Method Of In Situ Coal Gasification

Abstract

A method for distilling coal in situ through gasification to recover hydrocarbons and a calorific value gas by treating a rubblized bed of coal underground. A rubblized bed of coal is produced from the surface through wells or shafts sealed to convey fluids under high pressure to or from the coal bed. Superheated steam is injected into the chamber while volatized hydrocarbons and gaseous reaction products are simultaneously withdrawn. Subsequent to the removal of the volatile hydrocarbons, air is injected so as to initiate combustion of the remaining char and increase the chamber temperature to from about 1200.degree.F. to about 2000.degree.F. Air injection is then suspended with water or low temperature steam being introduced into the hot char in order to form superheated steam which is withdrawn and injected into a new rubblized chamber of coal to repeat the process.

Description

BACKGROUND OF THE INVENTION

This invention relates to the in situ combustion of a coal seam for the recovery of volatile hydrocarbons and a synthetic-calorific value gas. More particularly, the present invention is a method for producing volatile hydrocarbons from a rubblized zone of coal through the initial injection of a steam stream, with subsequent combustion of the coal zone, introduction of water in the burned zone and subsequent production of steam for the sustenance of the steam production for continued volatile hydrocarbon removal.

The production of coal energy by the use of wells through underground mining has been a continual subject of interest in the field of energy production. Coal gasification by use of above ground retorting is an old art, one of the better known methods being the Lurgi process developed in Germany prior to World War II. By this method, oxygen and steam are simultaneously injected into a field retort and upon combustion, an energy content gas, having values sufficient for commercial usage, and coal tar liquids are produced.

The idea of underground gasification is not new, however, many technological advances have been proposed. Efforts were primarily confined to the advancement of theory until substantial work and testing was done in Russia. Most of the Russian work involved considerable underground mining and construction in an effort to provide a passageway of air through the coal. Some efforts involved breaking up the coal underground to provide adequate air passage. The amount of evacuation encountered in this process is tremendous. Eventually, the art progresses to the drilling of holes into coal seams and charging the hole with dynamite. As the burning front progresses through the stratuum, the charges are automatically set off in an effort to break up and crush the coal and render the segment of the bed more permeable. This results in irregularities too great to sustain continued gas flow, with the gas produced containing large amounts of air, which considerably lowers the heating value of the produced gas. As this process is far too expensive and limited in scope, a shaft and borehole mining combination was devised for employment with steeply sloping seams near outcrops. In addition to being limited exclusively to deeply pitched beds, this process also required a large amount of excavation and mining.

Therefore, what is required is a method for gasification of coal for recovery of volatile hydrocarbons and gaseous synthetic-calorific value gases from subterranean coal deposits.

It is an object of the present invention to provide a method for the combustion of underground coal beds.

It is another object of the present invention to provide a method by which a rubblized zone of coal may be formed for subsequent steam injection for the recovery of volatile hydrocarbons and a synthetic calorific value gas.

It is still a further object of the present invention to provide a method by which volatile hydrocarbons and a synthetic-calorific value gas are obtained through the introduction of steam into a rubblized coal zone with subsequent combustion of the rubblized coal zone and further production of steam therefrom.

With these and other objects in mind, the present invention may be more fully understood through referral to the accompanying drawing and following description.

SUMMARY OF THE INVENTION

The objects of the present invention are accomplished through utilization of a process for the in situ recovery of volatile hydrocarbons and a synthetic gas having a high calorific energy value from a subterranean coal deposit. The process comprises completing one or more wells within a coal deposit and rubblizing the coal deposit about the well. Subsequent to rubblization, superheated steam is introduced into the rubblized coal deposit through the well with subsequent production of volatized hydrocarbons and a synthetic gas, having a high calorific energy value, from the rubblized coal deposit, through the well. Subsequent to hydrocarbon production, the rubblized coal deposit is burned and water introduced into the burned rubblized coal deposit in order to form a superheated steam.

