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$_{3}$ ). 

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 

Laboratory tests were carried out for screening of non-iron based crosslinkers and to evaluate their performance. 

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. 

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. 

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. 

Preliminary Screening Results in 28% HCl 

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. 

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. 

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. 

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. 

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. 

