Subterranean gasification system and method

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 Volatized hydrocarbons and gaseous reaction products are simultaneously withdrawn from a producer well from the subterranean formation to the surface.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.

FIELD OF THE INVENTION

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

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.

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

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.

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.

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.

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

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 (>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.

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.

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.

With reference toFIGS. 1-3there is representatively illustrated a subterranean gasification system10in accordance with an embodiment of the invention. The gasification system10includes a suitable subterranean formation12, an injection well14, and a production well16.

The subterranean formation12must be suitable to support gasification. A suitable subterranean formation12may 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 rock18and20, 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 well14to the production well16. 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.

The injection well14is shown run into the formation12and completed. The injection well includes a casing22that is preferably cemented24to retain it in place and to prevent fluid migration between subsurface formations. The injection well14further includes a feedstock string26, an oxidant string28, an igniter string30, and a wellhead32. The feedstock string26is run into the casing22, and the oxidant string28and the igniter string30are run into the feedstock string26.

As shown, the injection well14has an openhole completion. In embodiments, the injection well14may be completed with a downhole nozzle assembly34(shown in broken line) connected to the oxidant string28and possibly the feedstock string26to promote atomization of the feedstock and oxidant to further promote gasification.

Additionally, while injection well14is 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 string28to prevent soot from impeding injection.

The production well16is shown run into the formation12and completed. The production well includes a casing36that is preferably cemented38to retain it in place and to prevent fluid migration between subsurface formations. The production well16further includes a syngas string40and a water string42that are run into the casing36, and a wellhead44. As shown, the production well16has an openhole completion. In certain instances where the formation has high permeability, porosity, or both the production string16can be completed with a gravel or prop pack, or with a slotted liner or wire wrapped screen (not shown).

As further shown, the injection string14is completed so as to be in communication with a lower section of the formation12, while the production string16is 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 formation12and in a horizontal direction from the injection well14toward the production well16.

With reference to the injection well14, feedstock46is pumped from the surface down the feedstock string26and into the formation12. Similarly, an oxidant52is injected into the formation12through the oxidant string28. While atmospheric air could be used the oxidant, oxygen is the preferred oxidant because the produced syngas will not contain nitrogen. The igniter string30may be fitted with a downhole ignitor48, and in certain embodiments may provide for the injection of combustion fuel50into the formation12to initiate combustion within the formation to support gasification of the feedstock46. 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 fluid54, such as water or a non-condensable gas, is injected into the casing22to prevent fluid in the formation12from flowing upward through the casing, to cool the casing and to also monitor formation (downhole) pressure.

With reference to the production well16, gas56formed in the formation by the gasification of the feedstock46is recovered at the surface through syngas string40. Gas56is primarily syngas, but can also include other gas depending on the components of the feedstock. For example, gas56could also include methane as a result of anaerobic digestion of biomass contained in the feedstock. Similar to the injection well14, blanket fluid58, such as water or non-condensible gas, is injected into the casing36for cooling and to prevent fluid in the formation12from flowing upward through the casing. Additionally, water60can be injected into the formation12through water string42to quench the formation if the process needs to be shut down or to further cool and clean the syngas.

With reference toFIG. 4, gasification system10is shown with a fluid production well62run into the formation12and completed. In some iterations this may be accomplished with an additional string on the syngas production well. It may be desirable to include

fluid production well62in order to pump liquid such as slag/ash slurry and/or Pyrolysis liquids64from the formation12to 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.

InFIG. 5there is illustrated a block diagram of an exemplary gasification process including the gasification system10. The process illustrates gasification of a feedstock46with the system10to produce syngas56and then using the syngas in various downstream systems or plants.

Particularly, various feedstock components including water66and one or more of waste plastic68, petcoke70, coal72, and biomass74are feed to a feedstock preparation system76where the components and water are processed into feedstock slurry46. The feedstock46and oxidant52are injected into gasifier10. The feedstock46is gasified and syngas56is recovered from the gasifier10. The syngas56can be directly used, by a power generation plant78to produce electricity, for example. In addition or alternatively to power generation, the syngas56can be processed by methanization plant80, methanol plant82, or both. Additionally, product from the methanol plant82can be further processed to produce dimethyl ether84, which in turn can be used to produce gasoline86or propylene88and then polypropylene90. It is worth noting that excess CO2 can be converted to methanol potentially with hydrogen in the methanol plant80.

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.