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 and 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 proppant that is coated with a substantially cured, polyurea-type coating, the method comprising:
forming a thin, glassy film of said polyurea-type coating on a heated proppant core solid, having a size range of about 16 to about 100 mesh, by contacting said core simultaneously with:
(a) an isocyanate comprising at least 2 reactive isocyanate groups; and
(b) a mixture of water and a curing agent and/or catalyst in amounts sufficient to promote the formation of said thin, glass film, substantially cured, 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 and 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,
and wherein said coating exhibits an interparticle bond strength of at least 20 psi in unconfined compressive strength testing.
2. A method according to claim 1 , wherein said proppant core solid comprises sized sand or ceramic.
3. A method according to claim 1 , wherein said isocyanate comprises a polymer or an oligomer of an isocyanate.
4. A method according to claim 1 , wherein said catalyst comprises bis-dimethylaminoethylether or bis-morpholinium diethylether.
5. A method according to claim 1 , wherein the polyurea-type coating made by said method exhibits an interparticle bond strength of at least 100 psi in unconfined compressive strength testing.
6. A method according to claim 1 , wherein said heated proppant core solid has been preheated to a temperature within the range of 50-175° C.
7. A method according to claim 6 wherein said proppant core solid has been heated to a temperature within the range of 75-125° C.
8. A method according to claim 1 wherein said polyurea-type coating is present on said proppant within the range of 1-5 wt % based on total weight of coated proppant.
9. A method according to 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.
10. A method according to claim 1 , wherein said coating exhibits an interparticle bond strength of at least 20 psi in unconfined compressive strength testing.
11. The method according to claim 1 , wherein the coating is substantially foam-free.
12. The method according to claim 1 , wherein the coating is foam-free.
13. A method according to claim 1 , wherein said polyurea-type coating is disposed on an outermost skin layer on a polyurethane-coated proppant core solid.
14. A method according to claim 1 , wherein said polyurea-type coating is disposed on an outermost skin layer on a phenolic-resin-coated proppant core solid.
15. A method according to claim 1 , wherein said polyurea-type coating is cross-linked.