Heterogeneous Proppant Placement

Abstract

A method is given for inducing heterogeneous proppant placement in a hydraulic fracture in a subterranean formation by causing proppant aggregation through a gel phase transition or chemical transformation in the proppant carrier fluid. Proppant aggregation may be induced by causing or allowing syneresis of the polymer gel that viscosifies the fluid; formation of a polyelectrolyte complex from cationic and anionic polymers included in or created in, the fluid; and by increasing the temperature of the fluid above the critical solution temperature of a polymer in the fluid. The proppant carrier fluid may be formulated such that these transformations occur naturally during or after proppant injection, and the transformations may be chemically triggered or delayed.

1 . A method of inducing proppant aggregation in a hydraulic fracture comprising
 formulating a proppant carrier fluid viscosified by a first polymer gel that can undergo syneresis;   injecting a slurry of the fluid and proppant; and   triggering gel syneresis.   
     
     
         2 . The method of  claim 1  wherein the fluid further comprises fibers. 
     
     
         3 . The method of  claim 1  wherein at least a portion of the proppant is resin coated. 
     
     
         4 . The method of  claim 1  wherein the polymer gel is crosslinked. 
     
     
         5 . The method of  claim 1  wherein the gel is a borate crosslinked polymer gel and the syneresis is triggered by incorporation of a multivalent cation in the gel. 
     
     
         6 . The method of  claim 5  wherein the multivalent cation is a cation of a metal selected from the group consisting of Ca, Zn, Al, Fe, Cu, Co, Cr, Ni, Ti, Zr and mixtures thereof. 
     
     
         7 . The method of  claim 5  wherein the cation is incorporated by the dissolution of a salt, oxide or hydroxide of the cation. 
     
     
         8 . The method of  claim 5  wherein the cation is in the form of a hydroxide when it causes the syneresis. 
     
     
         9 . The method of  claim 1  wherein the syneresis is caused by overcrosslinking 
     
     
         10 . The method of  claim 9  wherein the overcrosslinking is delayed by a crosslink delay agent. 
     
     
         11 . The method of  claim 9  wherein the overcrosslinking is induced by an encapsulated crosslinker, a slowly dissolvable crosslinker, or a temperature-activated crosslinker. 
     
     
         12 . The method of  claim 1  wherein the syneresis is caused by including in the fluid, in addition to the polymer in the first polymer gel, a second polymer and a delayed crosslinker for the second polymer. 
     
     
         13 . The method of  claim 12  wherein the second polymer is at a concentration below its overlap concentration. 
     
     
         14 . The method of  claim 1  wherein the syneresis is caused by a superabsorbent polymer. 
     
     
         15 . The method of  claim 1  wherein the triggering is caused by a second fluid that contacts the proppant carrier fluid downhole. 
     
     
         16 . A method of inducing proppant aggregation in a hydraulic fracture comprising
 (1) formulating a proppant carrier fluid comprising (i) at least one anionic polyelectrolyte or the precursor to at least one anionic polyelectrolyte, and (ii) at least one cationic polyelectrolyte or the precursor to at least one cationic polyelectrolyte;   (2) injecting a slurry of the fluid and proppant; and   (3) triggering formation of a polyelectrolyte complex.   
     
     
         17 . The method of  claim 16  wherein the fluid further comprises fibers. 
     
     
         18 . The method of  claim 16  wherein at least a portion of the proppant is resin coated. 
     
     
         19 . The method of  claim 16  wherein the formation of the polyelectrolyte complex is induced by a pH change. 
     
     
         20 . The method of  claim 16  wherein the formation of the polyelectrolyte complex is induced by conversion of at least one polyelectrolyte precursor to a polyelectrolyte. 
     
     
         21 . The method of  claim 16  wherein the formation of the polyelectrolyte complex is induced by formation of a cationic polyelectrolyte downhole. 
     
     
         22 . The method of  claim 21  wherein the cationic polyelectrolyte is formed downhole by a method selected from Mannich reaction, Hofmann degradation of a polyacrylamide. 
     
     
         23 . The method of  claim 16  wherein the formation of the polyelectrolyte complex is induced by formation of an anionic polyelectrolyte downhole. 
     
     
         24 . The method of  claim 23  wherein the anionic polyelectrolyte is formed downhole by hydrolysis. 
     
     
         25 . The method of  claim 16  wherein at least one polyelectrolyte or polyelectrolyte precursor is initially present in the fluid in the internal phase of an emulsion. 
     
     
         26 . The method of  claim 16  wherein at least one polyelectrolyte or polyelectrolyte precursor is initially present in solid form. 
     
     
         27 . The method of  claim 16  wherein the formation of the polyelectrolyte complex is delayed by incorporating at least one polyelectrolyte in the fluid as a polyelectrolyte-surfactant complex. 
     
     
         28 . The method of  claim 16  wherein the triggering is caused by a second fluid that contacts the proppant carrier fluid downhole. 
     
     
         29 . A method of inducing proppant aggregation in a hydraulic fracture comprising
 (1) formulating a proppant carrier fluid comprising a polymer below its lower critical solution temperature; and   (2) injecting a slurry of the fluid and proppant into a subterranean formation that is above the lower polymer critical solution temperature.   
     
     
         30 . The method of  claim 19  wherein the fluid further comprises fibers. 
     
     
         31 . The method of  claim 29  wherein at least a portion of the proppant is resin coated.