 patent discloses both a cyclic process, wherein the injection and production occur in the same well, and a continuous drive process involving pumping steam down boreholes of wells surrounding the producing wells. In the continuous drive process, the &#39;867 patent teaches to extend the fractured zones to adjacent wells.  
       SUMMARY OF THE INVENTION  
       [0007]     The well is fractured to create a fractured zone of limited diameter. The fractured zone extends from the well and preferably does not intersect any drainage or fractured zones of adjacent wells. A downhole burner is secured in the well. The operator pumps a fuel, which may be hydrogen, and oxygen in separate conduits down the well to the burner, and burns the fuel in the burner. The operator also pumps partially saturated steam from the surface into the well The steam flows into and cools the burner. The heat exchange creates superheated steam, which then flows into the fractured zone along with residual unburned fuel and other products of combustion.  
         [0008]     The unfractured formation surrounding the fractured zone impedes leakage of these gaseous products from the fractured zone. After injecting the steam and other gaseous products for a selected time, the operator allows the fractured zone to soak for a selected time. During the soak interval, the operator may intermittingly pump fuel and steam to the burner to maintain a desired amount of pressure in the fractured zone. After the soak interval, the operator opens valves at the wellhead to cause the hydrocarbon to flow into the borehole and up the well. The viscous hydrocarbon, having undergone pyrolysis and/or hydrovisbreaking during this process, flows to the surface for further processing. Preferably, the flow occurs as a result of solution gas created in the fractured zone from the steam and residual hydrogen. A downhole pump could also be employed,  
         [0009]     When production declines sufficiently, the operator may repeat the procedure of injecting steam and combustion products from the burner into the fractured zone. The operator may also fracture the formation again to enlarge the fracturing zone.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic illustrating a well and a process for producing heavy oil in accordance with this invention.  
         [0011]      FIG. 2  is a schematic illustrating the well of  FIG. 1  next to an adjacent well, which may also be produced in accordance with this invention.  
         [0012]      FIG. 3  is a schematic illustration of a combustion device employed with the process of this invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Referring to  FIG. 1 , well  11  extends substantially vertically through a number of earth formations, at least one of which includes a heavy oil or tar formation  15 . An overburden earth formation  13  is located above the oil formation  15 . Heavy oil formation  15  is located over an underburden earth formation  17 . The heavy oil formation  15  is typically a tar sand containing a very viscous hydrocarbon, which may have a viscosity from 3,000 cp to 1,000,000 cp, for example. The overburden formation  13  may be various geologic formations, for example, a thick, dense limestone that seals and imparts a relatively high fracture pressure to the heavy oil formation  15 . The underburden formation  17  may also be a thick, dense limestone or some other type of earth formation.  
         [0014]     As shown in  FIG. 1 , the well is cased, and the casing has perforations or slots  19  in at least part of the heavy oil formation  15 . Also, the well is fractured to create a fractured zone  21 . During fracturing, the operator pumps a fluid down the casing, which flows through perforations  19  and imparts a pressure against heavy oil formation  15  that is greater than the parting pressure of the formation. The pressure creates cracks within formation  15  that extend generally radially from well  11 , allowing flow of the fluid into fractured zone  21 . The injected fluid to cause the fracturing may be conventional, typically including water, various additives, and proppant materials such as sand or ceramic beads.  
         [0015]     The operator controls the rate of injection of the fracturing fluids and the duration of the hydraulic fracturing process to limit the extent or dimension of fractured zone  21  surrounding well  11 . Fractured zone  21  has a relatively small initial diameter or perimeter  21   a . The perimeter  21   a  of fractured zone  21  is limited such that it will not intersect any existing or planned fractured or drainage zones  25  ( FIG. 2 ) of adjacent wells  23  that extend into the same heavy oil formation  15 . Further, in the preferred method, the operator will later enlarge fractured zone  21  well  11 , thus the initial perimeter  21   a  should leave room for a later expansion of fractured zone  21  without intersecting drainage zone  25  of adjacent well  23 . Adjacent well  23  optionally may previously have undergone one or more of the same fracturing processes as well  11 , or the operator may plan to fracture adjacent well  23  in the same manner as well  11  in the future. Consequently, fractured zone perimeter  21   a  does not intersect fractured zone  25 . Preferably, fractured zone perimeter  21   a  extends to less than half the distance between wells  11 ,  23 . Fractured zone  21  is bound by unfractured portions of heavy oil formation  15  outside perimeter  21   a  and both above and below fractured zone  21 .  
         [0016]     A production tree or wellhead  27  is located at the surface of well  11 . Production tree  27  is connected to a conduit for directing a mixture of fuel and steam down well  11 , as indicated by the numeral  37  The fuel may be hydrogen, methane, syngas, or some other fuel. The fuel may be a gas or liquid. Preferably, the steam is partially saturated steam, having a water vapor content up to about  20  percent. The water vapor content could be higher, and even water could be pumped down well  11  in lieu of steam, although it would be less efficient. A wellhead  27  is also connected to a conduit for delivering oxygen down well  11 , as indicated by the numeral  39 . Preferably the fuel and steam  37  is delivered separate from the conduit that delivers oxygen  39 . The conduits for fuel and steam  37  and oxygen  39  may comprise coiled tubing or threaded joints of production tubing. One of the conduits could comprise the annulus in the casing of well  11 .  
         [0017]     A combustion device or burner  29  is secured in well  11  for receiving the flow of fuel and steam  37  and oxygen  39 . As illustrated in  FIG. 3 , a packer and anchor device  31  seals and secures burner  29  to the casing of well  11 . Burner  29  has a combustion chamber  33  surrounded by a jacket  35 . Fuel and steam  37  enter combustion chamber  33  for burning the fuel. In this embodiment, at least some of the steam flows through jacket  35 , as indicated by the arrows  41 . If the fuel is hydrogen, some of the hydrogen can flow through jacket  35  along with steam.  
