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As mentioned above, acidizing oil and gas wells is a routine practice that has been used for a very long time. As a result, oil and gas operators and their service providers have considerable expertise and experience in safely and effectively conducting this work. Similarly, regulators that steward oil and gas operations have developed a well-founded regulatory framework to manage this work, protect the environment, and protect public health and safety. The volume of acid used in an acid job is generally determined by the length of the formation (footage) being treated in the well. Acid volumes used per foot of formation can vary depending on the design objectives and the characteristics of the specific formation. Typical acid volume ranges are between 10 and 500 gallons per foot. While a volume of 500 gallons per foot may appear to be large, in a matrix acid job, assuming 25% porosity, the acid would be displaced less than 20 feet from the wellbore. In fracture acid jobs, the acid will be displaced further, but is still limited by the fracture length. Fracture lengths are usually a few hundred feet at most. When acidizing, the acid is chemically consumed and neutralized as the target material is dissolved. In carbonate formations the reaction is relatively simple and occurs in a single step. The hydrochloric acid (HCl) reacts with the carbonate to form a salt, carbon dioxide, and water. When acidizing sandstones with HF the reactions are more complex, occurring in three stages. In the primary stage, the mud acid reacts with the sand, feldspar and clays to form silicon fluorides and aluminum fluorides. In the secondary stage the silicon fluorides can react with clay and feldspar to release aluminum and silicon precipitates, however with proper design, formation of these damaging precipitates, which can restrict flow of oil or gas through the formation, can be avoided. In the final stage the remaining aluminum fluorides react until all the remaining acid is consumed. Geologic formations are rarely homogeneous (pure carbonate, sandstone, or shale) but will be a blend of carbonate, sandstone, and clay minerals. As a result, most acid jobs are composed of both hydrochloric and hydrofluoric acid, with the ratios and strengths depending on the mineralogy and temperature of the formation being treated. Other types of acids can be used in more specialized situations (e.g., organic acids such as acetic and formic acid as alternatives to hydrochloric acid). Additionally, specialized additives can be included in cases where specific chemical reactions are anticipated to be particularly severe and require control or mitigation. A challenge in performing acid jobs is ensuring the acid goes where it can do the most good. To facilitate placement of the acid across the entire target interval in the well, operators often use coiled tubing units. A coiled tubing unit is a specialized piece of equipment that utilizes a reel mounted tubing string that can be run concentrically inside the well’s production tubing to the point directly across the interval that is targeted for treatment. The acid is pumped through the coiled tubing and into the productive formation. This equipment allows precise placement and pumping of the acid. It also provides the added benefit of not exposing the production tubing to the acid. When pumping any fluid into a well it will have a natural tendency to follow the path of least resistance and flow into those parts of the formation with the highest permeability. In an acid job, this is not the most desired result since the objective of an acid job is to improve the permeability of a well by dissolving material from lower permeability or plugged areas. To direct acid to the lower permeability parts of the formation, either chemical or physical flow diverters can be used. Use of diverters forces the acid into those lower permeability sections and thereby provides the potential for the most positive results. In all cases, once the acid job has been pumped the well is brought on production. When this is done, the spent acid is produced along with the oil, gas, and water in the formation. Since the acid is chemically consumed when it contacts the formation, the recovered fluid is relatively benign.


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