Retarded acid systems, emulsions, and methods for using in acidizing carbonate formations
In one embodiment, a retarded acid system comprises an aqueous acid and a retarding surfactant. The aqueous acid may comprise from 5 wt. % to 25 wt. % of a strong acid, that is, an acid having a K a greater than or equal to 0.01. The aqueous acid may further comprise from 75 wt. % to 95 wt. % water. The retarding surfactant may have the general chemical formula R—(OC 2 H 4 ) X —OH where R is a hydrocarbon having from 11 to 15 carbon atoms and x is an integer from 6 to 10. The retarding surfactant may have a hydrophilic-lipophilic balance from 8 to 16.
1. A retarded acid emulsion consisting of:
an internal phase consisting of an aqueous acid;
the aqueous acid comprising:
from 5 wt. % to 35 wt. % of strong acid having a Ka greater than or equal to 0.01; and
from 65 wt. % to 95 wt. % water;
an external phase consisting of a base oil; and
a retarding surfactant;
the retarding surfactant having the chemical formula R—(OC 2 H 4 ) x —OH, where:
R is a hydrocarbon having from 11 to 15 carbon atoms;
x is an integer from 1 to 4; and
the retarding surfactant has a hydrophilic-lipophilic balance from 3 to 7; and
where the ratio of the weight of retarding surfactant in the retarded acid emulsion to the weight of aqueous acid in the retarded acid emulsion is from 1:40 to 1:8.
2. The retarded acid emulsion of claim 1 , where R is a hydrocarbon having from 12 to 14 carbon atoms and x is 1.
3. The retarded acid emulsion of claim 1 , where the strong acid is an inorganic acid.
4. The retarded acid emulsion of claim 1 , where the strong acid is an organic acid.
5. The retarded acid emulsion of claim 1 , wherein the strong acid comprises hydrofluoric acid.
6. The retarded acid emulsion of claim 1 , where the base oil comprises mineral oil, safra oil, diesel, C12-C20 linear alpha olefins, or combinations thereof.
7. The retarded acid emulsion of claim 1 , where the retarded acid emulsion is free of hydrophobic organic alcohols.
8. A method for acidizing a carbonate formation, the method comprising:
adding a retarded acid emulsion to the carbonate formation;
the retarded acid emulsion consisting of:
an internal phase consisting of an aqueous acid comprising:
from 5 wt. % to 35 wt. % of strong acid having a Ka greater than or equal to 0.01; and
from 65 wt. % to 95 wt. % water;
a retarding surfactant having the chemical formula R—(OC 2 H 4 ) x —OH, where:
R is a hydrocarbon having from 11 to 15 carbon atoms;
x is an integer from 1 to 4; and
the retarding surfactant has a hydrophilic-lipophilic balance from 3 to 7; and
an external phase consisting of a base oil;
where the ratio of the weight of retarding surfactant in the retarded acid emulsion to the weight of aqueous acid in the retarded acid emulsion is from 1:40 to 1:8;
dissolving at least some of the carbonate formation with the retarded acid system; and
acidizing the carbonate formation.
9. The method of claim 8 , where R is a hydrocarbon having from 12 to 14 carbon atoms and x is 1.
10. The method of claim 8 , where the strong acid is an inorganic acid.
11. The method of claim 8 , where the strong acid comprises hydrofluoric acid.
12. The method of claim 8 , where the base oil comprises mineral oil, safra oil, diesel, C12-C20 linear alpha olefins, or combinations thereof.