Abstract:
A system and method for gasification of a feedstock in a subterranean formation to produce syngas is described. An injection well is completed in the formation to inject an oxidant, provide an ignition source and convey the feedstock that includes water and one or more of a biomass, waste plastic, coal, bitumen and petcoke. Volatized hydrocarbons and gaseous reaction products are simultaneously withdrawn from a producer well from the subterranean formation to the surface. This syngas product is treated at the surface for power generation or conversion to transportation fuels and/or plastics. This method provides a low capital cost gasification unit which is capable of processing a variety of feedstock mixtures.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to subterranean gasification, and more particularly, relating to a subterranean gasification system and method for the gasification of slurry injected into a subterranean formation and recovering syngas as a product of the gasification of the slurry. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gasification of organic material (biomass) or fossil fuel carbonaceous material (coal) into syngas is known. The gasification process converts these materials into a gaseous mixture including carbon monoxide, hydrogen, carbon dioxide and methane. This gaseous mixture is called syngas and is commonly used as a combustible fuel or in the manufacture of derivate products. 
         [0003]    Biomass gasification is generally conducted at the surface using gasifiers that are specially designed for biomass gasification. Coal gasification of mined coal may also be conducted at the surface using gasifiers that are specially designed for coal gasification. Non-mined coal may also be gasified using a process called in-situ coal gasification (ISCG), also referred to underground coal gasification, where coal is gasified in non-mined seams to produce syngas and methane. 
       SUMMARY OF THE INVENTION 
       [0004]    The system and method described herein provide a low cost, flexible feedstock method for the subterranean gasification of biomass, waste plastics, coal, bitumen, petcoke, or combinations thereof under pressure. The described system and method not only provides for a mechanism to generate renewable syngas for fuel and plastic processing but also the ability to dispose of waste without surface land fill. Objects of the present invention are accomplished through utilization of a system and method for the recovery of volatile hydrocarbons and a synthetic gas having a high calorific energy value from gasification of feedstock in a subterranean formation. 
         [0005]    In general, in one aspect a subterranean gasification method is provided. The method includes: completing an injection well and a production well in a suitable formation; injecting a feedstock and an oxidant through the injection well into the formation; causing gasification of the feedstock in the formation; and recovering syngas from the formation through the production well. 
         [0006]    The method may also include one or more of: injecting a blanket fluid through said injection well; injecting a blanket fluid through said production well; recovering methane from said formation through said production well; injecting water into said formation through said production well; injecting a combustion supporting fuel into said formation through said injection well; and causing a water-gas shift reaction in said formation between said combustion supporting fuel and water, for example. 
         [0007]    Additionally, in embodiments the feedstock may include water and one or more of the group consisting of biomass, petcoke, coal, and waste plastic. Further, in embodiments, the formation may be depleted hydrocarbon reservoir, a depleted coal seam, or a deep salt cavern. 
         [0008]    There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood and in order that the present contribution to the art may be better appreciated. 
         [0009]    Numerous objects, features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon a reading of the following detailed description of presently preferred, but nonetheless illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of descriptions and should not be regarded as limiting. 
         [0010]    As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
         [0011]    For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The following drawings illustrate by way of example and are included to provide further understanding of the invention for the purpose of illustrative discussion of the embodiments of the invention. No attempt is made to show structural details of the embodiments in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Identical reference numerals do not necessarily indicate an identical structure. Rather, the same reference numeral may be used to indicate a similar feature of a feature with similar functionality. In the drawings: 
           [0013]      FIG. 1  is a diagrammatic view of a subterranean gasification system (gasifier) constructed in accordance with the principles of an embodiment the present invention; 
           [0014]      FIG. 2  is a diagrammatic, partial view of an injection well of the gasifier of  FIG. 1 ; 
           [0015]      FIG. 3  is a diagrammatic, partial view of a production well of the gasifier of  FIG. 1 ; 
           [0016]      FIG. 4  is a diagrammatic view of a subterranean gasification system (gasifier) constructed in accordance with the principles of an alternative embodiment the present invention; and 
           [0017]      FIG. 5  is a diagrammatic view of a gasification process including a subterranean gasifier in accordance with the principles of an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Embodiments of the invention provide subterranean gasification of a feedstock slurry injected into a formation through an injection well and recovery of volatile hydrocarbons, syngas, or both from the gasification of the feedstock through a production well. The feedstock slurry is comprised of material that may be gasified in the formation. In the following discussion, the feedstock slurry may be referred to as the slurry or feedstock interchangeably. 
