A new technology that uses a high-energy, pulsed-plasma (HEPP) shock-wave generator to rehabilitate old wells will help public works departments avoid the cost of drilling new wells. Under-producing wells pose a challenge for municipal operations with large water volumes, and at peak pressures of more than 10,000 psi, the HEPP system offers an alternative to high-end explosive and chemical treatments.
For example, the technology may be the only viable treatment solution for abandoned or soon-to-be-abandoned wells. In those cases, the treatment cost would be considerably less than a new well. Aquifer storage and recovery (ASR) wells also could benefit from the technology: The accelerated buildup of fouling due to brackish water and saltwater limits the effectiveness of traditional treatment systems, and ASR wells typically use plastic materials that are easily damaged by impulse systems and harsh chemicals.
A NEW SOLUTION
The plasma-generated shock waves come from a tubular pulser that can be inserted in the well casing and operated at depths exceeding 1,500 feet. These shock waves radiate outward from a spark head mounted at the end of the pulser and impact the side of the well screen or pipe. The unit can be slowly moved down the well bore with a series of fluid-coupled shock waves to remove fouling and hard scaling, including the region behind the screens.
Standard 240 V alternating current from an on-board 5- to 10-kVa (kilovolt-ampere) generator or commercial power source is transferred through a power cable from the work truck. A pulse-forming network within the pulser stores electrical energy, which is switched in a 25- to 50-microsecond time increment across heavy-duty electrodes in the spark head of the pulser, producing a multimega-watt spark discharge.
This ability to “fast discharge” energy generates a powerful fluid-coupled shock wave without any steam bubble. The efficient conversion of electrical energy to shock-wave energy, which incorporates shock-wave focusing, penetrates the most difficult-to-treat part of the well — the production zone.
This technique produces strong secondary effects that can be useful for disinfection. Enhancements to the HEPP system are being developed that will enable the shock wave to disinfect the area behind the screens. Cleaning and disinfection are combined into a single application, eliminating the need for post-treatment chemicals. The system disinfects at distances of 2 feet from the HEPP discharge, making it effective for both aerobic and anaerobic bacteria.
The lack of an air or steam bubble prevents damage to the casing caused by over-pressurization. The ability to adjust power levels to match conditions is significant in the screen zones, which may require lower power levels to prevent damage. The system can reduce treatment time to a day or less, as it can be readily redeployed if visual inspections reveal a need for further cleaning. The process is safe for operators because it is not charged with electricity until it is inserted in the well for treatment.
The shock wave is the most important factor in removing hard scale deposits. The HEPP method also treats soft biofouling, such as algae on steel plates fouled with marine growth.
Processes traditionally used to rehabilitate water wells include mechanical brushing with organic acids, disinfection with chemicals such as chlorine or hydrogen peroxide, and explosive methods that generate shock waves to remove scaling and fouling.
These techniques have limitations. No single treatment application can effectively clean and disinfect in one step, and none of these processes cleans the production zone region behind the screens.
The use of chemicals is restricted by various state agencies and the U.S. EPA. Chemically treated water may be classified as point-of-source pollution, requiring treatment or disposal until acceptable chemical levels are reached.
The most effective cleaning technique is the shock wave, but explosive methods such as air impulse produce a gas bubble or over-pressurization, which is inefficient and may damage screens and casings. Explosives also are dangerous and require special handling precautions and licenses. They are also time-consuming to set up in the well. The operator has only one shot to determine the appropriate charge intensity in applications that may require different power levels depending on the condition, length, and location of screens in the pipe.
Tests of the HEPP tool on pipes and plates coated with scale, chemical encrustation, and marine life have demonstrated significant removal of fouling without damaging the underlying pipe and structures.
A field test was performed in May 2006 on an existing well located on the Mohave Resort Golf Course property in Laughlin, Nev. Since the 133-foot-deep, 18-inch-diameter well was drilled in 1997, the flow had decreased to 250 gallons per minute (gpm) from 1,000 gpm. Conventional acid and brushing treatments had increased the flow to 650 gpm, but a significant problem remained with the pumping level, which at 115 feet was almost to the bottom of the screens. The HEPP test was to be the last effort before drilling another well.
Because of the age of the well and mild-steel construction of the spiral screens, the HEPP device was operated at the low power levels that had been successful for the 10-inch test pipe. The flow rose to 775 gpm and the pumping level near the top of the screens to 94 feet. This single application dramatically improved the specific yield of the well and prevented its abandonment.
In addition, restoration of the pumping level to its original level reduced electrical costs for pumping. Eighteen months after the first treatment, Mohave reported data confirming that the treated well had maintained the same level of performance. This period included a summer of extreme heat during which the pumps ran 24 hours for six days a week without any pump letdown.
This test also confirmed the HEPP process did not force the debris and fouling into the well formation or cause damage to the gravel pack. This concern was the reason other impulse shock-wave generation methods were rejected for this application.
— Ayers is technical director of Advanced Conversion Technologies Inc. (ACTIX), Spring Valley, Calif.; Vivona is senior project manager with Jacobs Engineering Group Inc., Houston.
Water well treatment capabilities
Performance standards for commercial-use HEPP tools fall into four categories.
PRODUCTION CAPACITY Well-specific capacity returned to 90% of original
- Well flow rate returned to original
- Maintain above levels for a period of not less than one year.
All scale removed from the perforations and casings
- No structural damage to the well.
All water quality, including bacteriological, returned to background quality
- Disinfection behind the screens
- No sand production above American Water Works Association standards
- Water quality parameters sustained for a period of not less than one year.
Depths of up to 1,500 feet and diameter of 10 inches or more
- Ability to run on any 240 VAC, single-phase, 20-amp output generator or power source
- Treatment rate of 1 foot/four seconds
- Ability to adjust power levels via topside controls
- Telemetry and controls provided via separate wire line
- In-situ debris cleaning during treatment
- Ability to mount camera forward of spark head.