NATURAL GAS:
PART 7: COMPRESSION

Cooperative effort allows efficient, fast-track development of a gas field

An operator, its gas marketer and equipment supplier use ready-to-ship compressor packages and other equipment from inventory to bring the field to full production

Bill Bowers, The Hanover Company, Houston

otato Hills field in southeastern Oklahoma offers an innovative example of how natural gas can be brought to market efficiently through the cooperative efforts of an exploration and production company, the gathering system’s operating and construction firm, and the process equipment and services company. Each entity, by focusing on its core competencies and maintaining an open, clear line of communication with the other companies involved in the project, maximized its contribution to the project’s success.

Company Profiles

The GHK Co. is a privately held natural gas exploration and production company based in Oklahoma City. It was founded in 1959 by Robert A. Hefner III and two other partners who are no longer associated with the company. GHK’s major E&P activities have been in the Anadarko basin of western Oklahoma and the Texas Panhandle, the Arkoma-Ouachita fold-and-thrust province of eastern Oklahoma and the Eastern Cordillera fold-and-thrust belt of Colombia. From its inception through the mid 1980s, the company was best known for pioneering the deep, high-pressure gas development in the Anadarko basin, where the company led the industry in technological innovation to successfully drill and produce several of the world’s deepest, highest-pressure, natural gas wells. In the 1990s, GHK refocused its future on fold-and-thrust belts, with operations in Colombia and the Arkoma-Ouachita province of eastern Oklahoma.

Based in Tulsa, Oklahoma, Oneok, Inc., is a diversified energy corporation, with an assets stronghold that lies across the prolific Mid-Continent. The company is among the top 10 operators nationally for NGL production. Its marketing efforts reach more than 28 states, with key interests in the retail natural gas markets of Oklahoma and Kansas. Subsidiaries Oneok Gas Transportation and Oneok Producer Services offer gas gathering, transportation and producer services to assist in getting gas from field to market.

Based in Houston, The Hanover Co. specializes in full-service natural gas compression, and is a provider of service, financing, fabrication and equipment for contract natural gas handling applications. Hanover provides this equipment on a rental, contract compression and acquisition leaseback basis to natural gas production, gathering, processing and transportation companies. Founded in 1990, and a public company since 1997, Hanover’s customers include independent and major producers and distributors throughout the world.

Project Overview

GHK discovered, developed and operates Potato Hills field. Oneok constructed and operates the gas-gathering system, and markets the gas for GHK. Hanover manufactured, operates and maintains the gas compression and production equipment (under contract to Oneok) as an outsourcing service.

The Potato Hills discovery well was drilled in 1997. The Ratcliff 1-33 was drilled to a 12,597-ft TD, but was completed in the Jackfork formation at about 5,400 ft. Initial production was delayed until September 1998, when the gas gathering system was completed. The well initially produced in excess of 10 MMscfd.

Based upon the discovery well’s success, GHK contracted Oneok to construct a pipeline (Fig. 1) connecting the field gathering system to the ONG Pipeline. Construction of the original gathering system – which included 7 mi of 12-in. pipeline, dehydration and sales meter facilities – began in 1998 and was completed later that year. Most of this construction occurred in mountainous, rocky areas.

Fig. 1. A 7-mi, 12-in. pipeline was constructed to connect Potato Hills field to the gas sales line. Construction, most of which occurred in mountainous, rocky areas, began in 1998 and was completed later that year. The gathering system was later expanded to include 7 mi of 20-in. line paralleling (looping) the original 12-in.

While production from Ratcliff 1-33 was being established, GHK and Oneok contacted Hanover for dehydration equipment. This was to be a temporary installation until final production rates and pressures could be determined, so Hanover leased a wellhead-type dehydrator to Oneok.

Since the discovery well’s completion, GHK has drilled 32 additional wells, of which 23 are currently producing, with another four awaiting completion. The gathering system has grown to more than 29 mi of pipeline. During the same time period, the processing and compression facilities have been expanded to handle production rates to 175 MMscfd.

Gas Gathering / Conditioning

To handle radically increased production rates, the gathering system was expanded to include 7 mi of 20-in. line paralleling (looping) the original 12-in. line that connects the field to the compression site. Field flowlines include an additional 6 mi of 12-in., 3 mi of 16-in., 4 mi of 8-in. and 2 mi of 6-in. pipeline.

While the compressors were being installed, Hanover furnished and installed the gas dehydration equipment at a nearby site. Supplied from the company’s rental fleet, the gas dehydration equipment consisted of a 60-in. ID x 30-ft s/s 1,440-psig absorber-scrubber with eight trays, and a 750,000-Btu/hr glycol regenerator with pumps, Fig. 2. A horizontal coalescing filter-separator (48-in. OD x 20-ft s/s 1,440-psig) was later added to eliminate lube oil carry-over from the compressors to the dehydration station.

