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Wellhead Compressors Capture Casing Gas, Revive Marginal Wells And Aid In De-Watering
[Print Article: August 2002, by Pat Roche] Everyone wants more natural gas. No one wants more venting or flaring. So as gas demand grows, economics and environmental concerns are pressuring producers to recover volumes that once would have been ignored.

But these low volumes of gas -- whether from heavy oil properties, maturing shallow-gas wells or coalbed methane tests -- often don't have enough pressure to flow into a gathering system. And central compressors, which boost pressure on gathering systems serving several wells, are usually too costly for wellhead use.

The answer? Innovation. Compressor companies have been adapting and designing a wide range of small packages that are cheap enough to install on individual wells. For example, PC Compression Inc. of Lloydminster, Alberta, uses a design that for decades pumped Freon through household refrigerators. Stettler, Alberta-based Tornado Technologies Inc. has integrated a GM pickup truck into its system.

"We started building as economical a booster as we could that was portable and easy to install," says Dan Goodhope, PC Compression's business development manager. "What we wanted to do was to change the critical flow right at the wellhead. Rather than one compressor drawing on a multiple of wells, we wanted to have a compressor that focused on that one well and would increase the incremental gains enough to pay for that unit."

For example, a low-pressure gas well may stop producing because water accumulates in the wellbore. Often, by changing the critical flow, or downhole pressure, by as little as one or two pounds, a non-producing gas well will begin flowing again, says Goodhope. "So we don't really have to compress the gas that much at surface -- we need to change that downhole pressure enough for gas to start flowing and to lift that water."

As compressor firms introduce these smaller, cheaper, more-mobile compressors, oil and gas companies are snapping them up.

"What we are seeing as a trend is that the sub-100 horsepower is really becoming the compressor of choice (for lower pressure applications). Where we're the busiest now is in that sub-100-horsepower range," explains Robert Grandfield, president and general manager of Stettler-based Jiro Compression Ltd., a unit of Calgary-based Enerflex Systems Ltd.

One area where small compressors have chalked up impressive results is in the recovery of casing gas from heavy oil production. Traditionally vented to atmosphere, methane from many heavy oil wells is now used as fuel gas on site or sold at a profit.

In the Wabasca-Brintnell area in northern Alberta, Canadian Natural Resources Limited is recovering about 32 mmcf a day in casing gas from heavy oil wells, estimates Hal Rutz, a CNRL production foreman in the Wabasca area.

Wellhead compressors CNRL uses in that area range from 30-horsepower electric units up to 220-horsepower natural gas-powered packages, many of which are supplied by Jiro.

Last year Jiro introduced a new rotary vane compressor for casing gas and similar wellhead applications. Used for decades in air compression, rotary vanes are increasingly being used in low-pressure methane applications because they're smaller and hence cheaper than screw compressors, explains Jiro's Grandfield.

CNRL has been using about two dozen of Jiro's hydro vane compressors on casing gas applications for about 18 months, says CNRL facilities manager Dean Halewich. (A hydro vane uses an oil "bath," or reservoir, as its seal, whereas a rotary vane uses tighter mechanical tolerances with an oil film to form a mechanical seal.)

The company installed the small compressors, which are between 20 and 50 horsepower, at Alberta heavy oil properties such as Elk Point, Lindbergh and Wabasca-Brintnell. While CNRL is satisfied with the performance of the hydro vanes, the major limitation is that they can only go up to 145 psi discharge, says Halewich. "So you have to make sure your gathering line is below that or else your oil seal is not going to hold the pressure," he says. "And your maximum suction on them is 10 psi."

So if the gathering system is running at, say, 200 psi pressure, hydro vanes aren't powerful enough to pump into that line, so the alternative is the screw compressor, which is good for up to 300 psi, he says. Beyond that, a reciprocating compressor is needed.

On the shallow-gas side, Devon Canada Corporation has used PC Compression's small compressors to extend the life of marginal wells. In the Marten Hills area of northern Alberta, a gas well that had been producing nothing was boosted to an average of about 225 mcf a day for four months, says Darrel Hardes, a Devon production engineer.

Devon recently installed one of PC's compressors on a gas well that was plagued by liquids loading -- water in the wellbore was restricting gas production. The compressor installation doubled production to about 400 mcf a day, says Hardes. "It was a problem well that was taking a lot of the time of the operators -- they always had to go out and play around with this well," he recalls, adding that the operators haven't had to do anything with the well since the compressor was installed.

When it comes to promises of low maintenance, it's hard to beat Goodhope's claims about scroll compressors -- the ones based on refrigerator technology. "We expect this one to be an enormous seller (because it) has virtually no maintenance," he says, explaining that it would require between a quarter and one litre of oil once a year. "You tie it in, push the button and hopefully leave it alone. And we expect 20-25 years run time out of it -- very similar to a refrigerator."

The compact little scroll compressors can fit in the back of a pickup truck. The motor and compressor are built into a single canister that is about three feet long, two feet wide and about 30 inches tall.

More recently, PC has built even smaller compressors that are tiny enough to haul on a trailer behind an all-terrain vehicle or snowmobile. These units, which can move about 250 mcf a day, are geared toward the shallow-gas market, Goodhope says. "Our (intention) is to go after a lot of these wells that have been shut in and put them back online."

