Patent Publication Number: US-2012028201-A1

Title: Subsurface heater

Description:
BACKGROUND 
     There are extensive hydrocarbon reservoirs distributed throughout the world which, for the foreseeable future, represent key energy resources for the world&#39;s continued economic development. These reservoirs often contain a viscous hydrocarbon concoction, called “tar,” “heavy oil,” or “ultra heavy oil,” which typically has a viscosity in the range from about 3,000 to 1,000,000 centipoise when measured at around 37.5° C. Many hydrocarbon bearing geologic formations contain such hydrocarbon concoctions which do not permit a ready flow of the hydrocarbon content to a wellbore for extraction because of their high viscosity. In certain hydrocarbon reservoirs, for example, oil shale reservoirs, the hydrocarbon components must be thermally broken down into lower molecular weight compounds in order to effect their recovery from the reservoir. In certain instances, the reservoir must be heated to a temperature in excess of 300° C. in order to effect even the partial extraction of hydrocarbons from a hydrocarbon reservoir. 
     Three different types of processes are known to enhance hydrocarbon extraction from subterranean hydrocarbon reservoirs. These processes may be classified generally as thermal processes, chemical processes and miscible displacement processes. 
     A notable, known thermal process involves an “in situ” combustion technique in which the reservoir, serving as its own fuel source, is ignited through an injection well and a zone of combustion is propagated from the injection well towards a production well. The combustion can be somewhat controlled by the position of the injection well and the mode of delivery of the exogenous oxygen needed to effect combustion within the combustion zone. Because of the nature and complexity of the fuel involved, such in situ combustion techniques produce a complex variety of combustion product gases which must be carefully managed in order to prevent their uncontrolled release into the living environment. 
     Heat conduction phenomena within and around the reservoir may play a critical role in hydrocarbon recovery rates, and such rates may be further limited by a tendency of the hydrocarbon components of the reservoir to undergo coking. The heat transfer rate from a heat source to the reservoir may be limited by the coking temperature and the ambient temperature of the hydrocarbon bearing reservoir. Thus, methods involving heating of a hydrocarbon reservoir must balance the rate at which heat is introduced into the reservoir against the coking temperature of the hydrocarbon components of the reservoir and the rate at which the heat can be conducted from the heat source into the reservoir. 
     Therefore there is a need for subsurface heating devices which utilize clean fuels such as natural gas and effect a controlled delivery of substantial amounts of heat from the device to the reservoir such that coking may be minimized while maximizing the efficiency of hydrocarbon recovery. 
     BRIEF DESCRIPTION 
     In one aspect, the present invention provides a subsurface heater comprising: a combustible gas supply conduit; an oxygen supply conduit and a heat transmissive external housing encompassing the porous refractory medium. The combustible gas supply conduit and the oxygen supply conduit are configured as a concentric pair disposed within a porous refractory medium and coupled to a plurality of gas jets disposed within the porous refractory medium. The porous refractory medium having disposed within it a plurality of combustion product gas return conduits. The combustion product gas return conduits are configured to receive combustion product gases from the porous refractory medium. 
     In another aspect, the present invention provides a method for heating a subsurface zone, comprising: (a) creating an accommodation cavity for a subsurface heater; (b) installing the subsurface heater; and (c) operating the subsurface heater. The subsurface heater comprises a combustible gas supply conduit and an oxygen supply conduit configured as a concentric pair disposed within a porous refractory medium and coupled to a plurality of gas jets disposed within the porous refractory medium, the porous refractory medium having disposed within it a plurality of combustion product gas return conduits, the combustion product gas return conduits being configured to receive combustion product gases from the porous refractory medium; and a heat transmissive external housing encompassing the porous refractory medium. 
     In yet another aspect, the present invention provides a subsurface heater comprising: a combustible gas supply conduit; an oxygen supply conduit and a heat transmissive external housing encompassing the porous refractory medium. The combustible gas supply conduit and the oxygen supply conduit are configured as a concentric pair disposed within a porous refractory medium and coupled to a plurality of gas jets disposed within the porous refractory medium. The porous refractory medium having disposed within it a plurality of combustion product gas return conduits. The combustion product gas return conduits are configured to receive combustion product gases from the porous refractory medium. The plurality of gas jets are independently operable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a cross-section of a subsurface heater in accordance with an embodiment of the invention. 
         FIG. 2  is a sectional view of a subsurface heater in accordance with an embodiment of the invention. 
         FIG. 3  is a section of a subsurface heater in accordance with an embodiment of the invention. 
         FIG. 4  is a section of a subsurface heater in accordance with an embodiment of the invention. 
         FIG. 5  is a section of a subsurface heater in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following specification and the claims, which follow, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular feature of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Similarly, “free” may be used in combination with a term, and may include an insubstantial number, or trace amounts, while still being considered free of the modified term. 
