On the arid plains northwest of Fort Worth, TX, a Canadian firm is operating new high-tech water-treatment plants miles from the nearest electric utility lines.
The water being treated is a byproduct of drilling in one of North America’s largest reservoirs of natural gas, the Barnett Shale Formation. Because treatment takes place close to the wells from which the wastewater emanates, the company—Aqua-Pure Ventures Inc., of Calgary, AB—can power its plants with generators running on natural gas.
“The system is totally self-contained,” says Patrick Horner, Aqua-Pure’s process engineer. “It needs no external source of electric power. We draw natural gas directly from the well to run the compressor and to drive the generator, which produces electricity to power the pumps, instruments, and controls.”
Aqua-Pure has three of these plants operating in Wise County, TX, the first delivered in February 2005 and the others in September. Six more have been scheduled for delivery in 2006.
Each includes a Model GGHE 60 Hz spark-ignited 60kW (derated to 50kW for prime power application) generator set from Cummins Power Generation, of Minneapolis, MN, supplied through Cummins Western Canada in Edmonton, AB.
The engine in this generator set is a V10, naturally aspirated, 10-cylinder Ford Model WSG-1068. Sinisa Kuzmanovic, a power systems sales representative for Cummins Western Canada, says this genset is “a standard Cummins Power Generation product with a few modifications made in our Edmonton fabrication facility. Parts are readily available from any Cummins distributor worldwide.”
“We’ve had great experience with these generator sets used in other prime power applications,” Kuzmanovic says. “They’re very reliable. We’ve seen running hours as high as 20,000 logged in the field. With the extended service features we’ve added to these packages, the service intervals have been increased, allowing less downtime due to regular maintenance.”
Deep Underground
The natural gas in the Barnett Shale resides almost 2 miles beneath the surface. To fracture the gas-bearing rock strata and release the gas, producers perform a “frac job” by pumping 500,000 to one million gallons of water laced with chemicals down a well. It returns to the surface so polluted that conventional treatment methods haven’t been practical. Currently, most of this wastewater is hauled to a deep-well injection site for disposal, which is expensive and makes the water unavailable for any other use.
Aqua-Pure’s new water-treatment technology, the NOMAD 2000 Mobile Oilfield Evaporator, turns frac wastewater from a liability into an asset. Using a process called mechanical vapor recompression (MVR) evaporation, it recycles 85% of the wastewater into distilled water suitable for reuse in another frac job and concentrates the impurities into a brine. With a daily output of 2,000 barrels of reusable distilled water and 350 barrels of concentrated brine, a NOMAD unit cuts producers’ water supply, treatment, and disposal costs 50% to 60%.
Mechanical vapor recompression evaporation is a century-old technology with some inherent problems, including size. The traditional MVR evaporator employs a vertical shell-and-tube heat exchanger 8 to 10 feet in diameter and more than 60 feet tall. Because of the height, such units require firm foundations.
Modular and Mobile
But in the Barnett Shale region, individual wastewater sources typically don’t exist long enough to justify the cost of a fixed recycling unit nearby—or the cost of running electric utility power to a temporary site. To overcome these challenges, a treatment plant must have a small footprint and be readily transportable at intervals of several months while operating at enough capacity to function efficiently and cost-effectively.
“We’ve come up with an innovative modification enabling us to make a mobile MVR evaporator that provides higher capacity with a much smaller facility,” Horner says. “Its footprint is similar to that of a traditional MVR evaporator—about 2,500 square feet—but it is skid-mounted and designed for highway transport on three low-boy trailers without special permitting.”
When treatment at one site is finished, the operators can drain the system, load it on the trucks with a crane, haul it to a new site, and have it set up there within a couple of days.
The system consists of three modules, each 11.5 feet wide and 12.5 feet high:
- A pre-treatment module 40 feet long, weighing 25,000 pounds
- An evaporator module 37 feet long, weighing 42,000 pounds
- A compressor module 30 feet long, weighing 96,000 pounds
The NOMAD 2000 also includes interconnecting pipes, electrical connections, and the genset, which is 41.3 inches wide, 86.9 inches high, and 103.3 inches long (including 85.6 inches within the enclosure). The skids extend beyond the enclosure at both ends, a convenience when the moving crew wants to winch or hoist it onto or off of a truck. The genset’s dry weight is 1,966 pounds.
