Method of producing synthesis gas by the underground gasification of coal from a coal seam

1. A method of producing synthesis gas by the underground gasification of coal from a coal seam comprising:

(a) cementing casing string in a production well and an injection well which penetrates the coal seam by pumping into the wellbore a cementitious slurry having a density between from about 7.0 ppg to about 23.0 ppg, the cementitious slurry comprising a cement mix which comprises:

(i) between from about 10 to about 70 weight percent of a cementitious material selected from the group consisting of Portland cement, pozzolan cement, fly ash cement and silica fume;

(b) between from about 5 to about 70 weight percent of graphite; and

(c) between from about 5 to about 70 weight percent of an aluminum silicate,

and allowing the slurry to set to form a cement sheath in the wellbore;

(b) igniting the coal in the coal seam;

(c) introducing oxidant gas into the injection well; and

(d) generating syngas as a combustion front in the coal seam and advancing the combustion front through the production well, wherein the temperature of the combustion front is greater than or equal to 800° C. and further wherein the heat from the combustion front is distributed along the cement sheath in the wellbore.

2. The method of claim 1 , wherein the temperature of the syngas production stream is at least 40° C. less than the temperature of a syngas production stream produced in a production well having a casing cemented with a cementitious slurry other than that of step (a).

3. The method of claim 1 , wherein the aluminum silicate is metakaolin.

4. The method of claim 1 , wherein the cementitious material is a Portland cement.

5. The method of claim 4 , wherein the Portland cement is a API Class A, C, G or H cement or a Type I, II, III or V ASTM construction cement.

6. The method of claim 1 , wherein the cementitious slurry further comprises a strength retrogression additive.

7. The method of claim 6 , wherein the strength retrogression additive is silica.

8. The method of claim 1 , wherein the bottom hole static temperature of the production well at time of cement placement is less than or equal to 65° C.

9. The method of claim 8 , wherein the temperature of the combustion front is greater than or equal to 1000° C.

10. The method of claim 1 , wherein the temperature of the combustion front is in greater than or equal to 1000° C.

11. The method of claim 1 , wherein the set cement maintains its compressive strength and does not form hot spots within the cement sheath.

12. The method of claim 1 , wherein the cementitious slurry further comprises a density reducing agent or a density weighting agent.

13. A method of producing synthesis gas by the underground gasification of coal from a coal seam comprising:

(a) pumping into the wellbore of a production well and an injection well a cementitious slurry comprising between from about 10 to about 70 weight percent of Portland cement, between from about 5 to about 70 weight percent of graphite and between from about 5 to about 70 weight percent of an aluminum silicate and allowing the slurry to set form a cement sheath in the wellbore, wherein the density of the slurry is between from about 7 to about 23 ppg;

(b) igniting coal in the coal seam;

(c) introducing air and water through the injection well and forming a syngas; and

(d) transporting the syngas through the coal seam as a combustion front to the production well.

14. The method of claim 13 , wherein the temperature of the combustion front is greater than or equal to 800° C.

15. The method of claim 14 , wherein the temperature of the combustion front is greater than or equal to 1000° C.

16. The method of claim 13 , wherein at least one of the following conditions prevail:

(a) the aluminum silicate is metakaolin;

(b) the bottom hole static temperature of the production well at the time of slurry placement is less than or equal to 65 C;

(c) the density of the cementitious slurry is between from 7.5 to 14 and contains from about 5 to about 150% BWOC of a density reducing agent; or

(d) the density of the cementitious slurry is between from 16.5 to 23 ppg and contains from about 5 to about 150% BWOC of a density weighting agent.

17. A method of producing synthesis gas by the underground gasification of coal from a coal seam comprising:

(a) pumping into the wellbore of a production well and an injection well a cementitious slurry comprising Portland cement, graphite and metakaolin and allowing the slurry to set form a cement sheath in the wellbore, wherein the density of the slurry is between from about 14 to about 17.0 ppg;

(b) igniting coal in the coal seam;

(c) introducing air and water through the injection well and forming a syngas;

(d) transporting the syngas through the coal seam as a combustion front to the production well

wherein the bottom hole static temperature of the production well, at the time of slurry placement, is less than or equal to 65° C. and the temperature of the combustion front is greater than or equal to 800° C.

18. A method of producing synthesis gas by the underground gasification of coal from a coal seam comprising:

(a) pumping into the wellbore of a production well and all injection, well a cement mix comprising:

(i) graphite,

(ii) an aluminum silicate; and

(iii) a cementitious material comprising a mixture of two or more components selected from the group consisting of Portland cement, fly ash, slag, silica fume, gypsum, limestone and bentonite;

(b) allowing the slurry to set form a cement sheath in the wellbore, wherein the density of the slurry is between from about 7 to about 23 ppg;

(c) igniting coal in the coal seam;

(d) introducing air and water through the injection well and forming a syngas; and

(e) transporting the syngas through the coal seam as a combustion front to the production well.

19. The method of claim 18 , wherein the aluminum silicate is metakaolin.

20. The method of claim 18 , wherein the cement mix comprises between from about 10 to about 70 weight percent of the cementitious material; from about 7 to about 70 weight percent of graphite; and between from about 5 to about 70 weight percent of an aluminum silicate.