Proppant-free hydraulic fracturing

Abstract

A subterranean zone penetrated by a wellbore is hydraulically fractured. A well tool assembly is positioned within a casing installed in the wellbore. The well tool assembly includes a perforation tool and a resettable packer. The casing is perforated, such that fluid can be conducted from inside the casing to the subterranean zone through the perforations. The well tool assembly is positioned at a location within the casing that is downhole of the perforations. A portion of the casing that is downhole of the perforations is sealed with the resettable packer. A fracturing fluid is pulsed into the subterranean zone via the perforations by alternating between a first flow rate and a second flow rate. The second flow rate is in a range of from about 10% to about 40% of the first flow rate. The fracturing fluid is free of proppant.

What is claimed is: 
     
         1 . A method for hydraulically fracturing a subterranean zone penetrated by a wellbore, comprising:
 positioning a well tool assembly within a casing installed in the wellbore, the well tool assembly comprising a perforation tool and a resettable packer;   with the perforation tool, perforating the casing to form a plurality of perforations in the casing, such that fluid can be conducted from inside the casing to the subterranean zone through the plurality of perforations;   positioning the well tool assembly at a location within the casing that is downhole of the plurality of perforations;   with the resettable packer, sealing a portion of the casing that is downhole of the plurality of perforations from a remaining portion of the casing; and   pulsing a fracturing fluid into the subterranean zone via the plurality of perforations by alternating between flowing the fracturing fluid into the casing at a first flow rate that is equal to or less than 6 barrels per minute and flowing the fracturing fluid into the casing at a second flow rate that is in a range of from about 10% to about 40% of the first flow rate, wherein the fracturing fluid is free of proppant and has a viscosity that is substantially equal to that of water.   
     
     
         2 . The method of  claim 1 , wherein the plurality of perforations is a first plurality of perforations, and the method further comprises:
 unsetting the resettable packer to unseal the portion of the casing that is downhole of the first plurality of perforations;   positioning the well tool assembly at a location within the casing that is uphole of the first plurality of perforations;   with the perforation tool, perforating the casing to form a second plurality of perforations in the casing uphole of the first plurality of perforations;   positioning the well tool assembly at a location within the casing that is downhole of the second plurality of perforations;   with the resettable packer, sealing a portion of the casing that is downhole of the second plurality of perforations from a remaining portion of the casing;   pulsing the fracturing fluid into the subterranean zone via the second plurality of perforations by alternating between flowing the fracturing fluid into the casing at the first flow rate and flowing the fracturing fluid into the casing at the second flow rate; and   unsetting the resettable packer to unseal the portion of the casing that is downhole of the second plurality of perforations.   
     
     
         3 . The method of  claim 2 , wherein the well tool assembly remains within the casing throughout implementation of the method. 
     
     
         4 . The method of  claim 3 , wherein each plurality of perforations spans a portion of the casing that is at most 1.5 meters in longitudinal length. 
     
     
         5 . The method of  claim 4 , wherein each plurality of perforations comprises at most 12 perforations. 
     
     
         6 . The method of  claim 2 , wherein the fracturing fluid is flowed into the casing at the first flow rate for at most 5 minutes before alternating to the second flow rate. 
     
     
         7 . The method of  claim 6 , wherein the fracturing fluid is flowed into the casing at the second flow rate for at most 1.5 minutes before alternating to the first flow rate. 
     
     
         8 . The method of  claim 7 , wherein pulsing the fracturing fluid into the subterranean zone comprises alternating between flowing the fracturing fluid into the casing at the first flow rate and flowing the fracturing fluid into the casing at the second flow rate, such that each of the first flow rate and the second flow rate occur at least 5 times. 
     
     
         9 . The method of  claim 7 , pulsing the fracturing fluid into the subterranean zone comprises alternating between flowing the fracturing fluid into the casing at the first flow rate and flowing the fracturing fluid into the casing at the second flow rate until at least 2,000 barrels of fracturing fluid have been flowed into the subterranean zone via the respective plurality of perforations. 
     
     
         10 . A method for hydraulically fracturing a subterranean zone having a matrix permeability of at most 100 nanoDarcy and a clay content of at most 60 volume %, the method comprising:
 a) injecting a fracturing fluid into the subterranean zone at a first flow rate that is equal to or less than 6 barrels per minute, wherein the fracturing fluid is free of proppant and has a viscosity that is substantially equal to that of water;   b) injecting the fracturing fluid into the subterranean zone at a second flow rate that is in a range of from about 10% to about 40% of the first flow rate; and   c) sequentially repeating steps a) and b).   
     
     
         11 . The method of  claim 10 , wherein the fracturing fluid is injected into the subterranean zone at step a) for at most 5 minutes. 
     
     
         12 . The method of  claim 11 , wherein the fracturing fluid is injected into the subterranean zone at step b) for at most 1.5 minutes. 
     
     
         13 . The method of  claim 12 , wherein steps a) and b) are sequentially repeated at step c) at least 5 times. 
     
     
         14 . The method of  claim 12 , wherein steps a) and b) are sequentially repeated at step c) until at least 2,000 barrels of fracturing fluid have been injected into the subterranean zone. 
     
     
         15 . A system for hydraulically fracturing a subterranean zone penetrated by a wellbore, the system comprising:
 a well tool assembly comprising:
 a perforation tool configured to form a plurality of perforations in a casing installed within the wellbore; and 
 a resettable packer configured to reversibly seal a portion of the casing from a remaining portion of the casing while a fracturing fluid is injected into the subterranean zone via the plurality of perforations, wherein the well tool assembly is configured to remain within the casing during hydraulic fracturing; 
   a fracturing fluid that is free of proppant and has a viscosity that is substantially equal to that of water; and   a pump configured to flow the fracturing fluid into the subterranean zone via the plurality of perforations formed by the perforation tool by pulsing the fracturing fluid into the casing between a first flow rate that is equal to or less than 6 barrels per minute and a second flow rate that is in a range of from about 10% to about 40% of the first flow rate, thereby hydraulically fracturing the subterranean zone.   
     
     
         16 . The system of  claim 15 , wherein the perforation tool is configured to form the plurality of perforations in the casing across a portion of the casing that is at most 1.5 meters in longitudinal length. 
     
     
         17 . The system of  claim 16 , wherein the plurality of perforations comprises at most 12 perforations.