SELF-SUSPENDING PROPPANTS
Both the inside surface and the outside surface of the hydrogel polymer coating of a self-suspending proppant are surface crosslinked.
1 . A self-suspending proppant comprising a proppant substrate particle and a water-swellable coating made from a hydrogel polymer on the proppant substrate particle, wherein the water swellable coating defines an inside surface on the proppant substrate particle, an outside surface remote from the inside surface and a body section therebetween, wherein both the inside surface and the outside surface have been surface crosslinked.
2 . The self-suspending proppant of claim 1 , wherein the inside surface is crosslinked by a first crosslinking agent and the outside surface is crosslinked by a second crosslinking agent, and further wherein the number average molecular weights of both the first crosslinking agent and the second crosslinking agent are ≤1,000,000 Daltons.
3 . The self-suspending proppant of claim 2 , wherein the inside surface is crosslinked by applying a crosslinking agent to the proppant substrate particle and thereafter coating the proppant substrate particle with the hydrogel polymer.
4 . The self-suspending proppant of claim 2 , wherein the body section is also crosslinked.
5 . The modified proppant of claim 2 , wherein each of the inside surface and the outside surface are surface crosslinked by means of a covalent crosslinking agent which is independently selected from an epoxide, an anhydride, an aldehyde, a diisocyanate and a carbodiimide.
6 . The self-suspending proppant of claim 5 , wherein each covalent crosslinking agent is independently selected from an epoxide and a diisocyanate
7 . The self-suspending proppant of claim 2 , wherein the water swellable coating is formed from a polyacrylamide, a starch or both.
8 . The self-suspending proppant of claim 2 , wherein the modified proppant is made by (a) forming a polymer/particle mixture by combining an inverse emulsion of the hydrogel polymer with a proppant substrate particle that had previously been coated with a first covalent crosslinking agent, (c) continuing to mix the polymer/particle mixture until the hydrogel polymer coating is formed, and (d) drying the hydrogel polymer coating,
wherein a second covalent crosslinking agent is combined with the polymer/particle mixture before the hydrogel polymer coating is dried.
9 . The self-suspending proppant of claim 2 , wherein the hydrogel coating is made from a single hydrogel polymer.
10 . The self-suspending proppant of claim 2 , wherein the hydrogel coating is made from multiple different hydrogel polymers.
11 . The self-suspending proppant of claim 10 , wherein the hydrogel coating is formed from distinct coating layers, each coating layer being made from its own individual hydrogel polymer.
12 . The self-suspending proppant of claim 10 , wherein the hydrogel coating defines different regions in which the concentration of a first hydrogel polymer decreases while the concentration of a second hydrogel polymer increases from the inside surface of the coating to its outside surface.
13 . The self-suspending proppant of claim 2 , wherein the modified proppant exhibits a volumetric expansion of at least about 1.3 after being exposed to a simulated hard water containing 6,400 ppm hardness.
14 . The self-suspending proppant of claim 13 , wherein the modified proppant exhibits a volumetric expansion of at least about 1.75 after being exposed to a simulated hard water containing 6,400 ppm hardness.
15 . The self-suspending proppant of claim 2 , wherein the modified proppant exhibits a volumetric expansion of at least about 1.3 after having been subjected to shear mixing in a simulated hard water containing 6,400 ppm hardness at a shear rate of about 511 s −1 for 10 minutes.
16 . The self-suspending proppant of claim 15 , wherein the modified proppant exhibits a volumetric expansion of at least about 1.75 after having been subjected to shear mixing in a simulated hard water containing 6,400 ppm hardness at a shear rate of about 511 s −1 for 10 minutes.
17 . The self-suspending proppant of claim 2 , wherein the proppant is dry.
18 . An aqueous fracturing fluid comprising an aqueous carrier liquid and the self-suspending proppant of claim 1 .
19 . A method for fracturing a geological formation comprising pumping the fracturing fluid of claim 18 into the formation.