It is preferred that the superheated steam have a temperature of from about 1200.degree.F. to about 2000.degree.F. with the rubblized coal deposit being burned to raise the temperature therein from about 1200.degree.F. to about 2000.degree.F. Generally, it is preferred that at least two wells are completed within the coal deposit, with one well being utilized as an injection well and the other being utilized as a production well. The process of production of volatile hydrocarbons, burning, and introduction of water to form superheated steam may be repeated until no further depletion of hydrocarbon or synthetized gas production is achieved. It is a further preferred embodiment of the present invention that the superheated steam produced from the burned rubblized coal deposit be produced and injected into another rubblized coal deposit from which volatile hydrocarbons and synthetic gas may be produced.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be more readily understood by referral to the accompanying drawing in which a subterranean coal deposit is depicted with the process of the present invention practiced therein.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is a method for distilling coal in situ to recover hydrocarbons and a synthetic gas having a high calorific energy content. The resulting residual char remaining after the volatile hydrocarbons and synthetic gas are removed is utilized to produce steam for the sustenance of the process of the present invention. The initial step in the process of the present invention is to create a rubblized bed of coal underground through one or more completed wells drilled within the coal deposit. The initial step may be accomplished by any fracturing technique, for example by drilling a hole to a position below the coal seam and detonating a nuclear explosive, or drilling a hole or shaft below a coal seam, enlarging it to a chamber under the coal seam and fragmenting the overlying coal by detonation of conventional explosives. The rubblized bed is connected to the surface through one or more wells or shafts sealed to permit high pressure fluids to pass between the surface and the coal bed. As the chamber of rubblized coal is formed, superheated steam, having a temperature of from about 1200.degree.F. to about 2000.degree.F., is injected into the chamber with the volatized hydrocarbons and gaseous reaction products withdrawn. Generally, the synthetic gas formed is a water-gas shift reaction product as described in the following chemical equations:

C + H.sub.2 O.fwdarw. H.sub.2 + CO; and

2C + 2H.sub.2 O.fwdarw. CH.sub.4 + CO.sub.2.

A separate conduit may be provided to withdraw condensed steam. The withdrawn hydrocarbons and gaseous products of methane, hydrogen, carbon dioxide and carbon monoxide may be combined to produce pipeline gas by further conventional synthesis reactors at the surface.

Subsequent to the initial steaming of the rubblized coal deposit, air is injected so as to initiate combustion of the remaining char and increase chamber temperatures to within the range of about 1200.degree.F. to about 2000.degree.F. Air injection is then ceased and water or low temperature steam is injected so as to produce superheated steam by contacting the hot char and other solids in the chamber. The superheated steam produced may be utilized to further produce volatile hydrocarbons and synthetic gas from the rubblized coal chamber or may be withdrawn from the rubblized chamber or coal bed deposit to repeat the process within a freshly rubblized coal zone. Combustion air injection may be continuously alternated with water injection to maintain the chamber at the desired steam generation temperature until all char is consumed.

The present invention may be more fully understood by referral to the accompanying FIGURE in which a subterranean coal deposit 13 is being depleted of its hydrocarbon and calorific value through utilization of the process of the present invention. An injection well 30 is completed from the earth's surface 11 through overburden rock 12 into the coal deposit 13 by the cement completion 15 of a liner 14 having perforations 16 completed in the coal deposit 13. Injection well 30 is outfitted with a wellhead 17 having multiple injection means comprising an air injection system in which air 21 may be compressed by pump 20 with controlled air introduction through air line 18 into wellhead 17, for example by air valve 19. In conjunction with the air injection means are means for injecting steam and water comprising a water injection line 22 having a steam head 24 connected thereto with a steam control valve 23 through which steam 25 may be introduced upon demand. The water injection line 22 also contains a liquid water line 27 controlled through liquid water valve 26 connected to a water tank 28 having liquid water 29 contained therein.

Extending into the rubblized coal deposit 13 is a production well 31 comprising a liner 32 completed by cement 33 from the earth's surface 11, through overburden rock 12 and into the rubblized coal deposit 13. The liner 32 has perforations 34 completed within the rubblized coal deposit 13. The production well 31 is completed with wellhead 35, having gas-liquid separator 36 connected thereto such that gaseous overhead products 37 and liquid reformate 38 are produced.

