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The success of an acidizing operation can be compromised if the wellbore, tanks, or other equipment contain solids or other contaminants that could flow into the well or formation. Proper preparation is a key factor in a successful acid treatment.
Treating fluids must leave surface tanks, travel through surface pipe and well tubing, enter a wellbore, and pass through the perforations into the formation so that the solvent can react with the damaging solids. Each of these components through which the fluid travels must be properly cleaned before pumping acid into the formation. Surface tanks must be cleaned before being filled with acid. The best tanks are rubber lined and cleaned of any formerly contained materials before the new acid and additives are added to the tank. Surface lines through which the acid is pumped should be cleaned with acid before the treatment. A small amount of acid can be flushed through the lines and into waste containment before final hookup for the well treatment. This also can be accomplished in the step for acid cleaning well tubing.
The well should be adequately prepared before the service company arrives on site to perform the acid treatment. If possible, wellbore fill should be circulated out to remove any solids and sludge that have accumulated in the rathole and/or isolated by placing a heavy brine in the rathole prior to acidizing. If the formation pressure is very low, care must be taken to prevent the loss of accumulated sludge and other materials to the formation. Any fluid-loss additives selected should dissolve in the produced well fluids, such as oil-soluble resins or benzoic acid particulates.
Fluids used to load the well prior to injecting acid should be filtered to a “superclean” state to prevent any damage during injection testing before acidizing—typically to less than 50 ppm for solids and less than 2 microns for size. No produced lease water should be used because these produced waters usually are contaminated with emulsion breakers or corrosion inhibitors often found in water/oil separation facilities and may also contain suspended solid hydrocarbons and clay particles. Emulsion breakers and corrosion inhibitors in produced water can oil-wet the formation and reduce productivity, and suspended solids are very damaging.
Acid cleaning tubing
In addition to borehole cleanout, acid clean the tubing and surface piping before injecting acid into the formation to prevent plugging of the perforations by solids released from the tubing. Fig. 1 shows the characteristics of acid being pumped down tubing in a well. 
Fig. 1—Cleaning tubing with acid.
Pumping acid through tubing releases solids deposited on the pipe surface. Acid-insoluble solids like pipe dope, pararffin, asphalt, and gypsum or barite scales may plug the perforations and even fill the wellbore. Acid-soluble solids like calcium carbonate may just spend the acid, whereas dissolved iron oxide or iron sulfide may precipitate as the acid spends on other minerals in the formation. Either acid cleaning the tubing and reversing to surface containment or bypassing the production tubing with an acid-cleaned concentric tubing string prevents perforation plugging from tubing deposits.
The dissolution of mill scale and/or rust in the tubing can theoretically lead to concentrations as high as 75,000 ppm in acid, and field acid cleaning tests confirmed this. Iron complexing agents can prevent ferric hydroxide precipitation from acid with up to 10,000 ppm iron.
For high-pressure reservoirs, acid may be pumped down the tubing close to the bottom and then flowed back to the surface containment. If the reservoir pressure will not hold the acid hydrostatic column, foamed acid may be used to clean the tubing, or a work string can be run with a packer, isolation valve, and circulating tool to isolate the formation while acid cleaning the tubing. If a work string is not used and if the production tubing cannot be cleaned properly, it should be bypassed using a concentric tubing string to pump the acid.
A concentric tubing string can be used to circulate accumulated sludge below the perforated interval with clean brine before acid injection. Injection wells may have accumulated corrosion deposits and/or bacterial slimes. Producing wells may have loose scale deposits, hydrocarbon solids, or produced formation fines. Recent papers have provided additional guidance on tubing cleaning and pickling.
- McLeod Jr., H.O., Ledlow, L.B., and Till, M.V. 1983. The Planning, Execution, and Evaluation of Acid Treatments in Sandstone Formations. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October 1983. SPE-11931-MS. http://dx.doi.org/10.2118/11931-MS.
- Saylors, S.E. 1986. Iron: Minimizing Problems and Maximizing Treatment Effectiveness. Proc., 33rd Annual Southwestern Petroleum Short Course Assn., Lubbock, Texas, 148–156.
- Smith, B. 1990. Proper Treatment of Tubulars Key to Iron Control. Proc., 37th Annual Southwestern Petroleum Short Course Association, Lubbock, Texas, 115–122.
- Loewen, K., Chan, K.S., Fraser, M. et al. 1990. A Well Stimulation Acid Tube Clean Methodology. Presented at the Petroleum Society of CIM Annual Technical Meeting, Calgary, Alberta, Canada, 10–13 June. CIM 1990-047. http://dx.doi.org/10.2118/1990-047.
- Ashford, D.I. 1987. Effective Acid Pickling Increases Production. Presented at the Technical Meeting / Petroleum Conference Of The South Saskatchewan Section, Regina, Oct 6 - 8, 1987 1987. PETSOC-SS-87-13. http://dx.doi.org/10.2118/ss-87-13.
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