A vertical well has a history of problems which has contributed to its disappointing performance. A 15 wt% HCl acid treatment was performed in an attempt to remove drilling mud filter cake and improve wellbore to formation connectivity, however the solubility of the formation's cementation material (calcite) was overlooked and sand production resulted after the treatment. A screen liner running operation was then performed, with a need for sand control, and massive mud losses and polymer pills were lost to the formation during the process.

The workover fluids were displaced deep into the formation while loading the hole to maintain well control. The presence of biopolymers in the completion fluids enhanced the activities of sulfate reducing bacteria (SRB). These bacteria further compounded the problem, and the damaged environment was ideal to initiate the production of plugging FeS and biomass in the near wellbore region. Ultimately, the formation was not able to deliver the necessary flow rate to supply the ESP pump when the well was returned to production.

The damage mechanism is a combination of polymer residue, biomass and FeS plugging the screen liner. A three-stage treatment was designed to remove the damaging material. The first stage was based on tetrakishydroxy methyl phosphonium sulfate (THPS) and was designed to remove iron sulfide and hydrogen sulfide, and control the growth of bacteria. The second stage was based on formic acid and was intended to remove FeS and some of the polymer residue in the near wellbore area. The third stage was based on a two-part oxidative treatment and was designed to remove polymer damage deep in the formation. Analysis of well flowback samples before and after the treatments was used to confirm the damaging mechanisms and to evaluate the treatment.

This paper reviews the procedure used to identify the problem, and the unique treatment which successfully restored well productivity.

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