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Controlling precipitates due to acidizing
Methods to control the precipitates caused by acidizing are:
- Acid staging
- Lower acid concentrations
Preflush with either 5 to 15% HCl or 5 to 10% acetic acid. In formations with over 1% carbonate, an HCl or acetic acid preflush dissolves the carbonate to prevent waste of HF acid and formation of the insoluble precipitate calcium fluoride. Calcium and sodium chloride workover brine also must be flushed away from the wellbore with HCl acid or ammonium chloride brine. Preflushes also displace and isolate incompatible formation fluids (either brine or crude oil). Higher concentrations of ammonium chloride (> 3%) are recommended where swellable smectite and mixed layer clays are present.
Treat with an adequate volume of proper concentration HF acid. For successful HF acidizing, more than 120 gal/ft of HF/HCl acid is usually required. Less may be used where only shallow, moderate damage exists (e.g., 25 to 75 gal/ft is sometimes used on new perforations to remove damage or as a spearhead treatment in perforation breakdown prior to hydraulic fracturing in tight sandstone). The concentration, 3% HF to 12% HCl acid (often referred to as regular mud acid), is the usual concentration for damage removal in clean, quartzose sands. Concentrations of 0.5 to 1.5% HF are more effective in other clay containing sands. When the combined percentage of clay and feldspar is more than 30%, use 1.5% HF or less. In some low-permeability sandstone, HF concentrations as low as 0.5% HF have been used (e.g., the Morrow formation in Texas and New Mexico). If in doubt, consider an acid response test on a typical core or a geochemical acidizing simulator. See Table 1 for suggested acid concentrations that may be modified according to the information presented in the following sections.
Postflush or overflush
- Displaces unreacted HF acid into the formation
- Displaces HF-acid reaction products away from the wellbore
- Cleans corrosion inhibitors to restore a water-wet condition and good oil/gas effective permeability
- Re-establishes oil/gas saturation near the wellbore
Typical overflushes for HF acid treatments are 3% ammonium chloride brine, weak acid (3 to 7.5% HCl acid) and filtered diesel oil or aromatic solvent (oil wells only) or nitrogen (gas wells only). The volume of overflush should be equal to or greater than the HF acid stage volume. For most wells, an overflush of at least 200 gal/ft displaces spent acid past the critical flow radius of 3 to 5 ft. This large overflush reduces near wellbore precipitation of amorphous silica. At formation temperatures of 200°F or more, this precipitation occurs while the HF acid is being pumped into the formation. This precipitate is somewhat mobile at first but may setup as a gel after flow stops. Overflushing with 3% ammonium chloride or weak acid dilutes and disperses precipitate away from the wellbore. Often, the overflush is 3% ammonium chloride with 10% ethylene glycol monobutyl ether (EGMBE) and a polyquarternary amine clay stabilizer. However, high-cation capacity clays may swell as a result of injecting preflushes or overflushes of brines or acid with concentrations lower than 4%. Where significant quantities of smectite and mixed layer clays are found, Gdanski and Schuchart recommend the use of 5% ammonium chloride brine. This is supported by the work of Al-Anazi et al. Gidley et al. state that carbon dioxide preflushes and overflushes also have proven effective in some wells. Other chemicals can be added to acid to prevent or reduce the precipitation of some compounds (e.g., iron complexing agents, sulfate scale inhibitors, and sludge preventers). Table 2 summarizes the steps to prevent or control incompatibilities in acidizing different formations and formation fluids. 
Recent work has provided additional field cases of new types of acid damage from minerals in the formation such as zeolite,  chlorite,  and carbonate minerals precipitating aluminum fluoride complexes created by HF acid.  The experimental works of Shuchart and others provide a better understanding of HF acid chemistry and precipitation of HF acid reaction products. Shuchart summarized HF acid reactions into primary, secondary, and tertiary reactions. The primary reaction for HF acid dissolves damage and whole clay with no precipitation.
In the secondary reaction, fluosilicic acid (a product of the primary dissolution of clay or silica by HF acid) dissolves clay in formation and precipitates hydrous silica. This reaction can reduce clay damage deeper in the formation. Stronger acid (12% HCl and 3% HF acid) creates higher silica concentrations from the primary dissolution of clays and silica, which precipitate in subsequent reactions deeper in the formation. In higher-temperature formations, this silica precipitates closer to the wellbore and reduces permeability.
In the tertiary reaction, HCl acid and aluminum fluoride complexes react slowly to dissolve clays and precipitate hydrous silica but proceed faster at temperatures in excess of 00°F. This reaction exacerbates post-acid scale precipitation. The slower tertiary reactions occur in most acid treatments in the 8- to 24-hour time period that the acid system typically remains in the formation.
- Gdanski, R.D. and Schuchart, C.E.: “Advanced Sandstone-Acidizing Designs With Improved Radial Models,” SPEPF (November 1998) 272.
- Al-Anazi, H.A.: “Matrix Acidizing of Water Injectors in a Sandstone Field in Saudi Arabia: A Case Study,” paper SPE 62825 presented at the 2000 SPE/AAPG Regional Meeting, Long Beach, California, 19–23 June.
- Gidley, J.L., Brezovec, E.J., and King, G.E. 1996. An Improved Method for Acidizing Oil Wells in Sandstone Formations. SPE Prod & Oper 11 (1): 4-10. SPE-26580-PA. http://dx.doi.org/10.2118/26580-PA.
- McLeod, H.O. 1991. Acidizing Incompatibilities—A Review. Paper presented to the SPE Production Operations Study Group, Houston (27 February 1991).
- Underdown, D.R., Hickey, J.J., and Kalra, S.K. 1990. Acidization of Analcime-Cemented Sandstone, Gulf of Mexico. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 23-26 September 1990. SPE-20624-MS. http://dx.doi.org/10.2118/20624-MS.
- Simon, D.E. and Anderson, M.S. 1990. Stability of Clay Minerals in Acid. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, 22-23 February 1990. SPE-19422-MS. http://dx.doi.org/10.2118/19422-MS.
- Wehunt, C.D., Van Arsdale, H., Warner, J.L. et al. 1993. Laboratory Acidization of an Eolian Sandstone at 380F. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, Louisiana, 2-5 March 1993. SPE-25211-MS. http://dx.doi.org/10.2118/25211-MS.
- Shuchart, C.E. and Ali, S.A. 1993. Identification of Aluminum Scale With the Aid of Synthetically Produced Basic Aluminum Fluoride Complexes. SPE Prod & Oper 8 (4): 291-296. SPE-23812-PA. http://dx.doi.org/10.2118/23812-PA.
- Shuchart, C.E. 1995. HF Acidizing Returns Analyses Provide Understanding of HF Reactions. Presented at the SPE European Formation Damage Conference, The Hague, Netherlands, 15-16 May 1995. SPE-30099-MS. http://dx.doi.org/10.2118/30099-MS.
- Gdanski, R.D. 1994. Fluosilicate Solubilities Affect HF Acid Compositions. SPE Prod & Oper 9 (4): 225-229. SPE-27404-PA. http://dx.doi.org/10.2118/27404-PA.
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