New technology puts produced natural gas to use and lowers noise pollution at wellsites.
by Jared Oehring (U.S. Well Services, LLC)
March 11, 2016

In July 2014, a well servicing company deployed the first fully electric, fully mobile well stimulation system powered by natural gas in Antero Resources' sites in West Virginia. The fleet runs on electric power generated by three natural-gas-fueled turbine engine generators. Each produces 5.7 megawatts electric (MWe) of three-phase output power at 13,800 volts (V) for a total of 17.1 MWe. The mobile power unit consist of two trailers. The turbine engine and generator sit on the first trailer while the electric equipment room (EER) rides on the other. The EER holds the following an auxiliary, desktop computer; switchgear; carbon dioxide fire suppression system; and a motor control center. The trailers enable much faster moves than traditional power generation stations. Each power plant has a footprint of 52 by 28 feet.

Power Units

The mobile power units use a single-shaft, axial-flow, gas turbine engine. A turbine engine has three primary parts: the compressor, the combustion chamber and the power turbine. In the compressor section, the incoming air is compressed through 12 rotating and stationary compressor blades. In the annular type combustion chamber, fuel is added to the pressurized air molecules and ignited. The combustion chamber has 12 lean-premixed, dry, low-emission injectors, with a single torch ignitor system. As the molecules expand, they move into the turbine section at a high velocity. The turbine converts the energy from the high-velocity gas into rotational power through the expansion of the gas across three stages of turbine rotor blades that move clockwise. This rotation turns a generator that produces electrical power. The power generated by the turbines then runs through switchgear and is stepped down to 600 V using transformers. At this point, the electric powered fleet is similar to a conventional fleet, except that all the fleet's diesel engines and transmissions—more than 20 of them—are replaced by electric motors and variable frequency drives (VFDs). A VFD consists of a rectifier, a direct-current (DC) bus and an inverter. Incoming three-phase, 600-V power enters the six-pulse rectifier bridge converter section, which changes the alternating-current (AC) sine wave into unidirectional current pulses that then enter the DC bus section. This section has a passive capacitive filter to minimize ripple and hold a constant and steady DC. Next, the inverter section uses insulated gate bipolar transistors (IGBTs) for power switching to convert DC back to AC. The IGBTs switch the DC bus on and off at specific intervals, creating a variable frequency, three-phase, sinusoidal coded pulse width modulated (PWM) waveform. Alternating the positive and negative switches by the IGBTs allows the VFD to control the speed of the motor and the hydraulic fracturing pump.

Environmental Benefits

This electric-powered well stimulation fleet advances hydraulic environmental responsibility by reducing refueling traffic, site noise and emissions. Natural gas for the turbines is delivered straight from a pipeline. Using natural gas eliminates the 25 diesel fuel truck deliveries to the wellsite. In addition, using the produced field gas reduces fuel operating costs by 80 percent.

 Figure 1. The electric fleet's NOx emissions compared to diesel engine's (Graphics courtesy of U.S. Well Services, LLC) Figure 1. The electric fleet's NOx emissions compared to diesel engine's (Graphics courtesy of U.S. Well Services, LLC)

The fleet is decreasing emissions by 99 percent, mitigating environmental exposure to pollutants, such as nitrogen oxides (NOx) (see Figure 1). The reduction is primarily achieved by eliminating the conventional diesel engines and using natural gas as a fuel. The NOx levels were found to be near-zero at less than 0.036 grams per kilowatt hour (g/kW-hr). Natural gas is cleaner than diesel because it produces fewer potential pollutants. Natural gas turbine engines allow for a continuously burning combustion chamber that results in a more complete burn, which reduces emissions when compared with a diesel engine's four-stroke cycle. The gas turbines have another advantage because they use lean-premixed combustion technology to ensure a uniform air and fuel mixture and prevent the formation of regulated pollutants.

Noise Pollution

Finally, electric well stimulation reduces noise pollution, making the workplace safer and less disturbing to communities less. Conventional well stimulation sites are extremely loud with some locations measuring as high as 129.5 decibels one foot away from the equipment. The well servicing company employed a third party to perform a sound survey of the electric fleet. More than 140 total monitoring locations were used, and the result was up to a 69 percent reduction in the average sound pressure for the electric well stimulation site.

Figure 2. Sound pressure comparison at one foot away from the hydraulic fracturing skidsFigure 2. Sound pressure comparison at one foot away from the hydraulic fracturing skids

The frequency spectrum can compare each frequency band instead of only the total average sound pressure. Figure 2 compares measurements taken one foot away from a conventional fracturing pump trailer to those taken one foot away from an electric, dual-fracturing pump trailer. Two pumps are on the electric unit while the conventional trailer only holds one fracturing pump. The electric, dual-fracturing pump trailer showed reductions across the frequency spectrum from 12.5 to 20,000 Hz.

 Figure 3. Sound pressure of the electric unit as a percentage of the conventional unit Figure 3. Sound pressure of the electric unit as a percentage of the conventional unit

Figure 3 shows the electric unit's sound pressure as a percentage of the conventional unit in each frequency band. The sound pressure is dramatically reduced across all frequencies.