While some compressors are small enough to fit into the back of a pickup truck, Tornado's Compress X-press actually integrates the truck into the total package. A Comp-Air screw compressor runs off the truck engine, which General Motors Corporation designed specifically for natural gas.

"You drive it to the site on gasoline. You start it up on gasoline. And then you switch it over to natural gas," explains Miles Krowicki, Tornado's sales and marketing manager. "And it runs off the natural gas from the well. It's filtered four times before it enters the engine."

The operator sets the cruise control at the required speed -- say, 80 kilometres an hour -- and leaves the truck on the site ... for days or weeks or months. "It can't be stolen," says Krowicki, "because the operator disengages and removes part of the driveshaft."

But wouldn't this cost more than skid- and trailer-mounted units? Actually, it's cheaper, says Krowicki. In the case of trailer-mounted units, most companies will fabricate their own trailers, "which is not that cheap to begin with. And then you have to start mounting everything." But because Tornado's system uses the truck's engine and drive train, all it has to do is install the compressor on the vehicle. Consequently, the company says, its truck-mounted units rent to customers for 30% to 50% less than comparable skid- or trailer-mounted compressor packages.

Tornado says its design is also a safety improvement. Since the engine and compressor are in separate compartments, the danger of a gas leak igniting is greatly reduced. As well, the company says the overall unit has less vibration and noise -- common complaints of compressor operators. And installation at the wellsite is a breeze, says Krowicki. "It's a five-minute hookup."

Tornado has built two prototype models -- one to handle 600 mcf of gas a day and one capable of 1.2 mmcf. "Having completed development, we're now building our finished production-line model for rental or sale," Krowicki says. "We've already taken delivery of our first 20 trucks (from GM)."

While the unit could be used to de-water wells or boost pressure on a pipeline, Tornado expects the main application will be marginal gas properties.

"Many companies have significant downhole reserves remaining, but there's not enough pressure for them to flow by themselves into the pipeline. There's lots of money to be made but you have to compress (the gas to produce it). So the small mobile booster compressor market has taken off," says Krowicki. "It's a nice niche -- where we're giving producers the opportunity to recover that trapped gas. Why leave it trapped down there? You hook this thing up and five minutes later you're generating cash."

While marginal gas properties are an obvious application, small compressors also are used on some new wells. EnCana Corporation is using wellhead compressors on some coalbed methane evaluation wells on the Palliser block in southern Alberta. In many cases, no gathering system is available, so the only alternative is to flare the gas during testing. But where a gathering system is available, the small compressors enable EnCana to recover the gas instead of flaring it.

EnCana installed seven wellhead compressors from Calgary-based Startec Refrigeration Services Ltd., three from Oklahoma City-based Compressco Field Services, Inc. and four from Jiro.

The compressors range from about 30 to 90 horsepower and the average size is about 50 horsepower, says Edmond Soo, an EnCana facilities engineer. Startec supplied 35-horsepower screw compressors introduced this year in response to strong industry demand. These new compressors are designed for 150-800 mcf a day.

Compressco, which recently opened a Calgary office, is targeting wells producing less than 500 mcf a day. The company's 50-horsepower GasJack compressor is "about twice as efficient as a small screw package," says Kevin Book, Compressco's manager of Canadian operations. "We can move as much gas (as a 95-horsepower screw compressor) with half the horsepower."

CNRL's Halewich says the key message for small-compressor suppliers is to get the price down. "The less expensive those machines become, the more opportunity there will be because there is definitely a need -- and a very significant need -- for them," he says. "The challenge usually is finding a piece of equipment cheap enough -- because this gas isn't there forever. And it's also in small quantities."

If a well produces 50 mcf a day of casing gas at $3 an mcf, the gross revenue is only $150 a day, or about $55,000 a year. If a compressor package costs $50,000, Halewich says, site preparation and installation could bring the total capital cost to $100,000 -- and casing-gas production may last only for two or three years.

So if compressor companies continue to whittle down costs, more casing-gas recovery projects will become economic.

"The opportunity is huge -- but you have to find equipment that's economical," Halewich says. "It'd be excellent to have a $20,000 piece of equipment out there that we could use."

CONTACTS FOR MORE INFORMATION

Dan Goodhope, PC Compression, Tel: (780) 875-4546, E-mail: [email protected]

Robert Grandfield, Jiro Compression, Tel: (403) 742-5538, E-mail: [email protected]compression.com

Miles Krowicki, Tornado Technologies, Tel: (403) 263-8011, E-mail: [email protected]

Greg Nelson, Startec Refrigeration, Tel: (403) 295-5882, E-mail: [email protected]

Kevin Book, Compressco, Tel: (403) 589-7369, E-mail: [email protected]

  • Jiro has designed a wide range of small compressor packages that are cheap enough to install on individual wells.
  • Tornado's Compress X-press integrates the truck into the compressor package. A Comp-Air screw compressor runs off the truck engine.
  • PC Compression has built compressors small enough to haul on a trailer behind an ATV or snowmobile. The units, which can move about 250 mcf a day, are geared toward the shallow-gas market.
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