Methods for cementing thermal wells
Portland cement compositions for use in high-temperature, high pressure wells are designed such that the lime-to-silica molar ratio is between 0.5 and 1.0, and the alumina-to-silica molar ratio is between 0.05 and 0.10. After curing and setting at temperatures between 85° C. and 300° C., the cement compositions form tobermorite as an initial and permanent calcium silicate hydrate phase.
1. A method for cementing a subterranean well, comprising:
(i) preparing a pumpable cement slurry comprising water, a single hydraulic cement consisting of portland cement, lime, a source of silica and a source of alumina, wherein the source of alumina consists of Type F fly ash;
(ii) placing the cement slurry in the subterranean well; and
(iii) curing the cement slurry in the subterranean well at a temperature between 850 C. and 3000 C., whereupon the cement slurry forms a set cement;
wherein a lime-to-silica molar ratio between 0.5 and 1.0 and an alumina-to-silica molar ratio between 0.05 and 0.10 are maintained simultaneously, and an initial and permanent binding phase of the set cement is 11 A tobermorite, aluminum substituted tobermorite, or both.
2. The method of claim 1 , wherein the source of silica comprises crystalline silica.
3. The method of claim 2 , wherein the crystalline silica is present at a median particle size (d 50 ) between 3.0 μm and 400 μm.
4. The method of claim 1 , wherein the silica is present at a concentration between 20% and 120% by weight of cement.
5. The method of claim 1 , wherein the alumina is present at a concentration between 10% and 60% by weight of cement.
6. The method of claim 1 , wherein the alumina is present at a median particle size (d 50 ) between 1 μm and 100 μm.
7. The method of claim 1 , wherein the cement slurry further comprises iron dust, hausmannite, ilmenite, hematite, titanium dioxide, barite or calcite, or combinations thereof.
8. The method of claim 1 , wherein the cement slurry further comprises petroleum coke, delayed coke, fluid coke, calcined coke, asphalt, charcoal, coal, anthracite, graphite, flake coke, amorphous pitch coke, anode coke, metallurgical coke, amorphous graphite, lignite, bituminous coal, sub-bituminous coal, exinite, vitrinite, intertinite, fine kolite, activated carbon, gilsonite, ground rubber, an aqueous inverse emulsion of particles comprising a betaine group, poly-2, 2, 1-bicyclo heptene (polynorbornene), alkylstyrene, crosslinked substituted vinyl acrylate copolymers, diatomaceous earth, natural rubber, vulcanized rubber, polyisoprene rubber, vinyl acetate rubber, polychloroprene rubber, acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadiene rubber, ethylene propylene diene monomer, ethylene propylene monomer rubber, styrene-butadiene rubber, styrene/propylene/diene monomer, brominated poly(isobutylene-co-4-methyl styrene), butyl rubber, chlorosulfonated polyethylenes, polyacrylate rubber, polyurethane, silicone rubber, brominated butyl rubber, chlorinated butyl rubber, chlorinated polyethylene, epichlorohydrin ethylene oxide copolymer, ethylene acrylate rubber, ethylene propylene diene terpolymer rubber, sulfonated polyethylene, fluoro silicone rubbers, fluoroelastomers or substituted styrene acrylate copolymers, or combinations thereof.
9. The method of claim 1 , wherein the cement slurry further comprises nitrogen, air or microspheres or combinations thereof.
10. The method of claim 1 , wherein the cement slurry further comprises magnesium oxide.
11. The method of claim 1 , wherein the cement slurry has a solid volume fraction between 20% and 60%.
12. The method of claim 1 , wherein the set cement has a water permeability lower than 0.1 mD.
13. The method of claim 1 , wherein the set cement has a strength that does not vary more than 30% during a 6-month curing period.
14. The method of claim 1 , wherein the set cement has a compressive strength greater than 3.5 MPa.
15. A method for cementing a subterranean well, comprising:
(i) preparing a pumpable cement slurry comprising water, a single cement consisting of portland cement, lime, a source of silica and a source of alumina;
(ii) placing the cement slurry in the subterranean well; and
(iii) curing the cement slurry in the subterranean well at a temperature between 85° C. and 300° C., whereupon the cement slurry forms a set cement:
wherein a lime-to-silica molar ratio between 0.5 and 1.0 and an alumina-to-silica molar ratio between 0.05 and 0.10 are maintained simultaneously, and an initial and permanent binding phase of the set cement is 11 A tobermorite, aluminum substituted tobermorite, or both.