SALT-TOLERANT SELF-SUSPENDING PROPPANTS
A self-suspending proppant that resists the adverse effects of calcium and other cations on swelling comprises a proppant substrate particle and a gelatinized cationic starch coating on the proppant substrate particle.
1 . A self-suspending proppant comprising a proppant substrate particle and a gelatinized cationic starch coating on the proppant substrate particle, wherein the self-suspending proppant exhibits a volumetric expansion as determined by its Settled Bed Height (SBH) of at least 1.5 in simulated hard water containing 80,000 ppm dissolved CaCO 3 after having been subjected to shear mixing at a shear rate of about 550 s −1 for 10 minutes.
2 . The self-suspending proppant of claim 1 , wherein the self-suspending proppant is dry.
3 . A self-suspending proppant which is both durable and especially resistant to the adverse effects of calcium and other cations on swelling, this self-suspending proppant being made by mixing proppant substrate particles with a cationic starch which is at least partially gelatinized thereby forming discrete starch-coated substrate particles, and then drying the starch-coated substrate particles so formed.
4 . The self-suspending proppant of claim 3 , wherein
(a) the proppant substrate particle is treated with an adhesion promoter,
(b) the cationic starch is crosslinked, or
(c) both.
5 . The self-suspending proppant of claim 4 , wherein the cationic starch has a degree of substitution of 0.030 to 0.55 and contains about 5 to 30 wt. % of amylose units and about 70 to 95 wt. % of amylopectin.
6 . The self-suspending proppant of claim 3 , wherein the cationic starch is cationic due to the presence of quaternary ammonium groups.
7 . The self-suspending proppant of claim 3 , wherein the self-suspending proppant exhibits a volumetric expansion by a factor of ≧˜1.3 when exposed to a simulated hard water containing 80,000 ppm CaCO 3 for 10 minutes.
8 . The self-suspending proppant of claim 7 , wherein the self-suspending proppant exhibits a volumetric expansion by a factor of ≧˜1.75 when exposed to a simulated hard water containing 80,000 ppm CaCO 3 for 10 minutes.
9 . The self-suspending proppant of claim 8 , wherein when contacted with a water-based fracturing fluid the cationic starch swells to form a hydrogel coating which has a thickness 10% to 1000% of the average diameter of the proppant particle substrate.
10 . The self-suspending proppant of claim 3 , wherein gelatinization of the hydrogel coating is essentially complete within about 10 minutes of being contacted with an excess of tap water at 20° C.
11 . A process for making a self-suspending proppant that is especially resistant to the adverse effects of calcium and other cations on swelling, the self-suspending proppant comprising a proppant substrate particle and a gelatinized cationic starch coating on the proppant substrate particle, the process comprising chemically modifying the cationic starch, the proppant substrate particles or both to enhance coating adhesion, mixing the cationic starch with the proppant substrate particles while the starch is at least partially gelatinized thereby forming discrete starch-coated substrate particles, and then drying the starch-coated substrate particles so formed.
12 . The process of claim 11 , wherein chemical modification is accomplished by one or more of
(a) pretreating the proppant substrate particle with an adhesion promoter, and
(b) crosslinking the cationic starch.
13 . The process of claim 12 , wherein the cationic starch has a degree of substitution of 0.030 to 0.55 and contains about 5 to 30 wt. % of amylose units and about 70 to 95 wt. % of amylopectin.
14 . The process of claim 11 , wherein the cationic starch is cationic due to the presence of quaternary ammonium groups.
15 . The process of claim 11 , wherein the cationic starch is at least partially gelatinized by heating the cationic starch in the presence of water.
16 . The process of claim 15 , wherein the cationic starch is at least partially gelatinized by heating the cationic starch in the presence of water at a first temperature, wherein the cationic starch is combined with the proppant substrate particles either before, during or after being heated at the first temperature, and further wherein the mixture of proppant substrate particles and at least partially gelatinized cationic starch so formed is dried by heating at a second temperature which is higher than the first temperature.
17 . The process of claim 16 , wherein the combination of cationic starch and water which is heated at the first temperature has a water/cationic starch ratio of ≦2.5.
18 . The process of claim 16 , wherein the combination of cationic starch and water which is heated at the first temperature has a water/cationic starch ratio of ≦0.5.
19 . The process of claim 18 , wherein the combination of cationic starch and water is heated at the first temperature in an extruder.
20 . The process of claim 18 , wherein a cationic reagent capable of introducing cationic functionality into a starch is included in the combination of cationic starch and water which is heated at the first temperature.
21 . The process of claim 20 , wherein the cationic reagent is a quaternary ammonium compound including an epoxy moiety or a chlorohydrin moiety.
22 . The process of claim 16 , wherein the at least partially hydrolyzed cationic starch produced by heating at the first temperature is directly combined with the proppant substrate particles.
23 . The process of claim 22 , wherein the at least partially hydrolyzed cationic starch is produced in an extruder.
24 . The process of claim 11 , wherein the cationic starch is at least partially gelatinized by heating the cationic starch in the presence of water and a cationization reagent at a first temperature in an extruder, wherein the cationic starch exiting the extruder is directly combined with the proppant substrate particles, wherein the mixture so formed is mixed until starch-coated particles are formed, and further wherein the starch-coated particles so formed are dried.
25 . An aqueous fracturing fluid comprising an aqueous carrier liquid and the self-suspending proppant of claim 2 .
26 . A method for fracturing a geological formation comprising pumping the fracturing fluid of claim 25 into the formation.