     As discussed in detail below, embodiments of the present invention include a subsurface heater comprising: a combustible gas supply conduit; an oxygen supply conduit and a heat transmissive external housing encompassing the porous refractory medium. The combustible gas supply conduit and the oxygen supply conduit are configured as a concentric pair disposed within a porous refractory medium and coupled to a plurality of gas jets disposed within the porous refractory medium. The porous refractory medium having disposed within it a plurality of combustion product gas return conduits. The combustion product gas return conduits are configured to receive combustion product gases from the porous refractory medium. 
     In one embodiment of the present invention, as illustrated by  FIG. 1 , the subsurface heater  10  includes a combustion gas supply conduit  12  and an oxygen supply conduit  14 . In one embodiment, the combustion gas supply conduit  12  and oxygen supply conduit  14  form a concentric pair. In another embodiment, the combustion gas supply conduit  12  and oxygen supply conduit  14  can be placed parallel to each other (for example a side by side type of arrangement). In one embodiment, the combustion gas can be selected from the group consisting of natural gas, hydrocarbons such as methane, propane etc, a premix of methane and air, kerosene type jet fuel and the like. In another embodiment, the combustion gas supply conduit  12  forms an inner conduit of the concentric pair. In yet another embodiment, the combustion gas supply conduit  12  forms an outer conduit of the concentric pair. 
     In one embodiment, the oxygen supply conduit  14  can carry gas selected from air, inert gases such as argon, nitrogen, air enriched with oxygen, synthetic mixtures of oxygen and one or more gases, and the like. In another embodiment, the oxygen supply conduit  14  can carry gas that contains at least about 70 percent by weight of oxygen. In yet another embodiment, the oxygen supply conduit  14  can carry gas that contains at least about 90 percent by weight of oxygen. 
     The subsurface heater includes a heat transmissive external housing  18 . The heat transmissive external housing  18  encompasses a porous refractory medium  20 . In one embodiment, the area between the outer conduit of the concentric pair and the heat transmissive external housing  18  contains the porous refractory medium  20 . In one embodiment, the porous refractory medium  20  includes materials that are heat resistant. Non-limiting examples of the materials that can be present in the porous refractory medium  20  include ceramic materials such as alumina, silica, zirconia, silicon carbide, alumina-silicon dioxide (mullite), zirconia-alumina composites, metals balls including metals such as iron, or iron based alloys. In another embodiment, the porous refractory medium  20  includes at least one material selected from the group consisting of alumina, silica, carbon, and silt. 
     In one embodiment, the combustion gas supply conduit  12  and the oxygen supply conduit  14  that are configured as a concentric pair disposed within the porous refractory medium  20 . The combustion gas supply conduit  12  and the oxygen supply conduit  14  are coupled to a plurality of gas jets  24  (as shown in  FIG. 2 ). In one embodiment, the gas jets  24  can precess about a central axis defined by the combustion gas supply conduit  12  and the oxygen supply conduit  14 . Thus, the positions of the gas jets  24  and associated oxygen (air) nozzles  22  may vary with respect to a reference position defined for the axis defined by the combustion gas supply conduit  12  and oxygen supply conduit  14  and is referred to herein as a movement of precession. In one embodiment, the movement of precession can be comprised of regular rotational intervals with respect to a reference position. In one embodiment, one of the plurality of gas jets  24  represents the reference position and is denominated as 0 degrees of rotation, while a second adjacent gas jet is precessed with respect to the first gas jet by about 90 degrees of rotation, while the third gas jet adjacent to the second gas jet is precessed with respect to the first gas jet by about 180 degrees of rotation, while a fourth gas jet adjacent to the third gas jet is precessed with respect to the first gas jet by about 270 degrees of rotation, and a fifth gas jet adjacent to the fourth gas jet is precessed with respect to the first gas jet by about 0 degrees of rotation. Those of ordinary skill in the art will appreciate that various configurations of the plurality of gas jets  24  and associated oxygen (air) nozzles  22  are possible. In one embodiment, groupings of the plurality of gas jets  24  and associated oxygen (air) nozzles  22  may precess about the axis defined by the combustion gas supply conduit  12  and oxygen supply conduit  14 . 
     In another series of embodiments, the movement of precession can be random or discontinuous. In yet another embodiment, there is no precession of the plurality of gas jets  24  and associated oxygen (air) nozzles  22  about a reference position along the axis defined by the combustion gas supply conduit  12  and the oxygen supply conduit  14 . 
