US 9,399,728 B2  26  25  In addition, the fluids with aluminum-acetate crosslinker  successfully passed corrosion tests at 200°  F. and 225°  F.  Preliminary Screening for Non-Iron Based Crosslinkers  Initially following non-iron based crosslinkers were iden› tified as potential candidates for replacement of ferric chlo› ride in in-situ gelling acidizing system:  (a) Aluminum-based crosslinkers;  (b) Chromium-based crosslinkers;  ( c) Zirconium-based crosslinkers;  (d) Titanium-based crosslinkers; and  ( e) Cobaltous-based crosslinkers.  Chromium-based crosslinkers were promptly excluded  because certain chromium species can be toxic in nature.  Aluminum, titanium, cobalt, and zirconium-based crosslink- ers have no health, safety, or environmental concerns and are  very safe to handle. Hence the initial screening tests were  carried out with above mentioned crosslinkers for in-situ  gelling acidizing except for any chromium-based crosslinker.  Laboratory Chemicals  The materials used in the testing included:  (a) a representative strong acid, specifically concentrated  hydrochloric acid (HCl) in water;  (b) a representative viscosity-increasing agent, specifically  either:  (i) an anionic polymer tested at a concentration of 15 gpt in  a test fluid, more specifically a terpolymer of 2-acryla› mido-2-methylpropane sulfonic acid, acrylamide, and  acrylic acid; or  (ii) a cationic polymer tested at a concentration of20 gpt in  a test fluid, more specifically an acrylamide quaternary  ammonium salt of dimethyl ammonium chloride;  ( c) a representative water-soluble weak acid, specifically  glycolic acid 70% in water at a concentration of 2 gpt in the  test fluids;  ( d) a representative corrosion inhibitor, specifically either:  (i) a fluid including chloromethylnaphthalene quinoline  quaternary amine, isopropanol, and aldehyde at a con› centration of 10 gpt in the test fluids; or  (ii) a fluid of propargyl alcohol and methanol at a concen› tration of 10 gpt in the test fluids;  ( e) a representative surfactant, specifically a fluid mixture  of heavy aromatic petroleum naphtha and isopropanol at a  concentration of 1 gpt in the test fluids;  (f)  tested crosslinkers that are known to have no health,  safety, or environmental concerns and are safe to handle,  specifically:  (i) a conventional iron-based crosslinker, specifically ferric  chloride (FeC1$_{3}$ );  (ii) three different zirconium-based crosslinkers, specifi› cally "CL-23", "CL-29", and "CL-37", which are com› mercially available from Halliburton Energy Services,  Inc. in Duncan, Okla.;  (iii) two different titanium based crosslinkers, specifically  "CL-18" and "CL-11", which are commercially avail- able from Halliburton Energy Services, Inc. in Duncan,  Okla.;  (iv) a cobalt-based crosslinker, specifically "CAT-OS-1",  which is commercially available from Halliburton  Energy Services, Inc. in Duncan, Okla.; and  (v) aluminum-based crosslinkers, specifically either alu› minum chloride or an aluminum(III)-carboxylate  crosslinker, more specifically Al-monoacetate, which is  also identified as basic aluminum monoacetate or  Al(C$_{2}$H$_{3}$O$_{2}$)(OH)$_{2}$ ; and  (g) a representative carbonate for neutralizing the various  test fluids, specifically calcium carbonate (CaCO$_{3}$ ).  Fluids were tested lowering the acid concentration to 24%,  22%, and further to 20% HCl so as to optimize the acid  percentage and conduct further testing. But 24% HCl with  ferric chloride and Al-acetate acid blend also could not build  up the viscosity with acid spend and hence these concentra- tions were also not suitable for further testing. Finally 20%  HCl with ferric chloride acid blend could deliver the expected  in-situ crosslinking at around pH 2.2 with acid spend. With  carbonate neutralization of 20% HCl, a solid goo ball like  in-situ crosslinking was observed that was eventually broken  with further increase in pH. A non-iron based crosslinker like  aluminum acetate also showed similar kind of performance in  20%HCI.  Current in-situ crosslinking for acidizing systems using  ferric chloride crosslinker is designed mainly for 5% to 15%  HCl concentrations. It was believed that up to 28% HCl could  be used, but based on this laboratory testing no crosslinking  was observed with acid spend, either in ferric chloride or with  any other non-iron based crosslinker in 28% HCI. This is  mainly because of the high concentration of calcium chloride  generated during neutralization, which reduces the viscosity  of an in-situ crosslinked gel. As the calcium ion concentration  while neutralization is extremely high, no viscosity rise could  be seen in 28% HCI.  Similar to ferric chloride, this acid blend in 28% HCl  without additional chelating agent as shown below also could  not build up the viscosity with acid spend.  As a preliminary matter, it was determined that the use of  28% HCl in water for in-situ crosslinking for an acidizing  blend with existing ferric chloride was not feasible. While  neutralizing this test fluid with calcium carbonate, it was  observed that there is no increase in viscosity as acid spends,  even if pH is increased beyond 2.  In the beginning the tests were carried out with 28% HCl  and cationic polymer (15 gpt) as gelling agent at 200°  F. using  an existing ferric chloride solution in a concentration of 4.5  gpt as crosslinker. The blend was prepared as per the proce› dure described above. Also under similar test conditions, the  test was also carried out with aluminum acetate in a concen- tration of38.3 ppt.  Preliminary Screening Results in 28% HCl  After hydration of the polymer, the test fluid was neutral› ized at 200°  F. using calcium carbonate. The pH of each test  system was monitored continuously using pH meter during  neutralization process.  The test fluids with the cationic polymer were then  hydrated with high-speed stirring for 3 hrs. The test fluids  with the anionic polymer were then hydrated with high speed  stirring for 10 minutes.  For preparing a 100 ml test fluid, the desired amount of  water and acid was added to have the desired HCl concentra› tion in a warring blender. The additives such as the polymer,  the surfactant, the corrosion inhibitor, the weak acid, and a  crosslinker were added while continuously stirring the test  fluid.  Laboratory tests were carried out for screening of non-iron  based crosslinkers and to evaluate their performance.  The aluminum acetate exists as a solid powder form. In  order to make it more convenient during field operation, an  80% w/v aqueous solution was prepared and used in concen› tration of 4.5 gpt for all the performance tests (gel stability  and corrosion tests). This 80% w/v aqueous solution also gave  similar performance as that of powder form. Moreover, the  solution’s performance was found unaltered after 2 months of  preparation.  Screening Procedure 