Fracturing of subterranean formations

Abstract

Fracturing of a subterranean formation may be achieved using a hydrogel. A preferred gel is formed from a gel precursor formulation which comprises an aqueous formulation of poly(1,4-di(4-(N-methylpyridinyl))-2,3-di(4-(1-formylphenyl)butylidene), poly (vinyl alcohol) and a suitable proppant, in the presence of an acid catalyst. The gel may be prepared at the surface with a viscosity of greater than 200 cp measured at 100 s −1 and then injected into the subterranean formation under high pressure to fracture the formation.

1 . A method of hydraulically fracturing a subterranean formation, comprising the step of: 
 contacting a subterranean formation with a treatment fluid formulation at a flow rate and pressure sufficient to produce or extend a fracture in the formation, wherein the treatment fluid formulation comprises a third polymeric material which comprises a second polymeric material cross-linked by a first polymeric material, wherein said first polymeric material comprises:    (i) a first polymeric material having a repeat unit of formula                          wherein A and B are the same or different, are selected from optionally-substituted aromatic and heteroaromatic groups and at least one comprises a relatively polar atom or group and R 1  and R 2  independently comprise relatively non-polar atoms or groups; or    (ii) a first polymeric material prepared or preparable by providing a compound of general formula                          wherein A, B, R 1  and R 2  are as described above, in an aqueous solvent and causing the groups C═C in said compound to react with one another to form said first polymeric material.    
   
   
       2 . A method according to  claim 1 , wherein said first and second polymeric materials are reacted to form said third polymeric material prior to the treatment fluid formulation being injected via a well bore into the subterranean formation.  
   
   
       3 . A method according to  claim 1 , wherein said treatment fluid formulation has a viscosity at 25° C. in the range 50 to 500 cp at a sheer rate of 100 s −1 .  
   
   
       4 . A method according to  claim 1 , wherein said treatment fluid formulation has a viscosity at 200° F. in the range 20 to 100 cp, measured at a sheer rate of 100 s −1 .  
   
   
       5 . A method according to  claim 1 , wherein said treatment fluid formulation is aqueous and includes at least 90 wt % water.  
   
   
       6 . A method according to  claim 1 , wherein said treatment fluid formulation includes one or more proppants.  
   
   
       7 . A method according to  claim 1 , wherein said treatment fluid formulation includes breaker means for breaking the third polymeric material to reduce its viscosity and facilitate clean-up of the fracture.  
   
   
       8 . A method according to  claim 7 , wherein said breaker means is arranged to cleave chains of said third polymeric material.  
   
   
       9 . A method according to  claim 7 , wherein said breaker means is arranged to have a delayed action and includes means for restricting contact between an active material thereof and said third polymeric material.  
   
   
       10 . A method according to  claim 1 , which comprises selecting a said first polymeric material; selecting a said second polymeric material which includes a functional group which is able to react in the presence of said first polymeric material to form a third polymeric material; and causing the formation of said third polymeric material by a reaction involving said first and second polymeric materials.  
   
   
       11 . A method according to  claim 10 , wherein the ratio of the wt % of said first polymeric material to the wt % of said second polymeric material selected for preparation of said third polymeric material is less than 0.15 and is at least 0.01.  
   
   
       12 . A method according to  claim 11 , wherein the sum in said treatment fluid formulation of the wt % of the first and second polymeric materials selected for preparation of said third polymeric material is at least 1 wt % and is less than 8 wt %.  
   
   
       13 . A method according to  claim 1 , wherein, in the preparation of said third polymeric material, a catalyst is provided for catalysing the reaction of the first and second polymeric materials.  
   
   
       14 . A method according to  claim 1 , wherein one of A or B represents an optionally-substituted aromatic group and the other one represents an optionally substituted heteroaromatic group.  
   
   
       15 . A method according to  claim 1 , wherein said first polymeric material is of formula:  
     
       
         
         
             
             
         
       
     
     wherein n an integer.  
   
   
       16 . A method according to  claim 1 , wherein said second polymeric material is selected from optionally substituted polyvinyl alcohol, polyvinyl acetate, and polyalkalene glycols.  
   
   
       17 . A method according to  claim 1 , wherein said polymeric material includes at least one vinyl alcohol/vinyl acetate copolymer.  
   
   
       18 . A method of preparing a treatment fluid formulation comprising: 
 selecting a first polymeric material and a second polymeric material as described in  claim 1;  and    causing the formation of a said third polymeric material by a reaction involving said first and second polymeric materials.    
   
   
       19 . A method according to  claim 18 , which comprises contacting said first and second polymeric materials at a weight ratio of first to second in the range 0.025 to 0.067; and contacting the third polymeric material which forms with 5 to 20 wt % proppants.  
   
   
       20 . A treatment fluid formulation comprising: 
 water;    a third polymeric material according to  claim 1;  and    one or more proppants.    
   
   
       21 . A formulation according to  claim 20 , which comprises 1 to 5 wt % of said third polymeric material, 65 to 90 wt % water and 5 to 30 wt % of proppants.  
   
   
       22 . The use of a treatment fluid formulation as described in  claim 1  in hydraulically fracturing a subterranean formation.  
   
   
       23 . A method of breaking a third polymeric material as described in  claim 1 , the method comprising providing a formulation comprising said third polymeric material and a breaker means and causing said breaker means to change its state in order to release an active breaker material arranged to break the third polymeric material.  
   
   
       24 . An encapsulated breaker means for a third polymeric material as described in  claim 1  per se.  
   
   
       25 . A method of recovering oil from a subterranean formation comprising: 
 hydraulically fracturing a subterranean formation as described according to  claim 1;     allowing an area fractured to close down whilst being propped by a proppant; wherein as a result of said close down, a breaker means releases an active material which is arranged to lower the viscosity of the treatment fluid formulation of the first aspect; and