     In one embodiment, each one of the plurality of gas jets  24  and associated oxygen (air) nozzles  22  (together referred to as “burners”) are independently operable i.e. a burner can be switched on or off independently without affecting the status of other burners in the subsurface heater. Thus, in one embodiment, during operation a first plurality of burners located at a reference position denominated 0 degrees along the axis defined by the combustion gas supply conduit  12  and oxygen supply conduit  14  are “switched on” (i.e. the plurality of gas jets  24  and associated oxygen (air) nozzles  22  are open and the oxygen-fuel mixture emerging therefrom is burning) while a second plurality of burners located at a reference position denominated 180 degrees along the axis defined by the combustion gas supply conduit  12  and oxygen supply conduit  14  are “switched off” (i.e. gas jets  24  and associated oxygen (air) nozzles  22  are closed). In yet another embodiment, the amount of heat produced at any given time by any one of the plurality of gas jets  24  can be varied independently by varying parameters such as pressure of the combustion gas, pressure of the oxygen, or varying the ratio of the oxygen to the combustion gas. 
     In various embodiments, the plurality of gas jets  24  and associated oxygen (air) nozzles  22  may be controlled such that they are open, partially opened or closed depending on need. Conventional control systems may be employed. In one embodiment, the mechanical components of the burners (e.g. the gas jets  24 , the associated oxygen (air) nozzles  22 , and the burner igniter) and a set of operational sensors (flame on/off sensor, valve open/closed sensor, temperature sensor, pressure sensor, igniter on/off sensor) are linked to a controller via an insulated control cable arrayed along the axis of and within the combustion gas supply conduit  12 . 
     In one embodiment, the porous refractory medium  20  can include three zones (not shown) that include a mixing zone, an ignition zone and a reaction zone. The reaction zone can also be referred to as a combustion zone as the combustion occurs at the reaction zone. In another embodiment, the three zone present in the porous refractory medium  20  can be easily distinguishable. In one embodiment, the three zones can include porous refractory medium  20  having uniform particle size. In another embodiment, the particle size of the material in the porous refractory medium  20  can vary in the three zones. For example, the mixing zone can be packed with small size particles, the ignition zone can be packed with larger size particles. 
     In one embodiment, the porous refractory medium  20  includes a plurality of nozzles  22 , at times herein referred to as “air nozzles”, which are coupled to the oxygen supply conduit, release an oxygen-containing gas (e.g. air, oxygen, or a synthetic mixture of oxygen and one or more gases) into the porous refractory medium  20 . In another embodiment, the air nozzle  22  and the gas jet  24  can be form a concentric pair. In one embodiment, the mixing of the oxygen-containing gas supplied by the oxygen supply conduit and the combustion gas occurs in the vicinity of the plurality of gas jets  24 . In one embodiment, the mixture of combustion gas and oxygen can be ignited by an igniter, for example a small open flame burner, an electrically heated wire, or a spark device. Once ignited the flame may propagate into the combustion zone in the porous refractory medium  20 . 
     Disposed within the porous refractory medium  20  is a plurality of combustion product gas return conduits  16 . The combustion product gas return conduits  16  are configured to receive combustion product gases that are formed as a result of combustion in the porous refractory medium  20 . Combustion product gases are typically comprised of carbon dioxide and water nut may include other products as well. In one embodiment, the combustion product gas return conduits  16  are symmetrically disposed with respect to each other within the porous refractory medium  20 . In another embodiment, the combustion product gas return conduits  16  are located on the periphery of the porous refractory medium  20  adjacent to an inner surface of the heat transmissive external housing  18 . In yet another embodiment, the combustion product gas return conduits  16  can be disposed in a random or discontinuous manner throughout the porous refractory medium  20 . In another embodiment, the combustion product gas return conduits  16  can be disposed in a periodic manner in the porous refractory medium  20 . In yet another embodiment, the combustion product gas return conduits  16  can be spaced in a cluster in the porous refractory medium  20 . In one embodiment, the combustion product gas return conduits  16  include a porous outer surface that enables the flow of the combustion product gases to flow into the conduits from the porous refractory medium  20 . In one embodiment, the combustion product gas return conduits  16  can be independently operable. As used herein the term “independently operable” means that at any given time only some or all of the combustion product gas return conduits  16  can be operable to conduct the combustion product gas from the porous refractory medium  20 . In one embodiment, the combustion gas supply conduit  12  and oxygen supply conduit  14  disposed in the porous refractory medium  20  are configured to have an opposed flow with the heat generated as a result of combustion, i.e. the heat is conducted towards the central part of the porous refractory medium  20  while the combustion gas and the oxygen flow away from the center. In another embodiment, the combustion gas and the oxygen are maintained at a temperature of about 50° C. which aids in lower flow velocities and reduces pressure losses. 
