Proppant with polyurea-type coating
Proppants for hydraulic fracturing of oil and gas wells are coated with a polyurea-type coating. In a preferred embodiment, the polyurea-type coating is formed by contacting a polymeric isocyanate with an amount of water a blowing catalyst at a rate and quantity sufficient to generate a reactive amine in situ on the outer surface of the proppant which thereby reacts with unconverted polymeric isocyanate to form a thin polyurea-type surface coating that is substantially solid and lacks foam or substantial porosity. Alternatively, the polyurea-type can be produced by selecting reactive amine compounds and isocyanates to develop the coated proppant. The coated proppants retain the discrete, free-flowing character of the original core solids but with the beneficial effects of the polyurea-type coating of the present invention.
1. A method for the production of a coated proppant that is coated with a substantially foam-free polyurea-type coating, the method comprising:
contacting a heated proppant core solid with:
(a) an isocyanate comprising at least one reactive isocyanate group; and
(b) a mixture of water and a curing agent and/or catalyst in amounts sufficient to promote the formation of said substantially foam-free, polyurea-type coating,
wherein said water is contacted with said isocyanate at a rate sufficient to form a reactive amine that then reacts with additional isocyanate to form said polyurea-type coating on said proppant,
wherein the proppant core solid, water and isocyanate are added a rate sufficient to maintain a proportion of 5-30 wt % water to 95-70% isocyanate.
2. The method of claim 1 , wherein the isocyanate and the mixture of the water and the curing agent and/or catalyst are contacted with the heated proppant core simultaneously.
3. The method of claim 1 , wherein the isocyanate and the mixture of the water and the curing agent and/or catalyst are contacted with the heated proppant core sequentially.
4. The method of claim 1 , wherein said proppant core solid is sand or ceramic.
5. The method of claim 1 , wherein said isocyanate comprises a polymer or an oligomer of an isocyanate.
6. The method of claim 1 , wherein said catalyst comprises bis-dimethylaminoethylether or bis-morpholinium diethylether.
7. The method of claim 1 , wherein said heated proppant core solid has been heated to a temperature from 50-175° C. before being contacted with the isocyanate or water.
8. The method of claim 1 , wherein said proppant core solid has been heated to a temperature from 75-125° C. before being contacted with the isocyanate or water.
9. The method of claim 1 , wherein said polyurea-type coating is present on said proppant core solid in an amount of 1-5 wt % based on total weight of the coated proppant.
10. The method of claim 1 , wherein said method further comprises contacting said the heated proppant core solid with a silane adhesion promoter before contact with said isocyanate or said mixture.
11. The method of claim 1 , wherein the heated proppant core solid is a polyurethane-coated proppant core solid.
12. The method of claim 1 , wherein the heated proppant core solid is a phenolic-resin-coated proppant core solid.
13. The method of claim 1 , wherein said polyurea-type coating is cross-linked.
14. A coated proppant prepared according to a method of claim 1 .
15. A coated proppant prepared according to a method of claim 11 .
16. A coated proppant prepared according to a method of claim 12 .