With prices competitive with diesel, some drillers are turning to all-natural gas designs for generators.

With a recent decline in natural gas prices and increased environmental oversight regarding diesel engine emissions, some drill rig operators and contractors are looking at making the switch from diesel generators to natural gas units powered either by field gas or liquid natural gas (LNG) delivered to the site.

To serve that market, GE’s Waukesha gas engines and Waukesha-Pearce Industries are now offering the Waukesha VHP L7044GSI-EPA for drill rig applications. The unit provides a rich-burn engine that’s 100 percent natural gas fired, designed to reduce fuel costs while meeting state and federal mobile emissions guidelines.

“The greatest expense in operating drill rigs is fuel costs,” says Fred Stow, WPI sales director. “With all of the recent domestic natural gas exploration and development and a spike in supply, natural gas is currently finding itself in a position where it’s very competitive with diesel. The fuel cost for a diesel engine versus a 100 percent natural gas engine is currently about one third. In many cases, field gas gets flared on the drill site, so making the field gas available to operate the generator adds directly to the bottom line. Additionally, flaring gas will soon be prohibited. Since the units would largely operate on field gas that requires no transportation, to provide a more accurate comparison you would also need to count the transportation and liability costs associated with trucking diesel fuel to the site.”

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First drill rig generator application

While the engine model itself is not new, it’s the first time that it’s been employed on a drill rig. Antero Resources and Patterson-UTI Drilling are initially putting the gas generators through their paces with the first installation of three engines at a well in Harrison County, W. Va. The company has also announced plans to use gas engines on drill rigs in several other locations.

The switch-out of diesel for natural gas generators was “plug-and-play,” he says.

“What we did on the first rig at the Patterson site was to simply move the diesel skids out and drop the natural gas skids in using the exact same footprint,” says Stow.

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WPI notes that it’s not the only company currently offering a natural gas engine-powered generator that can operate on field gas.

“However, our model runs on a full range of field gases all the way up to propane,” says Stow. “Rich-burn engines operate with a wide margin for both knock and misfire, so that allows a high tolerance for different fuel compositions that might be available. If you had an interruption in natural gas flow from field gas, you could use alternate fuels such as trucked-in propane or LNG. If you were going to truck in LNG, the fuel delivery distance makes the difference in terms of economic advantage. If you’re within 500 miles of an LNG facility, then delivery makes economic sense when compared to diesel delivery.”

The Waukesha engine model also requires no ancillary equipment, such as a load bank. While the skid of the gas engine generator is identical in footprint to that of the diesel platform, the elimination of the load bank allows additional engine displacement. The natural gas engine’s 12 cylinders offer a total of 7040 cubic inches in displacement in a 21,000-pound unit.

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The natural gas engine generator is oversized to minimize voltage dip during heavy loading. Stow says that the resulting voltage response performance is similar to that of a diesel.

Stow also notes that diesel engines must be derated as they’re deployed in elevations in excess of 3,000 feet.

“Natural gas engines can be located as high up as 8,000 feet without being derated,” he says. “There are quite a number of oil and shale plays located up to 8,000 feet above sea level.”

Maintenance costs are competitive

Maintenance costs are about the same between natural gas and diesel engines over the life of the engine, says Tom Countryman, senior sales engineer with WPI. He notes, however, that natural gas engines require less routine maintenance than both diesel and other spark-ignited engines, in part because they operate at lower cylinder pressures.

He estimates that a similar diesel engine requires an oil change every 750 hours, while the natural gas engine requires one every 3,000 hours. He also says that diesels require top end overhauls approximately every 10,000 hours, while natural gas engines can run for 24,000 hours before requiring similar service. While diesels require a major overhaul every 24,000 hours, a natural gas engine can operate for twice that period.

Natural gas helps meet emissions requirements

The natural gas generators also meet stringent state and federal mobile emissions guidelines.

“As new rounds of emissions regulations go into effect, the EPA has challenged the natural gas industry to reduce levels of emissions during operation,” says Countryman. “The natural gas engines are very green and CO, NOx and volatile organic compounds emissions are significantly lower than diesel generators. US federal regulations reduced allowable NOx and CO emissions by 50 percent in 2010. However, some states are considering enacting tougher limits, particularly on NOx. The natural gas engines also don’t produce the black carbon and particulate matter – soot – that diesel engines are known to produce.”

At lower levels of NOx, lean-burn engines operate in a narrow window, limiting fuel flexibility and making them more sensitive to site conditions. In addition, fuel consumption increases and efficiency decreases as combustion deteriorates due to the high amount of excess air used to lower NOx. Rich-burn engines aren’t subject to the same trade-off.

In general, formaldehyde formation also increases as an engine is operated leaner and combustion temperature and exhaust temperature drop. Rich-burn engines produce up to 10 percent less formaldehyde than lean-burn engines.

Countryman also notes that the natural gas generators use Non-Selective Catalyst Reduction (NSCR) technology, commonly referred to as a Three Way Catalyst (TWC), which can achieve lower methane emissions.

“Rich-burn engines emit much less methane than lean-burn engines,” he says. “The EPA requires that both CO2 and methane be included in total carbon calculations. When we factor methane into the equation, rich-burn engines may be able to deliver a better greenhouse gas profile.”

The NSCR technology used in the gas engines is less expensive than the Selective Catalysis Systems used to meet emissions requirements in diesels.

The TWC needs to be cleaned or have its elements switched out every two to three years, says Countryman. The elements are user removable and can be used two to three times if cleaned.

EPA Mobile Certified

Two of GE’s Waukesha VHP Gas Engines recently received mobile certification from the EPA.

“With non-road mobile certification, these units can be used anywhere in the United States, other than California, and still retain their certification,” says Countryman.

Patterson has already begun to install Waukesha natural gas generators at a second location and has committed to using them at additional drill sites.

“When considering investment in new equipment or improvements to existing equipment, reduced emissions and cost-saving solutions have been identified by customers as a major area of opportunity,” says Stow. “GE and WPI worked with the customers to identify a solution that meet a range of key requirements, such as fuel gas tolerance and no load bank requirements and delivered additional operational benefits such as reduced fuel truck traffic at the drill site through consumption of a local Appalachian fuel source.”

Another drilling company has also ordered 12 GE Waukesha natural gas engines to be packaged by WPI to be used on its land rig platforms.

“The generators have undergone significant testing here at WPI,” says Stow. “This phase of the roll-out is designed to demonstrate that the natural gas engine-driven generators will do what they’re supposed to do in the field.”

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