Fig. 2. Initial gas dehydration equipment, supplied from the vendor’s rental fleet, included a 60-in. ID x 30-ft s/s 1,440-psig absorber-scrubber with eight trays, and a 750,000-Btu/hr glycol regenerator with pumps.

As production rates increased, a second dehydrator was installed. Later, a third unit for future expansion and backup to the existing units was added. Finally, a scrubber was installed downstream of the dehydrators to catch any glycol carry-over due to upsets.

Hanover also operates and maintains the dehydration and production equipment, relieving Oneok of the manpower and overhead for these functions and ensuring maximum up-time for the equipment.

Compression

Potato Hills gas is sweet and lean, and requires no treatment other than condensed liquid removal prior to compression. Gas from the field gathering system is piped to an inlet scrubber, consisting of a 60-in. ID x 10-ft s/s 1,440-psig vertical separator with vane mist extractor. A second, identical scrubber, like the one for the Latimer County wells, was added to handle additional gas from the Pushmataha wells.

Production from the field peaked at about 145 MMscfd in early 2001. Typical wellhead pressures range from 450 psig to 550 psig, depending upon the distance between the wellhead and the compressor station. Inlet (suction) pressure to the compressors is about 400 psig, and compressor discharge (sales line) pressure is typically 850 psig.

The first compressor was installed at Potato Hills in November 1999. Between that time and January 2001, six additional compressors were installed to keep up with increasing gas volumes flowing to the compressor stations, Fig. 3. The primary criterion for selecting the compressors was to ensure long-term flexibility by designing in the ability to fully use the units’ horsepower over a wide range of suction pressures. The first two compressor packages were single-stage units shipped from the company’s PDQ fleet in Houston, with the subsequent five units being configured to operate in one- or two-stage compression service.

Fig. 3. The first two compressor packages included Cat 3516 TALE (1,265-hp) engines with Ariel JGE4-1 single-stage compressors. Five subsequent units comprised Cat 3606 TALE (1,665-hp) engines with Ariel JGD4-2 two-stage compressors. The five latter units were set up initially as single-stage compressors, but can be switched to two-stage by merely reversing three blind flange paddles on the skid piping and cooler.

The Hanover PDQ program provides an immediate inventory of ready-to-ship compressors comprising one-, two- or three-stage standardized units with Ariel compressors and Caterpillar or Waukesha drivers ranging from 1,000 to 1,665 hp. These units are built on speculation, with a revolving inventory available for both onshore and offshore applications.

The initial two units selected for this project were Cat 3516 TALE (1,265-hp) engines with Ariel JGE4-1 single-stage compressors. The five subsequent units were all Cat 3606 TALE (1,665-hp) engines with Ariel JGD4-2 two-stage compressors. On the last five units, the cylinder selection, on-skid piping and cooler design were such that the units could be used initially as single-stage compressors. As field (suction) pressure declines, the units can be switched to a two-stage configuration by merely reversing three blind flange paddles on the skid piping and cooler.

The first-stage cylinders on these units were high-pressure PDQ cylinders that provide working pressures high enough to be used in the initial single-stage service required (850 to 950 psig discharge). Use of these smaller first-stage cylinders, typically found in offshore two-stage configurations, allowed the higher working pressure needed for single-stage service and a lower rod load over the entire range of design pressures. This cylinder selection also allows the compressors to operate fully loaded, as the field draws down and suction pressures decline.

Plant Controls

The dehydration plant and compressors are equipped with local control panels incorporating safety shutdowns, operator alarms and annunciators for cause of shutdown. Hanover field operating personnel are typically on site daily, and a backup remote monitoring system is incorporated in Oneok’s gas control center if a unit goes down after hours. Individual systems will automatically shut down for safety reasons and to protect the equipment from damage.

Future Plans

Potato Hills field now covers southern Latimer County and northern Pushmataha County of southeastern Oklahoma. To date, the field has produced more than 80 Bscf. Most of this production has been from the Jackfork formation.

In the near-future, GHK will continue to develop the Jackfork formation while also testing other, deeper reservoirs. Seismic data indicate that this subthrusted anticlinal play could ultimately produce in excess of 3 Tscf, if deeper pay zones are productive.

The author
	Bill Bowers is a product manager for The Hanover Company, working in the Integrated Systems Group at the Houston headquarters. He has previously worked for Peak Process, Inc., Conoco Specialty Products, and Vortoil Separations Systems in various engineering and management roles supporting applications, design and manufacture of liquids separation processes. Prior to beginning his oilfield career with Schlumberger in 1976, he earned a BS in electrical engineering from the Georgia Institute of Technology.

	Part 1: Exploration Methods
	Part 2: North America Outlook
	Part 3: HP/HT Drilling and Completions
	Part 4: Tight Formation Stimulation
	Part 5: Sour Gas Handling
	Part 6: Anomalously pressured zones
	Part 8: Monetizing Stranded Gas