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Acidizing safety and environmental protection
The main safety precautions for those on site during an acid treatment concern detection of leaks and proper handling of acid. Environmental hazards can be reduced or prevented by the proper choice of chemical additives at optimum concentrations.
The main safety precautions for those on site during an acid treatment concern detection of leaks and proper handling of acid. Pressure tests are performed with water or brine to ensure the absence of leaks in pressure piping, tubing, and packer. Leaks on the surface can endanger service personnel, and subsurface leaks can cause subsequent corrosion of tubing and casing in the annulus. Anyone around acid tanks or pressure connections should wear safety goggles for eye protection. Those handling chemicals and valves should wear protective gauntlet-type, acid-resistant gloves. Fresh water and spray washing equipment should be available at the job site. In case of acid contact with the eyes, immediately flush eyes with clean water and consult a physician. If acid contacts the skin, wash the area of contact with water for 15 minutes. Consult a physician immediately after flushing if hydrofluoric acid comes in contact with skin or eyes. Wear self-contained, full-face, fresh-air masks when potential hydrogen sulfide gas hazards exist. Also, testing equipment and appropriate safety equipment should be on hand to monitor the working area and protect personnel in the area. Special scrubbing equipment may be required for removal of toxic gases. Further information on safety with acid can be found in API Bull. D15, Recommendations for Proper Usage and Handling of Inhibited Oilfield Acids and in Data Sheet 634, Safe Well Stimulation by Acidizing from the National Safety Council. 
Proper handling and disposal of acid and spent acid products should be observed. Often, environmental hazards can be reduced or prevented by the proper choice of chemical additives at optimum concentrations. The acid flowbacks are normally processed in a test separator. Oil goes to the water/oil separation system, and the aqueous phase is filtered and treated with activated carbon for overboard disposal in accordance with regulatory guidelines of oil and grease measurements. This process, used in many offshore operations, is described in an article by Ali.  Regulatory guidelines are available to control and monitor discharges of well workover fluids containing oil or grease. Overboard discharges must meet 42 mg/L daily maximum and 29 mg/L monthly average oil and grease limits. There are no acute and chronic toxicity measurement requirements at present.
Quality control checks before, during, and after pumping increase the probability of acidizing success. Onsite supervisors are encouraged to check the equipment.
- Inspect all tanks that will be used to hold acid or water. The tanks must be clean. Small amounts of dirt, mud, or other debris can destroy any acid job.
- Make sure the service company has the equipment to circulate the acid tank prior to pumping. This must be done to avoid emulsion problems and to protect the tubing. Acid corrosion inhibitors and other additives can separate to the top of the tank in as little as 2 hours.
- The line to the pit or tank should be laid and ready to connect to the wellhead so the acid can be backflowed immediately after the end of the overflush.
When in doubt that the formation will take acid, inject a compatible “superclean” filtered brine to test the ability of the formation to take fluid. If the test shows severe damage, the operation may be changed to include an acid minisqueeze prior to the main acid job to make sure that the formation is open to fluid. Zhu and Hill showed that a pretreatment test could be used to evaluate permeability and skin factor prior to treatment. The monitoring program followed evolution of skin even with diversion effects. The program is reliable and flexible for:
- Acquiring and processing data
- Calculating skin
- Diagnosing matrix acidizing treatments
Sampling and titration
Sampling of all pumped fluids for solids content and acid titration for hydrochloric (HCl)- and hydrofluoric (HF)-acid concentration should be performed on site as a quality control measure. Samples of spent acid should be analyzed for pH immediately and then kept in airtight containers for chemical analysis. Large variations in acid concentrations delivered to the well site have been found. Delivered acid concentrations are usually more accurate and consistent when a known on-site titration program is to be used. Premixed acid should be rolled and circulated to make sure that all additives are properly dispersed and that none, especially corrosion inhibitors, have separated and floated to the top of acid tanks or sunk to the bottom. Titration of acid is an excellent test to see whether acid is well mixed. In one case, 15% HCl acid was sampled and titrated to show 6% HCl acid. The acid tank was “rolled” to mix well and titrated again. This time, it titrated as 15% HCl acid. Poorly-mixed acid can result in highly varied acid concentrations (5 to 25% in an average 15% HCl-acid mix) with similar variations in corrosion inhibitors and other additives. Such a variable mix will exacerbate corrosion, emulsion problems, and acid/formation interactions. Also, high acid strength can harm tubing and certain formations. Surfactants should be checked to ensure they leave the rock minerals in a water-wet condition for optimum oil flow.
The key to successful job execution is thorough and effective job supervision. The operating company responsible for supervising the job must:
- Prepare the well before the service company administers the acid treatment
- Monitor the progress of the project before, during, and after the treatment
- Properly evaluate the results
The most important tasks associated with job supervision are those related to safety, well preparation, and quality control.
- API Bull. D15, Recommendation for Proper Usage and Handling of Inhibited Oilfield Acids, first edition. 1985. Washington, DC: API, Washington.
- Data Sheet 634, Safe Well Stimulation by Acidizing, National Safety Council.
- Ali, S.A. et al. 1997. Process, Optimized Acidizing Reduce Production Facility Upsets. Oil & Gas J. 95 (6): 44.
- GMG290000, General Permit for the Western Portion of the Outer Continental Shelf (OCS) of the Gulf of Mexico, 58, No. 231. Washington, DC: NPDES, Federal Register.
- King, G.E. and Holman, G.B. 1985. Quality Control at Well Site Optimizes Acidizing Economics. Oil & Gas J. 83 (11): 139.
- McLeod, H.O.J. 1984. Matrix Acidizing. J Pet Technol 36 (12): 2055–2069. SPE-13752-PA. http://dx.doi.org/10.2118/13752-PA.
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