         [0019]    The feedstock includes biomass, waste plastics, coal, bitumen, petcoke or combinations thereof admixed with water. In aspects, biomass includes plant or animal based biological material derived from living or recently living organisms. The addition of coal, bitumen, petcoke , or a combination thereof to the biomass may increase the heating value of the biomass. The feedstock is prepared at the surface utilizing methods or devices known in the art used to produce feedstock for surface gasifiers. 
         [0020]    The feedstock along with an oxidant is injected into the formation for gasification within the formation. Gasification of the feedstock is achieved by reacting the feedstock at high temperatures (&gt;700° C.) with a controlled amount of oxygen and/or steam. The oxygen supports a limited amount of combustion, which heats up the feedstock and boils both the natural formation water present along with injected water, to generate steam. In essence, a limited amount of oxygen or air is introduced into the reactor to allow some of the organic material to be “burned” to produce carbon dioxide and energy, which drives a second reaction that converts further organic material to hydrogen and additional carbon dioxide. 
         [0021]    The water can be saline as opposed to fresh water. The resultant conditions (high temperature, high pressure by virtue of the formation depth, and the presence of steam) cause a number of chemical reactions to occur whereby the injected feedstock slurry is converted into a gas, which consists primarily of synthesized methane, carbon dioxide, hydrogen and carbon monoxide. This gas is then conducted up to the surface via the vertical production well, where the gas can then be processed. 
         [0022]    It is contemplated that a benefit may be gained by replacement of some or all of the slurry water with supercritical CO2 with lower viscosity and density. 
         [0023]    With reference to  FIGS. 1-3  there is representatively illustrated a subterranean gasification system  10  in accordance with an embodiment of the invention. The gasification system  10  includes a suitable subterranean formation  12 , an injection well  14 , and a production well  16 . 
         [0024]    The subterranean formation  12  must be suitable to support gasification. A suitable subterranean formation  12  may include a depleted oil and gas reservoir, a depleted coal seam, or a depleted salt cavern, for example, of suitable integrity. A formation of suitable integrity preferably includes a formation that is of a certain depth and with adequate overburden and underburden formation rock  18  and  20 , respectively, to prevent fluid migration into groundwater. The formation must also have a sufficient permeability and porosity to allow syngas to migrate through the formation from the injection well  14  to the production well  16 . It is contemplated that a formation with limited permeability but otherwise having suitable overburden and underburden formation rock might benefit from hydraulic fracking to encourage communication between the injection and the production wells. 
         [0025]    The injection well  14  is shown run into the formation  12  and completed. The injection well includes a casing  22  that is preferably cemented  24  to retain it in place and to prevent fluid migration between subsurface formations. The injection well  14  further includes a feedstock string  26 , an oxidant string  28 , an igniter string  30 , and a wellhead  32 . The feedstock string  26  is run into the casing  22 , and the oxidant string  28  and the igniter string  30  are run into the feedstock string  26 . 
         [0026]    As shown, the injection well  14  has an openhole completion. In embodiments, the injection well  14  may be completed with a downhole nozzle assembly  34  (shown in broken line) connected to the oxidant string  28  and possibly the feedstock string  26  to promote atomization of the feedstock and oxidant to further promote gasification. 
         [0027]    Additionally, while injection well  14  is shown as a vertical well, in certain instances where ash or soot accumulation could be problematic, the injection well could be formed as a horizontal well and could include casing or another string portion extending beyond the end of the oxidant string  28  to prevent soot from impeding injection. 
         [0028]    The production well  16  is shown run into the formation  12  and completed. The production well includes a casing  36  that is preferably cemented  38  to retain it in place and to prevent fluid migration between subsurface formations. The production well  16  further includes a syngas string  40  and a water string  42  that are run into the casing  36 , and a wellhead  44 . As shown, the production well  16  has an openhole completion. In certain instances where the formation has high permeability, porosity, or both the production string  16  can be completed with a gravel or prop pack, or with a slotted liner or wire wrapped screen (not shown). 
         [0029]    As further shown, the injection string  14  is completed so as to be in communication with a lower section of the formation  12 , while the production string  16  is completed so as to be in communication with an upper section of the formation. This arrangement is to encourage gasification to flow in a general vertical direction from the bottom of the formation toward the top of the formation  12  and in a horizontal direction from the injection well  14  toward the production well  16 . 
