Water Well Journal
Issue link: http://read.dmtmag.com/i/440561
W ater well maintenance is a broad topic and a term used to encompass all manner of work performed on troubled well systems. One of the more common methods used in well mainte- nance is the combined use of chemicals and mechanical agita- tion to target fouling present downhole. Chemicals are used in water well rehabilitation to facilitate the removal of fouling mechanisms that can impact the production capability of well systems, reduce well efficiency, and contaminate or degrade the well system in some manner. Contamination can be the result of chemical infiltration into the well environment or the appearance of bacteria, viruses, or other microorganisms. Well fouling can be caused by mineral and biological buildup as well as sediment migra- tion, each individually or combined impacting the screens, pump intake, pore spaces of the gravel pack, and near-well formation. An acid is a substance that has a pH of less than 7 and has a high concentration of hydrogen ions when dissolved in water. Acids are used to aid in the destruction of mineral scale representing the physically harder portion of the complex accumulation of biomass, mineral scale, and sediment found plaguing many well systems. Second to chlorine usage, acids are one of the more com- mon classes of chemicals employed during well cleaning. Acids vary greatly in their chemical composition, their reactivity, and even their physical state (powder vs. liquid). Mineral Acids Mineral acids are used to dissolve mineral precipitates such as calcium carbonate, calcium sulfate, magnesium hydroxide, iron oxide, and manganese oxide alone or in various combina- tions. Carbonates are the most easily dissolved, and will result in the release of carbon dioxide. The following example shows the action of hydrochloric acid (HCl) on calcite (calcium carbonate [CaCO 3 ]): 2 HCl + CaCO 3 → CaCl 2 + H 2 CO 3 H 2 CO 3 → H 2 O + 2 CO 2 ↑ In this reaction, the hydrochloric acid (HCl) breaks apart the calcite (CaCO 3 ) into calcium chloride salt (CaCl 2 ) and carbonic acid (H 2 CO 3 ), which further breaks down into water (H 2 O) and carbon dioxide (CO 2 ), which bubbles off. In the reaction, part of the carbonate molecule is removed from the reaction by the evolution of CO 2 . This reaction is carried out by most mineral acids against carbonate scale, and accounts for the rapid development of gas during the cleaning of wells containing these deposits. When gypsum (calcium sulfate) and other non-carbonate mineral deposits are dissolved by mineral acids, the byprod- ucts of the reaction will accumulate and can be observed in the cleaning solution. The example below shows the action of hydrochloric acid (HCl) on calcium sulfate (CaSO 4 ): 2 HCl + CaSO 4 → CaCI 2 + H 2 SO 4 CaCI 2 → Ca ++ + 2 Cl − H 2 SO 4 → 2H + + SO 4 -2 In this reaction ions, or byproducts of the "dissolving" re- actions, remain in the cleaning mixture and result in the accu- ACID USE DURING WELL MAINTENANCE ACID USE DURING WELL MAINTENANCE SCHNIEDERS continues on page 20 Acid and biodispersants must be separated and maintained safely prior to use in well cleaning. Selecting the right acid and using it properly is critical to a well's performance. By Michael Schnieders, PG, PH-GW Twitter @WaterWellJournl WWJ January 2015 19