Solid sandstone dissolver

Abstract

A dry solid composition is disclosed that, when added to an aqueous liquid, provides a slurry that can be used in sandstone acid fracturing. The slurry generates hydrofluoric acid downhole to etch the sandstone fracture faces created. The chemical and physical properties of the composition result in very uneven etching of the fracture faces, enhancing the fluid conductivity of the final fracture. Optionally, inert masking materials may be included in the dry composition, or added to the slurry, to increase the inhomogeneity of the etching and further increase the fluid conductivity.

1 . A water-free composition comprising particles of a solid acid-precursor and particles of a solid that releases hydrogen fluoride in the presence of aqueous acid.  
   
   
       2 . The composition of  claim 1  wherein said solid acid-precursor is selected from the group consisting of lactide, glycolide, polylactic acid, polyglycolic acid, copolymers of polylactic acid and polyglycolic acid, copolymers of glycolic acid with other hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containing moieties, copolymers of lactic acid with other hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containing moieties, and mixtures thereof.  
   
   
       3 . The composition of  claim 1  wherein said solid acid-precursor is coated with an effective amount of a material that slows hydrolysis of said solid acid-precursor when said solid acid-precursor is contacted with an aqueous fluid.  
   
   
       4 . The composition of  claim 1  wherein said solid that releases hydrogen fluoride in the presence of aqueous acid is selected from the group consisting of ammonium fluoride, ammonium bifluoride, polyvinylammonium fluoride, polyvinylpyridinium fluoride, pyridinium fluoride, imidazolium fluoride, sodium tetrafluoroborate, ammonium tetrafluoroborate, salts of hexafluoroantimony, TEFLON™ synthetic resinous fluorine-containing polymer, and mixtures thereof.  
   
   
       5 . The composition of  claim 1  wherein said particles of said solid acid-precursor and said particles of said solid that releases hydrogen fluoride in the presence of aqueous acid differ in one or more than one of properties selected from size, shape, surface area, and hydrolysis rate.  
   
   
       6 . The composition of  claim 1  further comprising particles of an inert solid.  
   
   
       7 . The composition of  claim 6  wherein said inert solid particles are selected from the group consisting of plastic, glass, polyacrylamide, phenol formaldehyde polymer, nylon, wax, natural rubber, synthetic rubber, vermiculite, organic seeds, organic shells, mica, cellophane flakes, starch, rock salt, benzoic acid, metals, naphthalene, and mixtures thereof.  
   
   
       8 . The composition of  claim 1  slurried in a hydrocarbon.  
   
   
       9 . The composition of  claim 1  further comprising a chelating agent for one or more than one of the ions selected from calcium, aluminum and magnesium.  
   
   
       10 . The composition of  claim 1  wherein the solid that releases hydrogen fluoride is coated to slow the release of hydrogen fluoride.  
   
   
       11 . An aqueous fluid made by mixing with water a water-free composition comprising particles of a solid acid-precursor and particles of a solid that releases hydrogen fluoride in the presence of aqueous acid.  
   
   
       12 . The composition of  claim 11  buffered to a pH of 6.0 or greater.  
   
   
       13 . The composition of  claim 11  wherein said fluid further comprises inert solid particles.  
   
   
       14 . The composition of  claim 13  buffered to a pH of 6.0 or greater.  
   
   
       15 . The composition of  claim 11  wherein said fluid further comprises a viscosifying agent.  
   
   
       16 . The composition of  claim 15  buffered to a pH of 6.0 or greater.  
   
   
       17 . The composition of  claim 11  further comprising a chelating agent for one or more than one of the ions selected from calcium, aluminum and magnesium.  
   
   
       18 . The composition of  claim 11  further wherein either the solid acid precursor or the solid that releases hydrogen fluoride or both are coated with hydrocarbon.  
   
   
       19 . A method of creating a fracture in a subterranean formation penetrated by a wellbore comprising: 
 a. preparing a fluid comprising a solid acid precursor and a solid that releases hydrogen fluoride in the presence of aqueous acid,    b. injecting said fluid into said formation above fracture pressure, and    c. allowing at least a portion of said solid acid-precursor to hydrolyze.    
   
   
       20 . The method of  claim 19  wherein said fluid is buffered to a pH of 6.0 or greater.  
   
   
       21 . The method of  claim 19  wherein said fluid further comprises particles of an inert solid.  
   
   
       22 . The method of  claim 21  wherein said fluid is buffered to a pH of 6.0 or greater.  
   
   
       23 . The method of  claim 19  wherein said fluid further comprises a viscosifying agent.  
   
   
       24 . The method of  claim 23  wherein said fluid is buffered to a pH of 6.0 or greater.  
   
   
       25 . The method of  claim 19  wherein said fluid further comprises a chelating agent for one or more than one of the ions selected from calcium, aluminum and magnesium.  
   
   
       26 . The method of  claim 19  wherein said step of injecting above fracture pressure is preceded by injection of a fluoride-free pad comprising a component selected from a dissolution agent and a clay control agent.  
   
   
       27 . The method of  claim 26  wherein said pad comprises a viscosifying agent.  
   
   
       28 . The method of  claim 19  wherein the solid that releases hydrogen fluoride is coated to slow the release of hydrogen fluoride.  
   
   
       29 . The method of  claim 19  wherein the solid acid-precursor is coated with an effective amount of a material that slows hydrolysis of said solid acid-precursor when said solid acid-precursor is contacted with an aqueous fluid.  
   
   
       30 . The method of  claim 28  wherein the solid that releases hydrogen fluoride is coated with hydrocarbon.  
   
   
       31 . The method of  claim 29  wherein the solid acid-precursor is coated with hydrocarbon.  
   
   
       32 . A method of creating a fracture in a subterranean formation penetrated by a wellbore comprising: 
 a. preparing a fluid comprising a solid acid precursor, particles of an inert solid, and hydrofluoric acid,    b. injecting said fluid into said formation above fracture pressure, and    c. allowing at least a portion of said solid acid-precursor to hydrolyze.