         [0018]     Burner  29  ignites and burns at least part of the fuel, which creates a high temperature in burner  29 . Without steam or water as a coolant, the temperature would likely be too high for burner  29  to withstand over a long period. The steam flowing into combustion chamber  33  reduces that temperature. Also, preferably there is an excess of fuel flowing into combustion chamber  33 . The excess fuel does not burn, thus also lowers the temperature in combustion chamber  33 . Further, the steam and fuel  41  flowing through jacket  35  cools combustion chamber  33 . A downhole burner for burning fuel and injecting steam and combustion products into an earth formation is shown in U.S. Pat. No. 5,163,511.  
         [0019]     The steam and excess fuel lower the temperature within combustion chamber  33 , for example, to around 1600 degrees F., which increases the temperature of the partially saturated steam flowing through jacket  35  and through combustion chamber  33  to a superheated level. The gaseous product  43 , which comprises superheated steam, excess fuel and other products of combustion, exits burner  29  preferably from about 550 to 700 degrees F. The hot, gaseous product  43  flows into fractured zone  21 . The fractures within fractured zone  21  increase the surface contact area for these fluids to heat the formation and dissolve into the heavy oil to lower the viscosity of the oil and create solution gas to help drive the produced oil. The unfractured formation  15  is substantially impenetrable by the gaseous product  43  because the heavy oil or tar is not hot enough to be displaced. The surrounding portions of heavy oil formation  15  thus create a container around fractured zone  21  to impede leakage of hot gaseous product  43 .  
         [0020]     In the preferred method, the delivery of fuel, steam and oxygen into burner  29  and the injection of hot gaseous product  43  into fractured zone  21  occur simultaneously over a selected period, such as seven days. While gaseous product  43  is injected into fractured zone  21 , the temperature and pressure of fractured zone  21  increases. At the end of the injection period, fractured zone  21  is allowed to soak for a selected period, such as  21  days. During the soak interval, the operator may intermittingly pump fuel, steam and oxygen to burner  29  where it burns and the hot combustion gases are injected into formation  15  to maintain a desired pressure level in fractured zone  21 . Other than pressure maintenance, no further injection of hot gaseous fluid  43  occurs during the soak period.  
         [0021]     Then, the operator begins to produce the oil, which is driven by reservoir pressure and preferably additional solution gas pressure. The oil is preferably produced up the production tubing, which could also be the same tubing through which the fuel and steam or oxygen is pumped. Preferably, burner  29  remains and place, and the oil flows through burner  29 . Alternately, well  11  could comprise two boreholes a few feet apart, preferably no more than about 50 feet, with the oil flowing up a separate borehole from the one containing burner  29 .  
         [0022]     The oil production will continue as long as the operator deems it feasible, which could be up to 35 days or more. When production declines sufficiently, the operator may optionally repeat the injection and production cycle either with or without additional fracturing. It may be feasible to fracture again after one or more injection and production cycles to increase the perimeter  21   a  of fractured zone  21 , then repeat the injection and production cycle described above. Preferably, this subsequent fracturing operation can take place without removing burner  29 . The process may be repeated as long as fractured zone  21  does not intersect fractured zones or drainage areas  25  of adjacent wells  23  ( FIG. 2 ). By incrementally increasing the fractured zone  21  diameter from a relatively small perimeter up to half the distance to adjacent well  23  ( FIG. 2 ), the operator can effectively produce the viscous hydrocarbon formation  15 . With each new fracturing operation, the previously fractured portion would provide flow paths for the injection of hot gaseous product  43  and the flow of the hydrocarbon into the well. Also, the previously fractured portion retains heat from the previous injection of hot combustion gases  43 . The numeral  21   b  in  FIGS. 1 and 2  indicates the perimeter of fractured zone  21  after a second fracturing process. The operator could be performing similar fracturing, injection, soaking and production cycles on well  23  at the same time, if desired.  
         [0023]     Before or after reaching the maximum limit of fractured zone  21 , which would be greater than perimeter  21   b , the operator may wish to convert well  11  to a continuously driven system. This conversion might occur after well  11  has been fractured several different times, each increasing the dimension of the perimeter. In a continuously driven system, well  11  would be either a continuous producer or a continuous injector. If well  11  is a continuous injector, downhole burner  29  would be continuously supplied with fuel and steam  37  and oxygen  39 , which burns the fuel and injects hot gaseous product  43  into fractured zone  21 . The hot gaseous product  43  would force the oil to surrounding production wells, such as in an inverted five or seven-spot well pattern. Each of the surrounding production wells would have fractured zones that intersected the fractured zone  21  of the injection well. If well  11  is a continuous producer, fuel and steam  37  and oxygen  39  would be pumped to downhole burners  29  in surrounding injection wells, as in a normal five or seven-spot pattern. The downhole burners  29  in the surrounding injection wells would burn the fuel and inject hot gaseous product  43  into the fractured zones, each of which joined the fractured zone of the producing well so as to force the oil to the producing well.  
         [0024]     The invention has significant advantages. The unfractured heavy oil formation surrounding the fractured zone serves as a container to impede leakage of excess fuel, steam and other combustion products into adjacent formations or to the surface. The container maximizes the effects of the excess fuel and other hot gases flowing into the fractured zone. By reducing leakage from the fractured zone, the expense of the fuel, oxygen, and steam is reduced. Also, containing the excess fuel increases the safety of the well treatment.  
         [0025]     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, although the well is shown to be a vertical well, it could have a horizontal component extending through the heavy oil formation The fractured zone could be one or more vertical fractures in that instance.