Many wells are damaged in the course of drilling and workover by the use of drilling muds, drill-in fluids, and killpills that contain, amongst other things, polymeric constituents. The latter may consist of a single polymer or may consist of mixtures of polymers in aqueous solution/suspension which are added for the purposes of viscosification, leak-off control, lubrication, etc. Typically, the polymers used include xanthans, starches, celluloses, guars, and derivatives of these main groups. In addition, most of these fluids contain some form of particulate to impart density and to improve fluid-loss control, commonly calcium carbonate. It has been established that well productivity can be significantly impaired by the use of these mixtures, due to the persistence of residues. Their removal can result in substantial improvements in production. There are those who believe that such damage can be removed simply by applying sufficient drawdown but, generally, chemical methods using acids and/or oxidisers like sodium hypochlorite are used. Neither of these approaches is effective alone at removing all the damage caused by cake components. This often necessitates multistage soak treatments, with large volumes of treating fluid, in extended reach wells. Such procedures complicate the operation and cause additional expense due to the time involved. This paper describes a novel chemical system, capable of removing all common mud polymers while, simultaneously, dissolving the calcium carbonate and accomplishing this in a single one-step treatment. These systems are safe and effective and can remove essentially 100 percent of all residues, restoring permeability to its original level.
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