Coated and cured proppants
Solid proppants are coated with a coating that exhibits the handling characteristics of a pre-cured coating while also exhibiting the ability to form particle-to-particle bonds at the elevated temperatures and pressures within a wellbore. The coating includes a substantially homogeneous mixture of (i) at least one isocyanate component having at least 2 isocyanate groups, and (ii) a curing agent comprising a monofunctional alcohol, amine or amide. The coating process can be performed with short cycle times, e.g., less than about 4 minutes, and still produce a dry, free-flowing, coated proppant that exhibits low dust characteristics during pneumatic handling but also proppant consolidation downhole for reduced washout and good conductivity. Such proppants also form good unconfined compressive strength without use of an bond activator, are substantially unaffected in bond formation characteristics under downhole conditions despite prior heat exposure, and are resistant to leaching with hot water.
1. A method for producing coated proppant solids having a polyurethane resin coating that forms interparticle bonds under downhole conditions, the method comprising:
coating proppant solids coated with a silane with a coating formulation,
reacting the coating formulation onto said proppant solids under conditions sufficient to coat said proppant solids with the polyurethane resin, and
contacting the coated proppant solids with an anticaking agent, wherein said anticaking agent comprises amorphous silica,
wherein applied amounts of the anticaking agent are within the range from about 0.001 wt % to about 1 wt % based on the dry weight of the proppant solids,
wherein the coating formulation comprises an isocyanate component having at least 2 isocyanate groups and a polyol component, wherein the formulation comprises an excess weight of the isocyanate component relative to the polyol component,
wherein said coating and reacting occurs in a period of time of less than about four minutes to produce the coated proppant solids, and
wherein the method is performed free of solvents.
2. The method of claim 1 , wherein said coating formulation comprises an isocyanate component that is about 200% to about 300% in excess weight than the polyol component.
3. The method of claim 1 , wherein said proppant solids exhibits an average particle size from about 50 μm to about 3000 μm.
4. The method of claim 1 , wherein said polyol component comprises an aliphatic polyol.
5. The method of claim 1 , wherein said coating occurs at a temperature from about 75° C. to about 150° C.
6. The method of claim 1 , wherein said coating formulation further comprises an ion exchange resin or dissolvable solid that when dissolved releases a scale inhibitor.
7. The method of claim 1 , wherein the coating is a cured coating.
8. The method of claim 1 , wherein the proppant solids are preheated prior to being coated.
9. The method of claim 1 , wherein the proppant solids are preheated to a temperature of about 50° C. to about 150° C.
10. The method of claim 1 , wherein the coating and reacting steps are performed in a mixer.
11. The method of claim 1 , wherein the coating and reacting steps are performed simultaneously.
12. The method claim 1 , further comprising coating the proppant solids with a silane prior to coating the proppant solids with the coating formulation.
13. The method of claim 1 , wherein the silane is a bifunctional silane.
14. The method of claim 1 , wherein the silane comprises a reactive primary amino group and a hydrolysable ethoxysilyl group.
15. The method of 1 , wherein said amorphous silica is a dry form of amorphous silica or a dispersion of nanometer-sized fumed silica.