Compositions and methods for well completions
Incorporation of carbonaceous materials in a cement slurry increases the linear thermal-expansion coefficient of the set cement. When placed in a subterranean well having at least one casing string, cement sheaths with linear thermal-expansion coefficients similar to that of the casing will be subjected to lower compressive and tensile stresses during downhole-temperature changes. Such cement slurries are particularly advantageous in the context of thermal-recovery wells.
1. A method, comprising:
(i) selecting at least one casing string having a casing linear thermal expansion coefficient (LCTE);
(ii) preparing a set cement slurry that contains at least one carbonaceous material, and measuring a set cement LCTE of the set cement slurry,
wherein the carbonaceous material is one or more members selected from the group consisting of delayed coke, fluid coke, calcined coke, asphalt, charcoal, coal, anthracite, graphite, flake coke, amorphous pitch coke, anode coke, tar coke, metallurgical coke, amorphous graphite, lignite, bituminous coal, subbituminous coal, exinite, vitrinite, intertinite, gilsonite, kolite, needle coke and activated carbon;
(iii) comparing the casing LCTE to the set cement LCTE;
(iv) if the set cement LCTE is higher than the casing LCTE, and is between 11·10 −6 /° C. and 15·10 −6 /° C., proceed to step (v); if not, repeat steps (ii) and (iii) by adjusting the set cement LCTE, such adjustment comprising varying a type, a concentration or a particle size of the at least one carbonaceous material until the set cement LCTE is higher than the casing LCTE and between 11·10 −6 /° C. and 15·10 −6 /° C.;
(v) installing the casing string in a thermal recovery well;
(vi) performing a cementing operation such that the cement slurry is placed adjacent to the at least one casing string, and allowing the cement slurry to set in the thermal recovery well; and
(vii) applying heat to the thermal recovery well such that a well temperature varies between 150° C. and 700° C., causing the at least one casing string and the set cement slurry to undergo linear expansion without the formation of a microannulus between the at least one casing string and the set cement.
2. The method of claim 1 , wherein the carbonaceous-material concentration is between 10% and 60% by weight of cement.
3. The method of claim 1 , wherein the carbonaceous-material particle size is between 1 μm and 1200 μm.
4. The method of claim 1 , wherein the cement slurry further comprises one or more members selected from the group consisting of accelerators, retarders, dispersants, fluid-loss additives, extenders, swellable materials, pozzolans, fibers and antifoam agents.
5. The method of claim 1 , wherein the cement slurry density is adjusted by adding one or more members selected from the group consisting of glass microspheres, ceramic microspheres, hematite, ilmenite, barite and manganese tetraoxide.