When performance monitoring data suggest decreases in injection or extraction well efficiency below acceptable levels, the well will require rehabilitation by chemical and/or mechanical means to return it to optimal flow rates.
The injection or extraction well should be shut down a minimum 24 hours prior to the removal of equipment in order to allow the well to return to a static condition. Power to the pumps should be secured using lock-out and tag-out procedures to prevent shock or electrocution. Injection well valves should be turned off to prevent water from leaking into the well vault. The electrical wires connecting the transducer or pump should be disconnected. The wellhead equipment will need to be disconnected from the well casing. Once completed, this equipment needs to be hoisted from the well using the development rig.
Following removal, the field crew should document any discoloration on the piping, biological deposition or encrustation, mineral growth and/or deterioration to the downwell equipment in the field notebook, and take pictures as necessary. The removed equipment should be staged near the well site on plastic sheeting to minimize the potential for surface contamination.
Ideally, a video log will be taken in order to determine the well condition. The extent of biofouling, sedimentation and encrustation will be documented. Notes will be taken, noting the depth to water, percentage of screen blockage (include depths, color and type of material), overall water quality, and damage to casing or screen.
Data ComparisonAfter the completion of the video log, physicochemical and biological assay data analyses will be collected. The results of these analyses will be compared to the baseline data collected during initial development of the well, and used to determine the nature of the problems and the countermeasures. Based on these data, rehabilitation and maintenance measures can be fine-tuned, based on observed changes.
The physicochemical (inorganic) data analyses include phenolphthalein alkalinity, pH, chlorides, total dissolved solids, total hardness, carbonate hardness, non-carbonate hardness, calcium, magnesium, phosphate, iron, copper, nitrate, tannin/lignin, sulfate, silica as SiO2, manganese, saturation index, and redox potential. The iron, manganese and sulfur are analyzed in order to predict the clogging potential, presence of biofouling, and redox potential shifts. The analyses of pH indicate acidity or basicity, which predicts corrosion or encrustation. In addition, pH is combined with redox potential to determine the likely metallic mineral states present. Conductivity is used to indicate the total dissolved solids content, and is a component of the corrosivity assessment.
The biological assay is used to determine the presence of biofouling, the types of organisms that are present, and if the well has increased or decreased in biofouling over time (based on long-term monitoring). The biological assay analyses will include assessment of the total bacterial count per milliliter, the anaerobic bacterial load on the system, the presence of sulfur-reducing bacteria, the presence of iron oxidizing bacteria, branching or filamentous bacteria, protozoans, and the identification of the two largest populations of bacteria present. Ground water samples will be collected from the injection water stream, screened zone (aquifer), and the extraction water stream prior to rehabilitation. The injection and extraction samples should be collected from the sampling ports, and the aquifer sample should be collected using a submersible pump. Biological assay samples do not require refrigeration if they are received by the laboratory within 24 hours of sampling. In addition, no preservatives are to be used in the sampling bottles.
Bailing and BrushingDuring the bailing process, a suction bailer is lowered into the well until it fills with water and sediment. It then is pulled to the surface and emptied. Water from the aquifer then will flow toward the well, and bring in more drilling fluid. The bailer’s up and down motion causes a surging action, which initiates development in the area around the screen. Prior to initiating the bailing procedure, the water level and total depth of well will be measured, noting the amount of sediment in the well. Next, the sediment will be removed from the bottom of the well using the suction bailer, and the contents will be emptied into a 55-gallon drum. The color, clarity and smell of the water will be noted in the field notebook. After the sediment has been removed from the bottom of the well, the total well depth will be measured again. Throughout this process, purge water present in the 55-gallon drum will be decanted to a water storage container, and the amount of material (i.e. sand/silt, etc.) present in the bottom of the drum will be noted (i.e., the amount of sediment removed from the well). The amount of accumulated sediment in the well should progressively decline following successive rehabilitation events.
Brushing simply involves running a hard bristle brush up and down the length of the well screen to remove sediment encrusted on the well casing and screen. Similar to bailing, the up-and-down movement of the brush produces a surging effect, continuing the development process.
Biofouling TreatmentFollowing the completion of brushing and bailing, a chemical solution is applied to the well to facilitate the breakdown of biofilm, and to disperse mineral salts that are present in the well screen and the filter pack. The chemical treatment will be applied following the manufacturer’s recommendations.
To ensure the proper distribution of the chemical treatment in the screened zone, the treatment mixture will be tremied into the upper, middle and bottom thirds of the well screen. To improve the effectiveness of the chemical treatment, the well will be swabbed every two hours. The agitation caused by swabbing ensures the distribution of the chemical treatment through the screened zone and filter-pack. The solution should remain in the well between 18 hours and 48 hours, depending on the severity of the biofouling and/or encrustation. In addition, it’s important that the pH of the treatment solution stay below 2.0. If the chemical mixture is left in the well overnight, a water and acid mixture should be added to the well to ensure that the pH remains below 2.0.
The following morning, the pH should be tested, and if it is greater than 2.0, it should be lowered with the acid and water mixture. After this has been completed, the well should be swabbed for at least one hour. Next, an airlift pump system will be lowered into the bottom of the well to remove the chemicals, biofilm and mineral salts. During the airlift pumping process, the pH of the discharge water will be monitored, and the airlifting will continue until the pH of the discharge water is less than 5.0. Upon completion of the airlifting, a second chemical treatment will be performed.
Mud DispersantIn contrast to initial well development, during well rehabilitation, only one mud dispersant treatment will be performed (however, in some cases [e.g., excessive drilling mud present in the well], a second treatment will be necessary). In theory, the mud dispersant treatment will follow the completion of the initial rehabilitation steps of brushing, sediment removal and biofouling treatment. In order for the mud dispersant to penetrate into the filter pack and beyond, fouling materials (i.e. biological growth, mineral encrustations, etc.) need to be removed, and the available flow paths restored.
Dual-swab AirliftingDuring well rehabilitation, dual-swab airlifting will be utilized after the mud dispersant treatment application to remove accumulated sediment and ground water containing the treatment chemicals. Ground water containing the treatment chemicals will be removed to the extent practical to minimize potential impacts to the treatment system (e.g., downgradient extraction).
Initially, the dual-swab tool will be inserted into the well and set at the top 10 feet of the screened zone. Airlift pumping will occur at the maximum rate possible for a period of approximately 40 minutes. Due to the turbidity of the airlifted water, the initial water will be pumped into a settlement tank before being transferred to a containment tank. The dual-swab airlift equipment will be raised and lowered throughout the entire screened zone. During the airlift pumping, the air supply will be periodically shut off, allowing the water column to flow back through the screen. The combination of swabbing, pumping and surging enhances the removal of trapped material in the screened zone and filter-pack. Upon completion of dual-swab airlifting, a second mud dispersant treatment/dual-swab airlifting process may follow.
Conventional Surge PumpingThis stage of rehabilitation involves pumping and surging the wells at a variety of rates until no further sand, turbidity and drawdown declines are observed.
Equipment Re-installationUsing the development rig, the PVC casing (injection well) and steel casing and pump (extraction well) will be reset into the well. Once the equipment is lowered into place, the wellhead flange plates will be bolted, and associated piping reconnected to the injection or extraction line. The electrical wires connecting the transducer or pump should be re-connected, and tested. Finally, the equipment should be leak tested and injection and extraction rates tested.
This article is provided through the courtesy of NASA’s Jet Propulsion Laboratory at the California Institute of Technology.