Formatted Title
Well Maintenance Techniques to Improve Injectability over Time for Long-Term In Situ Bioremediation
Background/Objectives
A treatability study was conducted at a site located in the western United States to evaluate the effectiveness of in situ bioremediation (ISB) to reduce the flux of perchlorate mass in groundwater. This treatability study was implemented on a large scale using 25 permanent injection wells to periodically inject a slow-release carbon substrate, namely, emulsified vegetable oil (EVO), nutrients, and distribution water over eight periodic injection events conducted from 2017 through 2021. The subsurface in the treatability study area is comprised largely of silty sands and sandy silts with interbedded sandy gravels. This relatively permeable subsurface initially accepted injectate at pressures generally less than 15 pounds per square inch (psi) at rates averaging 6.5 gallons per minute. After four injection events over two years, approximately one third of the injection wells indicated reduced performance with increased injection pressures ranging from 25 to 35 psi accompanied by decreasing injection rates compared to previous events, including two injection wells that no longer accepted injectate solution. Downhole video logging and sampling revealed that the injection well screens and surrounding filter packs accumulated biomass, inorganic precipitates, and intermediate byproducts of EVO hydrolysis, which was the primary cause of changes to the injectability (i.e., increases in injection pressures and/or decrease in injection rates required for subsurface distribution). Because periodic well maintenance is an important corrective measure to sustain injectability, promote injection well longevity, and ensure successful long-term operation of ISB, a variety of well maintenance techniques were evaluated to develop a more in-depth understanding of the long-term operation and maintenance requirements of injection well networks associated with ISB systems.
Approach/Activities
Injection wells were selected for well maintenance based on visual observations in during a video logging survey of the injection wells and trends in injection rates and pressures observed during the periodic injection events. The techniques used for well maintenance included surge and bail, hydrojetting, and a combination of both surge and bail and hydrojetting with the addition of chemicals. Hydrojetting used high pressure water with either fixed or rotating nozzles to remove any incrustation from well screens. Acidic agents AQUA-CLEAR® MGA (dry blend of granular sulfamic acid and sodium chloride used to remove mineral precipitates) and AQUA-CLEAR® AE (liquid blend of hydroxyacetic acid formulated to control biomass) were selected to support chemical well maintenance. To provide a good comparison of techniques and resulting outcome (i.e., improved injectability), three injection wells were selected for the testing of each well maintenance procedure. Improvement in injection well efficiency was evaluated based on the injection pressures and rates observed during subsequent ISB injection events performed after well maintenance activities. Reduced injection pressures and/or increased injection rates compared to the previous injection events performed prior to well maintenance and the associated magnitude of these injection pressure reductions and injection rate increases were used to evaluate the effectiveness of the various well maintenance techniques.
Results/Lessons Learned
Based on analysis of solids collected from injection wells and visual observations during video logging and injection well maintenance activities, accumulation of material within the injection wells was generally limited to microbial biomass and/or chemical precipitates, primarily consisting of calcium carbonate or calcium oleate. Although all three well maintenance techniques improved the injectability of wells, the most aggressive approach of hydrojetting with chemical addition was more effective than traditional surge and bail alone. Injection wells that needed more aggressive cleaning to improve injectability appeared to be comprised primarily of calcium precipitates. This presentation will include field observations of accumulated solids during video logging and well maintenance activities and data and graphical depictions of the effectiveness of the injection well maintenance techniques and improved injectability over time.