Strength retrogression preventer

1. A method of cementing a subterranean formation penetrated by an oil or gas well comprising placing into the subterranean zone of the formation to be cemented a cementitious slurry comprising a cementitious material and a bismuth salt or derivative and setting, wherein strength retrogression of the set cement is controlled or prevented by the presence of the bismuth salt in the cementitious slurry.

2. The method of claim 1 , wherein the bismuth salt is selected from the group consisting of bismuth trioxide, bismuth nitrate, bisnmith chloride, bismuth citrate, bismuth iodide, bismuth lactate, bismuth hydroxide, bismuth nitrate, bismuth oxynitrate, bismuth oxalate, bismuth sesquioxide, bismuth oxybromide, bismuth oxychloride, bismuthic acid, bismuth oxycarbonate, bismuth acetate, bismuth benzoate, bismuth bromide, bismuth tartrate, bismuth potassium tartrate, bismuth sodium tartrate, bismuth ammonium citrate, bismuth trichloride, bismuth oxynitrate, and bismuth aluminate, Bi 2 S 3 Bi(VO 3 ) 3 , BiVO 4 , Bi 4 (V 2 O 7 ) 3 , BiO, Bi 2 O 5 ), BiS, and Bi 2 O 4 .

3. The method of claim 2 , wherein the bismuth salt is bismuth trioxide.

4. The method of claim 1 , wherein the amount of bismuth salt or derivative in the slurry is between from about 0.50 to about 25 percent by weight.

5. The method of claim 4 , wherein the amount of bismuth salt or derivative in the slurry is between from about 1.25 to about 10 percent by weight.

6. The method of claim 5 , wherein the amount of bismuth salt or derivative in the slurry is between from about 2.5 to about 5.0 percent by weight.

7. The method of claim 1 , wherein the cementitious material is a hydraulic cement comprised of calcium, aluminum, silicon, oxygen and/or sulfur.

8. The method of claim 1 , wherein the cementitious material is a hydraulic cement, slag or a blend thereof.

9. The method of claim 1 , wherein the cementitious slurry further comprises a strength enhancer selected from the group consisting of silica powder, silica flour, fumed silica, silica sand and crystalline silica.

10. The method of claim 1 , wherein the cementitious slurry further comprises a density-modifying agent.

11. The method of claim 10 , wherein the density-modifying agent is hollow or porous microspheres.

12. The method of claim 1 , wherein the cementitious slurry further comprises a foaming agent.

13. A method of increasing the compressive strength of cement during cementing of a subterranean formation which comprises pumping into the formation a cementitious slurry comprising a cementitious material and a bismuth salt or derivative.

14. The method of claim 13 , wherein the bismurh salt or derivative is bismuth trioxide.

15. The method of claim 13 , wherein the amount of bismuth salt or derivative in the cementitious slurry is between from about 1.25 to about 10 percent by weight.

16. The method of claim 15 , wherein the amount of bismuth salt or derivative in the cernentitious slurry is between from about 1.25 percent by weight to about 7.5 percent by weight.

17. The method of claim 16 , wherein the amount of bismuth salt or derivative in the cementitious slurry is between from about 2.5 to about 5.0 percent by weight.

18. A method of controlling compressive strength retrogression of cement during cementing of a subterranean formation which comprises pumping into the formation a cementitious slurry comprising a cementitious material and a bismuth salt or derivative.

19. The method of claim 18 , wherein the bismuth salt is selected from the group consisting of bismuth trioxide, bismuth nitrate, bismuth chloride, bismuth citrate, bismuth iodide, bismuth lactate, bismuth hydroxide, bismuth nitrate, bismuth oxynitrate, bismuth oxalate, bismuth sesquioxide, bismuth oxybromide, bismuth oxychloride, bismuthic acid, bismuth oxycarbonate, bismuth acetate, bismuth benzoate, bismrith hiomide, bismuth tartrate, bismuth potassium tartrate, bismuth sodium tartrate, bismuth ammonium citrate, bismuth trichloride, bismuth oxynitrate, and bismuth aluminate, Bi 2 S 3 Bi(VO 3 ) 3 , BiVO 4 , Bi 4 (V 2 O 7 ) 3 , BiO, Bi 2 O 5 ), BiS, and Bi 2 O 4 .

20. The method of claim 19 , wherein the bismuth salt is bismuth trioxide.

21. The method of claim 18 , wherein the amount of bismuth salt or derivative in the slurry is between from about 0.50 to about 25 percent by weight.

22. The method of claim 21 , wherein the amount of bismuth salt or derivative in the slurry is between from about 1.25 to about 7.5 percent by weight.

23. The method of claim 18 , wherein the cementitious slurry further comprises a density-modifying agent.

24. The method of claim 18 , wherein the cementitious slurry further comprises a strength enhancer.

25. The method of claim 24 , wherein the strength enhancer is selected from the group consisting of silica powder, silica flour, fumed silica, silica sand and crystalline silica.

26. A method of cementing a pipe string in a wellbore which comprises introducing a cementitious slurry in an annulus between the pipe string and the wall of the welibore in which the pipe string is disposed, and allowing the cementitious slurry to set, wherein the cementitious slurry comprises a cementitious material and a bismuth salt or derivative.

27. The method of claim 26 , wherein the bismuth salt is selected from the group consisting of bismuth trioxide, bismuth nitrate, bismuth chloride, bismuth citrate, bismuth iodide, bismuth lactare, bismuth hydroxide, bismuth nitrate, bismuth oxynitrate, bismuth oxalate, bismuth sesquioxide, bismuth oxybromide, bismuth oxychloride, bismuthic acid, bismuth oxycarbonate, bismuth acetate, bismuth benzoate, bismuth bromide, bismurh tartrate, bismuth potassium tartrate, bismuth sodium tartrate, bismuth ammonium citrate, bismuth trichloride, bismuth oxynitrate, and bismuth aluminate, Bi 2 S 3 Bi(VO 3 ) 3 , BiVO 4 , Bi 4 (V 2 O 7 ) 3 , BiO, Bi 2 O 5 ), BiS, and Bi 2 O 4 .

28. The method of claim 27 , wherein the bismuth salt is bismuth trioxide.