         [0030]    With reference to the injection well  14 , feedstock  46  is pumped from the surface down the feedstock string  26  and into the formation  12 . Similarly, an oxidant  52  is injected into the formation  12  through the oxidant string  28 . While atmospheric air could be used the oxidant, oxygen is the preferred oxidant because the produced syngas will not contain nitrogen. The igniter string  30  may be fitted with a downhole ignitor  48 , and in certain embodiments may provide for the injection of combustion fuel  50  into the formation  12  to initiate combustion within the formation to support gasification of the feedstock  46 . A water-gas shift reaction between the combustion fuel and water in the formation may be caused to increase the calorific content of produced gas. A blanket fluid  54 , such as water or a non-condensable gas, is injected into the casing  22  to prevent fluid in the formation  12  from flowing upward through the casing, to cool the casing and to also monitor formation (downhole) pressure. 
         [0031]    With reference to the production well  16 , gas  56  formed in the formation by the gasification of the feedstock  46  is recovered at the surface through syngas string  40 . Gas  56  is primarily syngas, but can also include other gas depending on the components of the feedstock. For example, gas  56  could also include methane as a result of anaerobic digestion of biomass contained in the feedstock. Similar to the injection well  14 , blanket fluid  58 , such as water or non-condensible gas, is injected into the casing  36  for cooling and to prevent fluid in the formation  12  from flowing upward through the casing. Additionally, water  60  can be injected into the formation  12  through water string  42  to quench the formation if the process needs to be shut down or to further cool and clean the syngas. 
         [0032]    With reference to  FIG. 4 , gasification system  10  is shown with a fluid production well  62  run into the formation  12  and completed. In some iterations this may be accomplished with an additional string on the syngas production well. It may be desirable to include 
         [0033]    fluid production well  62  in order to pump liquid such as slag/ash slurry and/or Pyrolysis liquids  64  from the formation  12  to promote gasification within the formation that would otherwise be hindered by built up solids and/or liquids. This is done by shutting down the gasifier and purging the well with high pressure water. This can alternatively be accomplished on-line through gas-lift or similar downhole pump. 
         [0034]    In  FIG. 5  there is illustrated a block diagram of an exemplary gasification process including the gasification system  10 . The process illustrates gasification of a feedstock  46  with the system  10  to produce syngas  56  and then using the syngas in various downstream systems or plants. 
         [0035]    Particularly, various feedstock components including water  66  and one or more of waste plastic  68 , petcoke  70 , coal  72 , and biomass  74  are feed to a feedstock preparation system  76  where the components and water are processed into feedstock slurry  46 . The feedstock  46  and oxidant  52  are injected into gasifier  10 . The feedstock  46  is gasified and syngas  56  is recovered from the gasifier  10 . The syngas  56  can be directly used, by a power generation plant  78  to produce electricity, for example. In addition or alternatively to power generation, the syngas  56  can be processed by methanization plant  80 , methanol plant  82 , or both. Additionally, product from the methanol plant  82  can be further processed to produce dimethyl ether  84 , which in turn can be used to produce gasoline  86  or propylene  88  and then polypropylene  90 . It is worth noting that excess CO2 can be converted to methanol potentially with hydrogen in the methanol plant  80 . 
         [0036]    Other embodiments are possible. For example, in some cases where biomass slurry will be the only feedstock it could be of benefit to modify the injector well design to accommodate anaerobic digestion in lieu of a gasification reaction. The oxidant injector string and ignitor are not required, the oxidant string is instead replaced with a downhole gas production string. The producer well for anaerobic digestion may not be required. Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. There are four key biological and chemical stages of anaerobic digestion—hydrolysys, acidogenesis, acetogenesis and methogenesis. A simplified generic chemical equation for the overall processes outlined above is as follows: C6H12O6→3CO2+3CH4. The process produces a biogas, consisting of methane, carbon dioxide and traces of other ‘contaminant’ gases. Methogenesis is sensitive to both high and low pH and occurs between pH 6.5 and pH 8 that the pH of the feedstock may require adjustment through use of a base or acid at surface. The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the digestate which may be pumped to surface using a producer well if able to build up within the formation. In some cases the two processes can be combined in which methogenesis is encouraged beyond the high temperature gasification reaction in the formation through occasional shutting down of the gasifier and the downhole pumping of slurry to encourage anerobic digestion outside the gasification reaction zone and in the formation matrix. 
         [0037]    A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.