Drilling: A Look Beneath the Surface
By Dan Piller, Fort Worth Star-Telegram, Texas
May 01–Tarrant County and surrounding counties are in the midst of a drilling boom in the Barnett Shale natural gas field, about a mile or more below the North Texas prairie. The rigs, especially at night, are visible for miles. And many of us are getting up-close views of the operations.
The public sees the derrick standing over the well. But the completion of a natural gas well of the type being drilled in and around Fort Worth is a multistage operation that lasts for decades.
Below are several frequently asked questions about drilling. Technical information was provided by Jimmy Thomas, a Weatherford-based geologist and veteran drilling consultant.
Question: It seems that a lot of gas would escape as soon as drilling begins. Is that true?
Answer: Not really. Only minuscule amounts of gas escape because it is trapped inside the rock. Blowout preventers are put over the wellhead during drilling. The drilling phase is the safest period for a well.
Q: After the well hole is drilled, what is the next step?
A: The casing, or steel pipe, is put into the well and held in place with concrete. Then, through the use of small explosive charges, perforations are put into the sides of the pipe to make it ready for fracturing.
Q: What is fracturing?
A: Fracturing is the high-pressure injection of water into the well. The water shoots through the perforations into the rock, making little cracks so the gas trapped in the rock can escape. Granular sand is included in the water to keep the fractures open.
Q: What happens to the water?
A: About 60 percent of it comes back to the surface. The rest gradually flows into underground reservoirs. But there will be water in the well for a while. The fracture water that comes back up is contained in those round tanks that can be seen standing next to completed wells.
Q: Is that good or bad?
A: The water would keep the risk of a fire down. Because the rock is hot, a new well contains a lot of gas and steam vapor.
Q: Does gas start flowing after fracturing?
A: Yes. The period after fracturing is the most delicate time for the well because the pressure inside the well is rising and there isn’t any place for the gas to go until the well is hooked up to a pipeline.
Q: How is the gas contained?
A: The pipe is rated strong enough to hold the gas. Occasionally gas is vented to relieve the pressure.
Q: How soon is the well attached to a pipeline?
A: Ideally, no longer than a few days. Many drillers have the pipe to the well laid even before the well is finished. The pipeline from the well is attached to a larger gathering system that moves the gas to a processing plant or directly to customers.
Q: Does the gas flow automatically through the pipelines or does it need help?
A: A field and its pipeline system normally have several compressor stations, which generate more pressure to boost the gas along the pipeline.
Because the compressor and its big motors are there for the duration — unlike the drilling derrick, which is usually up no more than three weeks — the compressor’s neighbors are most likely to be the unhappiest of everyone who lives near a well.
Q: How much pressure does a newly drilled well make?
A: Up to about 3,000 pounds per square inch. But that pressure will decline by at least 50 percent in six to eight months.
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Dan Piller, (817) 390-7719 danpil@star-telegram.com
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Copyright (c) 2006, Fort Worth Star-Telegram, Texas
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