“We’ve built the highest-capacity system that is still easily mobile,” Horner says. “If we were to build one much larger, it would be much harder to move. This is the optimal balance between capacity and mobility.”
Gathering the Gas
Aqua-Pure pays market price—$7 to $8 per thousand cubic feet—for the natural gas to run its generators. That’s far less than the cost of building an electric power line every time a NOMAD has to be relocated, even assuming that electric utilities would be willing to build the lines and supply electricity on such a short-term basis.
“Using a diesel generator also would be much more expensive than a natural gas generator,” Horner says. “We would have to truck in the diesel fuel, making the cost of diesel to produce an equivalent amount of electricity considerably higher than natural gas.”
Each evaporator is located within a few hundred feet of a natural gas wellhead. Where gas from the well flows into a pipeline, Aqua-Pure installs a tap to draw off the gas it needs. This supply goes into a line heater that regulates its temperature and pressure.
“When you drop the gas pressure quickly, its temperature goes down,” Horner explains. “In very cold weather, or from dropping the pressure, the line can freeze. Our line heater prevents that.
“Because the evaporator is located so close to the wellhead, we just pipe the gas over. It goes right into the engine. Texas gas is lean and clean. It requires no treatment whatsoever.”
Separate Power Supply
Each of the gensets used in the NOMADs includes a Stamford alternator, Model UCI 224F, made by a subsidiary of Cummins Inc. This alternator has a permanent magnet generator (PMG), its own small generator that supplies power to the voltage regulator.
“By having a separate power supply,” Kuzmanovic says, “you end up with better motor starting performance, lower voltage dip when you apply the load, and better immunity from harmonics in the main alternator output induced by non-linear loads. If you have computers and uninterruptible power supplies, they put noise in the power grid and create a disturbed voltage wave form. Having a PMG on the alternator works to eliminate that.
“The alternator is designed with a two-thirds pitch—the way the windings are built—to produce a more true sine wave by eliminating third-order harmonics, the largest harmonics you can have.”
The alternator is rated at 57 kW at 80° C, but it could actually run as high as 79 kW for a short time. “Prime power units have some overload built into them,” Kuzmanovic says. “In a decent environment, this alternator could last double the engine life. This alternator is designed to run at a relatively low temperature, which maximizes its life. You can get a temperature rise of 150° C out of an alternator for a standby application in a high-rise building, but if you run it there all the time, it won’t last long. Standby gensets 30 years old typically don’t have more than 300 hours on them.”
Protective Controls
The gensets used in the NOMADs have a Cummins Power Generation control panel, the PowerCommand 2100 with AmpSentry Protection. It operates the entire genset and protects all components from a wide variety of faults and abnormalities.
“The control panel monitors more than 35 separate parameters, of which the most important are overcurrent and short-circuit protection,” Kuzmanovic says. “If a fault occurs, it shuts off the field in the alternator so zero amps are produced and therefore no power will be produced. This protective device is particularly important, as it will react quicker than a standard circuit breaker.”
Other parameters being monitored include abnormal engine speed, low oil pressure, high coolant temperature, low coolant level, over and under voltage, and over and under frequency.
“It will also tell you if a sensor has failed,” Kuzmanovic says. “It knows the difference between a faulty sensor and an actual fault.”
Helpful Modifications
The genset’s standard oil capacity is 10.4 liters (just under 11 quarts), but these NOMAD installations have a 210-liter (55.4-gallon) tank integrated into the skids to extend the oil-change interval.
“Aqua-Pure’s goal was to run 2,500 hours without an oil change,” Kuzmanovic explains. “That would give them 104 days—more than three months—of running time before they have to shut down for maintenance. It’s something they wanted because downtime would cost them money.
“To achieve that, the operator is responsible for a proper oil-analysis program, which determines whether they can go that long. We recommend changing the oil every 250 hours until you get an idea of how your oil is doing, because there are so many variables, such as the quality of the natural gas you’re burning, the amount of dust in the air, and whether or not acid mist is in the air.”
Another modification for the NOMADs is a sound-attenuation enclosure. The first installation lacked one and ran at 86 decibels at seven meters. Aqua-Pure added an enclosure, and specified enclosures for all subsequent units to reduce the noise level to 77 decibels at seven meters.