Selective fracture face dissolution

Abstract

A method is given for acid fracturing a subterranean formation for improving the flow of fluids. The principal source, optionally the sole source, of the acid is a solid acid-precursor, optionally injected with an additional solid that is inert and that masks a portion of the newly created fracture faces so that the fracture face etching by the acid is not uniform. The method ensures a good flow path for fluids between the fracture tip and the wellbore.

What is claimed is: 
     
         1 . A method of acidizing a subterranean formation, comprising:
 creating a fracture in the subterranean formation penetrated by a wellbore;   masking a portion of a face of the fracture with a protecting material to create a balance of masked and unmasked areas along the fracture face;   etching the unmasked areas with an acid in the absence of a proppant;   removing, degrading, dissolving, or hydrolyzing the protecting material; and   forming at least one supporting pillar, comprising formation material, in the fracture to help keep the fracture open.   
     
     
         2 . The method of  claim 1 , wherein the protecting material is soft and deformable particles. 
     
     
         3 . The method of  claim 1 , wherein the protecting material is non-deformable particles having large aspect ratios. 
     
     
         4 . The method of  claim 1 , wherein the protecting material is in a shape of ribbon, film, sheet, flake, platelet, and mixtures thereof. 
     
     
         5 . The method of  claim 1 , wherein the protecting material is selected from the group consisting of glass, phenol formaldehyde polymer, wax, natural rubber, synthetic rubber, vermiculite, organic seeds, organic shells, mica, cellophane flakes, starch, rock salt, metals, naphthalene and mixtures thereof. 
     
     
         6 . The method of  claim 1 , wherein the acid is a hydrolyzed product of particles of a solid acid-precursor. 
     
     
         7 . The method of  claim 6 , wherein the 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. 
     
     
         8 . The method of  claim 7 , wherein said solid acid-precursor is polylactic acid. 
     
     
         9 . The method of  claim 7 , wherein said solid acid-precursor is polyglycolic acid. 
     
     
         10 . The method of  claim 6  wherein said solid acid-precursor has a shape selected from the group consisting of beads, ribbons, platelets, fibers and mixtures thereof. 
     
     
         11 . The method of  claim 6 , wherein said solid acid-precursor is coated to hinder hydrolysis. 
     
     
         12 . The method of  claim 1 , wherein the protecting material and the acid are introduced into the subterranean formation in a single fluid. 
     
     
         13 . The method of  claim 12 , wherein the fluid further comprises a viscosifying agent. 
     
     
         14 . The method of  claim 12 , wherein the fluid further comprises an agent capable of dissolving at least one component of said formation. 
     
     
         15 . The method of  claim 14 , wherein said agent capable of dissolving at least one component of said formation is selected from the group consisting of hydrochloric acid, formic acid, acetic acid, lactic acid, glycolic acid, sulfamic acid, malic acid, tartaric acid, maleic acid, methylsulfamic acid, chloroacetic acid, aminopolycarboxylic acids, polyaminopolycarboxylic acids, 3-hydroxypropionic acid, salts thereof and mixtures thereof. 
     
     
         16 . The method of  claim 14  wherein said agent capable of dissolving at least one component of said formation is retarded by a method selected from emulsifying and encapsulating. 
     
     
         17 . A method, comprising:
 creating a fracture in a subterranean formation penetrated by a wellbore;   masking a portion of a face of the fracture with a first solid acid-precursor that is capable of hydrolyzing and dissolving to generate a first acid at a first rate, said masking creating a balance of masked and unmasked areas along the face of the fracture;   etching the unmasked areas with a second acid generated by hydrolysis of a second solid acid-precursor at a second rate, wherein the first rate is slower than the second rate;   forming at least one supporting pillar, comprising formation material, in the fracture to help keep the fracture open, in the absence of a proppant.   
     
     
         18 . The method of  claim 17 , further comprising allowing the first solid acid-precursor to hydrolyze and dissolve after etching the unmasked areas with the second acid. 
     
     
         19 . The method of  claim 17 , wherein said first and second solid acid-precursors differ in one or more than one of the properties selected from size, shape, surface area, and hydrolysis rate. 
     
     
         20 . The method of  claim 17 , wherein at least one of said first and second solid acid-precursors are coated to hinder hydrolysis.