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Preparing a Well for Production

Newly drilled wells must be properly completed.

A newly drilled well must be properly completed to allow natural gas to enter the well and move to the surface. The bottom of the well ends either in open rock in the target formation or in casing. In the former case, the formation is already accessible to the well. However, when a well is cased through the gas-producing section, openings in the casing must be made to permit the natural gas to enter the well. The holes or perforations ("perfs") are made in the casing using small explosive charges or guns that are lowered to the desired depth on a cable. The perfs allow gas to enter the well. After creating the perfs, the well is stimulated by hydraulic fracturing to allow gas to flow from the formation into the well.

Hydraulic Fracturing

Because shale gas is held within a nonporous medium, it is necessary to fracture the shale so that the gas has a conduit or pathway to move from the shale to a production well. The fracturing process (a "frac job") injects water, sand, and possibly other ingredients at very high pressure into the well. The high pressure creates small fractures in the rock that extend out as far as 1,000 ft laterally away from the well. The vertical extent varies but is related to the thickness of the shale layer. After the fractures are created, the pressure is reduced. Water is removed from the well, but the sand grains remain in the rock fractures, effectively propping the fractures open and allowing the gas to move. Fracturing is a critical step in producing the Fayetteville Shale wells.

Various types of "frac fluids" have been used. Some of the earlier Fayetteville Shale wells were fracced using nitrogen foam; however it was found that wells fracced in that way did not produce the fracture system the companies hoped to achieve. Many wells are now being fracced using "slick water" - water containing some surfactant additives to help the flow-back water return from the well at the end of the frac job.

Frac jobs on horizontal Fayetteville Shale wells are often conducted in stages. The length of the horizontal section of the well is divided into several sections by plugs. The outermost section is fracced first. First water is injected at increasingly higher pressure until a pressure chart shows that the bottom-hole pressure has made a sudden drop, indicating that the rock has fractured. At this point, sand is added to the injected water and the pressure is maintained until a desired dimension of fraccing is completed. The plug separating the last section from the next-to-last one is sealed, and the frac job is continued for the next-to-last section. This sequence continues until all sections are fracced.

Immediately following completion of the frac job, formation pressure causes the water used as part of the frac fluid to come back out of the well at a rate of 100 to 150 bbl per hour. The volume of this "flow-back water" declines over time. It is collected in 500-bbl frac tanks and is either reused or disposed of.

When the frac job is finished, the plugs that separated the sections of the horizontal leg of the well are drilled out.

In a final well preparation step, small-diameter pipe called tubing is lowered into the well to the depth at which fluids have accumulated. An inflatable packer is installed near the bottom of the tubing. A packer looks like a large donut that fills the space between the tubing and the next layer of casing. This configuration creates a tight seal and prevents materials from entering the well bore except through the tubing. Completion fluid is added to the annular space above the packer and between the tubing and casing to prevent corrosion. Although most Fayetteville Shale gas wells flow by natural reservoir pressure, later in the life of a well, some wells may require pumping systems. Operators can install plunger lift systems to produce the natural gas as the need arises.

Logistical Requirements

Frac jobs require a great deal of heavy equipment and supplies. The entire frac job for one well may take a full day and uses 50,000 to 80,000 bbl (2 to 3 million gallons) of water and 1 to 1.5 million pounds of sand. When more than one well is located at the same well pad, fraccing operations can last several days, and the water and sand volumes increase proportionally. Sand is hauled onsite by a fleet of trucks. Water is either piped in from local ponds, streams, or reservoirs constructed by the natural gas operators in cooperation with local communities and landowners or can be trucked in when local water sources are not available. Many of the local county roads in the Fayetteville Shale region are unimproved dirt roads. These roads were not designed to accommodate heavy traffic by large trucks and other oil field machinery.

Even without numerous tank trucks carrying water, the pad area at a well being fracced is crowded with heavy equipment. The service companies conducting the frac job will bring multiple engines and pumps, monitoring vans, frac tanks for making the frac fluid mixture and for capturing flow-back water, sand-hauling trucks, and other support equipment. These figures show a well site in the midst of a frac job.

   
Water Pipelines
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Typical Area Road
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Click the thumbnail images below to see enlarged photos of a frac job in the Fayetteville Shale play.



Frac Job Trucks



Sand Hauling Trucks



Frac Tanks



Sand Storage Tanks



Well Fraccing



Well Fraccing



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