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 free-flowing, substantially cured, coated proppant solids having a substantially cured polyurethane resin coating that forms interparticle bonds under downhole conditions, the method comprising:
coating proppant solids coated with a bifunctional silane having a reactive primary amino group and a hydrolysable ethoxysilyl group, wherein said solids have been heated to temperature of about 50 to about 150° C. with a curable coating formulation without the use of an organic solvent;
reacting the curable coating formulation coated onto said proppant solids under conditions sufficient to substantially cure said curable coating formulation,
wherein the curable coating formulation comprises an isocyanate-functional component having at least 2 isocyanate groups, and a polyol-functional component comprising a polyether polyol, wherein the formulation comprises an excess functional equivalent weight of the isocyanate-functional component relative to the polyol-functional component;
wherein said coating and reacting occurs simultaneously and in a period of time of less than about four minutes to form the free-flowing, substantially cured, coated proppant solids.
2. The method according to claim 1 , wherein said curable coating formulation comprises a functional equivalent amount of said isocyanate-functional component that is about 200 to about 400% more than the polyol-functional component.
3. The method according to claim 1 , wherein said proppant solids exhibits an average particle size from about 50 μm to about 3000 μm.
4. The method according to claim 1 wherein said curable coating formulation comprises an aliphatic polyol.
5. The method according to claim 1 wherein said coating occurs simultaneously with said curing at a temperature from about 75° C. to about 150° C.
6. The method according to claim 1 wherein said method comprises adding to said coating as it cures an ion exchange resin or dissolvable solid that when dissolving releases a scale inhibitor.
7. The method according to claim 1 , the method further comprising contacting the substantially cured, coated proppant solids with an anticaking agent.
8. The method according to claim 7 wherein said anticaking agent comprises amorphous silica.
9. A method for producing free-flowing, substantially cured, coated proppant solids having a substantially cured polyurethane resin coating that forms interparticle bonds under downhole conditions, the method comprising:
coating proppant solids coated with a bifunctional silane having a reactive primary amino group and a hydrolysable ethoxysilyl group, wherein said solids have been heated to temperature of about 50 to about 150° C. with a curable coating formulation without the use of an organic solvent; and
reacting the curable coating formulation to react under conditions sufficient to substantially cure said proppant solids coated with said curable coating formulation,
wherein the curable coating formulation comprises an isocyanate-functional component having at least 2 isocyanate groups, and a polyol-functional component comprising a polyether polyol, wherein the formulation comprises an excess functional weight % of the isocyanate-functional component relative to the polyol-functional component;
wherein said coating and reacting occurs simultaneously and in a period of time of less than about four minutes to form the free-flowing, substantially cured, coated proppant solids, and
wherein the isocyanate-functional component has an excess functional weight % of about 110 to about 430% relative to the polyol-functional component.