Drilling

When natural gas companies are confident that they have located economically viable resources, they begin to drill wells to access the natural gas resource.

When natural gas companies are confident that they have located economically viable resources, they begin to drill a series of wells that allow access to the portions of the formations where the resources reside. A useful overview of the drilling process is offered through Argonne’s Drilling Waste Management Information System (DWMIS) website. Portions of that description are excerpted here.

Basics of the Drilling Process

Natural gas wells are constructed with multiple layers of pipe known as casing (an entire length of casing is known as a casing string). Traditional wells are not drilled from top to bottom at the same diameter but rather in a series of progressively smaller-diameter intervals. The top interval is drilled starting at the surface and has the largest diameter hole. After a suitable depth has been reached, the hole is lined with casing that is slightly smaller than the diameter of the hole. Then cement is pumped into the space between the wall of the drilled hole and the outside of the casing. Next, a smaller-diameter hole is drilled to a lower depth inside the previous string of casing. Another casing string is installed to that depth and cemented. This process may be repeated several more times. The final number of casing strings depends on the regulatory requirements in place at that location. It reflects the total depth of the well and the strength and sensitivity of the formations through which the well passes.

The figure shows a well cross-section. The well incorporates surface casing plus two additional casing strings set to deeper depths. It also shows the recirculating system of drilling fluid or mud that is used to lubricate the rotating drill bit. Drilling mud is pumped downward through the hollow drill pipe and exits through holes in the bit. The mud helps to convey the ground-up rock (drill cuttings) to the surface through the annular space between the drill pipe and the drilled hole.

At the surface, the mixture of mud and cuttings is passed over a vibrating screen known as a shale shaker. After passing through the screens, the liquid mud is recirculated back to mud tanks where mud is withdrawn for pumping downhole. The drill cuttings remain on top of the shale shaker screens; the vibratory action of the shakers moves the cuttings down the screen and off the end of the shakers to a point where they can be collected and stored in a reserve pit for later disposal.

   
Gas Well
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Directional Drilling

Historically, oil and gas wells were drilled to be relatively vertical. They were completed at a depth to intersect a single formation. Thus, one full well was required for each completion. Modern technology offers modifications to several aspects of this procedure, thereby allowing more natural gas production with less drilling and waste generation. In the mid-1970s, new technologies included steerable downhole motor assemblies and measurement-while-drilling tools became more prevalent and enabled drilling to proceed at angles off of vertical. Drillers could more easily turn the well bore to reach targets at a horizontal offset from the location of the wellhead. This opened up many new possibilities for improving production. Three variations of drilling to offset targets are shown in the figure. They include multi-lateral drilling, horizontal drilling, and directional drilling. (While shown here as a specific case, the term "directional drilling" is typically used for the broad class of drilling to offset targets).

   

Advanced Drilling Techniques
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Fayetteville Shale Drilling

The larger operators in the Fayetteville Shale follow similar drilling programs to develop wells that will maximize gas production. First, a relatively large diameter hole is drilled from the surface down to about a 40-ft depth. This is lined with large casing called conductor pipe; the conductor pipe is cemented in place. Next, an air-drilling rig is used to drill a hole to about a 500- to 550-ft depth. Surface casing is installed and cemented in place. Drilling continues with an air-drilling rig until the hole is a few hundred feet above the top of the shale.

At this point, larger conventional drilling rigs are brought to the well pad to drill the horizontal section. The larger rig uses drilling mud to lift cuttings from the well, lubricate the drill bit, and to serve other functions. Unlike regular drill pipe, which is straight, the drilling motor has a slight curvature (this is the blue tubular device in the photo). It is placed on the leading end of the rotating drill string and gradually bends the well bore until the desired angle is reached. Many Fayetteville Shale wells are currently being constructed with lateral extended reaches of 3,000 to 4,000 ft.

Many of the horizontal sections of the wells are drilled using water-based muds (WBMs). When WBMs are used for drilling, the drill cuttings are placed in the reserve pit. At the end of the drilling job, the cuttings are stabilized with fly ash and then are buried in place after all liquids have been removed. The used WBMs are disposed of using land application.

In some of the wells toward the eastern side of the Fayetteville Shale, the formations are more difficult to drill. There, operators use oil-based muds (OBMs) to drill the horizontal sections. Because OBMs pose more environmental risk than WBMs, wells drilled with OBMs put the muds and cuttings into a closed system of tanks rather than putting muds and cuttings into a reserve pit. At the end of drilling a well with OBMs, the OBMs are recycled, and the cuttings are hauled offsite for disposal, typically at a municipal landfill, with permission from the landfill operator.

   

Drilling Rig
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Horizontal
Drilling Motor that Allows Directional Drilling.
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