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Oil and gas operators have safely and successfully used acid to improve productivity of oil and gas wells for nearly 120 years. Today, acidizing is one of the most widely used processes for stimulating oil and gas wells. • Two types of acids are most commonly used; hydrochloric acid in all formation types and hydrofluoric acid in sandstones and certain shales. Other types of acids, such as organic acids, may also be used in specialized situations. • Since geologic formations are never homogeneous, blends (particularly for sandstone formations) of HCl and HF are usually pumped with the blend ratios based on the formation mineralogy. • All aspects of the regulatory framework surrounding the use of acid in oil and gas wells are well developed and mature as are the operational and safety practices employed by operators and service providers. • When the acid reacts with formation materials it is largely consumed and neutralized. • Spent acid that is recovered when a treated well is brought on production is treated and safely disposed of in essentially the same way as produced water. References 1. Halliburton, Effective Sandstone Acidizing, Best Practices Series 2. Halliburton, Carbonate Matrix Acidizing Treatments, Best Practices Series 3. Halliburton, Fracture Acidizing, Best Practices Series 4. Kalfayan, L.J. Production Enhancement with Acid Stimulation (Second Edition). 2008. PennWell. 5. Schlumberger, Reservoir Stimulation (Third Edition). 2000. John Wiley & Sons. 6. Schlumberger, Trends in Matrix Acidizing Considerations As already mentioned, the oil and gas industry has been using acids for well treatment for well over 100 years. As a result, the industry has a great deal of experience with the safe and environmentally sound handling and management of these fluids both before and after their use. Operator, service companies, and regulatory agencies have sound procedures in place that protect both workers and the public. Acids must be transported and used with proper precautions, safety procedures, and equipment. Transportation of the acid and related materials must be done in USDOT (or equivalent) approved equipment and containers, properly labeled, and follow approved routes to the work site. Personnel working directly with the acids must utilize the personal protective equipment (PPE) specified in the Safety Data Sheet (or equivalent) and be properly trained and experienced in the use of these materials. All equipment used in pumping the acid should be well maintained and all equipment components that will be exposed to pressure during the acid job should be tested to pressures equal to the maximum anticipated pumping pressure plus an adequate safety margin prior to the start of pumping operations, in accordance with industry standards and pressure pumping service provider operating guidelines. The operator should consider the use of barricades to limit access to areas near acid and additive containers, mixing and pumping equipment, and pressure piping. After the acid job is successfully pumped and the well is brought to production, the operator should consider using separate tanks or containers to isolate the initial produced fluids (spent acid and produced water). The fluids that are initially recovered will contain the spent acid (acid that is largely chemically reacted, neutralized, and converted to inert materials) and it will typically have a pH of 2-3 or greater, approaching neutral pH. These fluids can be further neutralized to a pH>4.5 prior to introduction into the produced water treatment equipment, if necessary. Once neutralized, the spent acid and produced water can be handled with other produced water at the production site. Most produced water, including spent acid, is treated as needed and then injected via deep injection wells that are permitted by the jurisdictional regulatory authority

 

Article Source https://www.api.org/~/media/files/oil-and-natural-gas/hydraulic-fracturing/acidizing-oil-natural-gas-briefing-paper-v2.pdf