     As will be appreciated by those of ordinary skill in the art, the fuel and air tubes (i.e. the combustible gas supply conduit and the oxygen supply conduit) may be in close proximity to the combustion zone of the porous refractory medium and there is a tendency of heat to flow toward the center of the subsurface heater as well as being radiated outwardly from the subsurface heater. As a result of the outward flow of the combustible gas and the oxygen containing gas from the combustion gas supply conduit and the oxygen supply conduit respectively, the temperature within each of the conduits can be maintained at relatively low temperature during operation of the subsurface burner. Lower flow velocities and lower pressure losses are a result, of the relatively low temperatures prevailing within the fuel and oxygen containing gas supply conduits. 
     In one embodiment, the subsurface heater can further include a plurality of temperature sensors (not shown). In one embodiment, the temperature sensor can be disposed within the subsurface heater. In another embodiment, the temperature sensor can be disposed outside an outer surface of the heat transmissive external housing  18  of the subsurface heater. In another embodiment, the temperature sensor is configured to provide data to a control system. 
       FIG. 2  is a section  30  of the subsurface heater according to one embodiment of the invention. As illustrated in  FIG. 2  the pressurized combustion gas from the combustion gas supply conduit  12  and the oxygen from the oxygen supply conduit  14  are contacted with the porous refractory medium  20  through the gas jet  24  and the air nozzle  22  respectively. As illustrated in  FIG. 2  the combustion product gas return conduits  16  are disposed periphery of the porous refractory medium  20  adjacent to an inner surface  26  of the heat transmissive external housing  18 . 
       FIG. 3  is a section of the subsurface heater  50  according to one embodiment of the invention. As shown in  FIG. 3  the oxygen  52  and the combustion gas  54  flow into the reaction zone  56  in the porous refractory medium  20 . In one embodiment, the propagation of the flame is radial. The equivalence ratio contour which is defined as an estimated contour along which the combustion gas to the oxygen ratio is equal to the stoichiometric ratio of the combustion gas to the oxygen. This indicates that the reaction or combustion occurs stoichiometrically along the contour. In one embodiment, the highest flame temperature can be experienced in the region defined by the contour. 
       FIG. 4  depicts the temperature profile along a section of the subsurface heater according to one embodiment of the invention. The temperature profile along the heater and external to it in the area surrounding the heater for example shale oil, is shown in  FIG. 4 . In the illustrated embodiment the temperature is found to be dependent upon the distance from the axis defined by the center of the combustion gas supply conduit  12 .  FIG. 5  provides data  80  demonstrating the temperature contours or isotherms  82 ,  84 ,  86 ,  88  and  90  within a subsurface heater during operation according to one embodiment of the invention. 
     In one embodiment, the subsurface heater can be operated in a pressurized environment. In another embodiment, the can be operable at varying combustion gas and oxygen pressures over several thousands of feet in length. In one embodiment, the heat released from the combustion product gas return conduits  16  is relatively low, for example when the average temperature of the combustion product gases within and along the length of the combustion product gas return conduits  16  is less than about 200° C. Under such conditions the generation of NOx may be minimal. In one embodiment, and the combustion product gases comprise less than about 2 ppm NOx. 
     Another aspect of the invention provides a method for heating a subsurface zone, comprising: (a) creating an accommodation cavity for a subsurface heater; (b) installing the subsurface heater; and (c) operating the subsurface heater. 
     In one embodiment, the accommodation cavity can be created in a hydrocarbon reservoir. As used herein the term “hydrocarbon” is defined as compounds comprising carbon and hydrogen. However, hydrocarbon-containing reservoirs may contain a host of components comprising elements other than carbon and hydrogen, for example halogens, nitrogen, oxygen, metals, sulfur, and selenium. Non-limiting examples of components which may be present in a hydrocarbon reservoir include, straight chain and branched hydrocarbons, for example eicosane (a C 20  straight chain hydrocarbon) and phytane (a C 20  branched hydrocarbon), bitumen, oil tars, minerals, asphaltites, kerogen, and the like. The hydrocarbon reservoir is typically contained within a geologic matrix, such as sedimentary rock, sands, silicilytes, carbonates, diatomites, and the like. In one embodiment, the hydrocarbon reservoir is a subterranean, viscous oil-containing formation. In one embodiment, the hydrocarbon reservoir is contained within a heavy oil tar sand formation. In another embodiment, hydrocarbon reservoir is contained within a shale oil formation. In one embodiment, the accommodation cavity can be subterranean, located under tundra, under sea or inland based wells. The methods provided by the present invention may be practiced in conjunction with a wide variety of hydrocarbon recovery techniques including vertical recovery, horizontal recovery, and steam assisted gravity drainage (SAGD) techniques. In another embodiment, the accommodation cavity can be created in a near-surface zone. Examples of applicable near-surface zones include but are not limited to construction activity zones, water containment zones, water transport zones (e.g. municipal water delivery and waste water removal), and water treatment zones such as municipal water treatment plants. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.