In the normal utilization of the process of the present invention for the in situ recovery of volatile hydrocarbons and gas having a high calorific energy value from the subterranean coal deposit, generally, the injection wells 30 and 31 are completed within the coal deposit 13. The coal deposit is rubblized about the well by hydraulic fracturing, nuclear stimulation, explosive stimulation or other means so that superheated steam 25 may be introduced into the rubblized coal deposit through steam valve 23, steam line 24, water line 22, injection wellhead 17, injection well 30 and into the coal deposit 13. As the steam contacts the rubblized coal 13, volatile hydrocarbons and a synthetized gas formed through a water-gas shift reaction are produced through perforation 34 and the production well 31 so that volatilized hydrocarbons and synthetized gas are produced through the wellhead 35 and separated in a gas-liquid separator 36 into liquid hydrocarbons and synthetized gas.

Subsequent to depletion of the hydrocarbons and synthetized gas through steam injection, air is introduced into the rubblized coal deposit 13 through pump 20, control valve 19, air line 18, wellhead 17, injection well 30, and perforations 16. It is preferred that the temperature be raised from about 1200.degree.F. to about 2000.degree.F. in order to provide an activated char into which water 29 is injected. Superheated steam is formed from water contact with the burned char produced through perforations 34 and wellhead 35 of production well 31 and separated into a superheated steam stream and water separated as a liquid stream.

The process of the present invention may further comprise repeating the steps of introduction of superheated steam, the burning of the coal deposit and the introduction of water and production of steam therefrom for further production of volatile hydrocarbons and synthetized gas from the coal deposit and for the complete depletion of the coal deposit. The process may also comprise the production of the super-heated steam and its reintroduction into a rubblized coal zone of another injection well pattern. Generally, it is preferred that the superheated steam be utilized in a cyclic reintroduction into other rubblized coal deposits and have a temperature fraom about 1200.degree.F. to about 2000.degree.F. in order to adequately rubblize the hydrocarbons and cause the water-gas shift reaction to occur within the rubblized zone. In the preferred embodiment of the present invention, the simultaneous introduction of air into the rubblized coal may be utilized; in order to both burn, char and simultaneously form superheated steam until the coal deposit is consumed.

Therefore, the present invention as it applies to the art of in situ combustion of coal deposits provides a significant process for the total depletion of hydrocarbon content and energy value from a coal deposit in order that greater coal consumption and energy derivation may be obtained. The invention enhances the art of in situ combustion of coal deposits by presenting an economic method for the combustion and reclamation of energy from these deposits.

While the invention has been described above with respect to certain embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as set forth herein.

Claims

Therefore, I claim:

1. A process for the in situ recovery of volatile hydrocarbons and a synthetic gas having a high calorific energy value from a subterranean coal deposit, which comprises:

a. completing one or more wells within the coal deposit;

b. rubblizing the coal deposit about one of said wells;

c. introducing superheated steam into the rubblized coal deposit through one of said wells;

d. producing volatized hydrocarbons and a synthetized gas, having a high calorific energy value, from the rubblized coal deposit through one of said wells;

e. burning the rubblized coal deposit through air introduction via one of said wells subsequent to depletion of the hydrocarbons; and

f. introducing water via one of said wells into the burned coal deposit in order to form superheated steam.

2. The process of claim 1 in which:

a. the superheated steam has a temperature of from about 1200.degree.F. to about 2000.degree.F.; and

b. the rubblized coal deposit is burned to raise the temperature therein to from about 1200.degree.F. to about 2000.degree.F.

3. The process of claim 2 in which at least two wells are completed within the coal deposit, one or more wells being utilized as injection wells and one or more wells being utilized as production wells.

4. The process of claim 3 further comprising the continued introduction of air and water and the production of steam until the coal deposit is consumed.

5. The process of claim 1 further comprising repeating steps (c) through (f) until no further hydrocarbon or synthetic gas production is achieved.

6. The process of claim 1 further comprising recovering through one of said wells the superheated steam produced in step (f) and injecting it into another rubblized coal deposit.

7. The process of claim 6 in which:

a. the superheated steam has a temperature of from about 1200.degree.F. to about 2000.degree.F.; and

b. the rubblized coal deposit is burned to raise the temperature therein to from about 1200.degree.F. to about 2000.degree.F.

8. The process of claim 7 in which at least two wells are completed within the coal deposit, one or more wells being utilized as injection wells and one or more wells being utilized as production wells.

9. The process of claim 8 further comprising the continued introduction of air and water, and the production of steam until the coal deposit is consumed.