#AskCabot Recap: Why and how we case a well

Last week’s #AskCabot question was, “How much pipe do you use when drilling a well?” We responded with, “It takes more than 100 tons of pipe to drill a well 10,500 foot total in length.”

We thought this was a great opportunity to take a closer look to a process used when drilling our wells called casing.

Safe and responsible natural gas development is the first and foremost priority of Cabot Oil & Gas Corporation. One particular safe guard Cabot, and the natural gas industry, utilizes to protect ground water is multiple layers of casing in our wells.

Why Case a well?

Drilling a natural gas well not only connects a reservoir such as the Marcellus Shale to the surface, it also creates a pathway between previously unconnected formations found in between. Through the well, any fluids or gases trapped in one formation could begin migrating to another. Conversely, the same type of movement could occur from the surface and migrate down the wellbore where it could enter formations previously secluded from such activity. In shallow formations fresh water is commonly found and retrieved through personal or municipal well systems. Casing is the mechanism used by Cabot, and the industry, to prevent possible undesired interaction between formations.

How does casing work?

Casing a well will reestablish the barrier between formations that existed for millions of years.  It separates the water table from future developmental activities imperative to producing gas from a desired formation. Casing will also prevent the wellbore from becoming an inter-formation connection or pathway for migration.

What makes up the casing?

Cabot utilizes a combination of steel pipe and cement to establish its casings. To protect the shallow water formations as best as possible, Cabot uses four layers of casing to create an eternal barrier to the well bore. An additional steel pipe, commonly referred to as production tubing is placed in the wellbore when it officially starts producing natural gas. This production tubing is not permanent and can be changed throughout the life of the well.

Why use four layers of casing?

Each casing has a distinct purpose which, together, provides maximum protection to the fresh water table. All casing is cemented into place from the bottom up and set into solid rock that is capable of holding the structure together.

  • Conductor Casing
    • This first-string of casing establishes the wellbore and protects the integrity of it from lose material (glacier till) commonly found at Earth’s surface from collapsing the wellbore in on itself. This string of casing is set at a depth of 150ft.
  • Surface Casing
    • Set past the deepest fresh water aquifer, this string of casing is designed to protect drinking water from development and completion of the rest of the wellbore. This casing is 13 3/8th thick and it is set at a depth 400 – 900ft.
  • Intermediate Casing
    • Designed to prevent migration from deeper layer up into the shallow water tables. This casing is set to a depth of 2000 ft. The casing thickness is 9 5/8ths inch and is set into solid rock.
  • Production Casing
    • The final layer of casing is set at the toe of the well. This means the absolute end of the wellbore; taking into account both vertical and horizontal sections. The purpose of this case is to prevent natural gas in the Marcellus Shale from migrating anywhere but the wellbore. Total length of this casing can be upwards of 10,000ft. The thickness of this casing is 5 ½ inch. This particular pipe too is cemented into place from the bottom up. Unlike each of the other layers of casing, the production layer of casing is cemented into place from the toe point up to roughly 2,000 ft below Earth’s surface.

If you still have questions about casing or would like to see a visual, come to the Cabot Community Picnic this Saturday from 10 a.m. to 2 p.m. at the Harford Fairgrounds and speak with Cabot’s Drilling team. They will have a model of the steel pipes and cement using in the casing process on hand to demonstrate how all of the layers work together.

Remember that today is an #AskCabot Thursday. We’ll take your questions, tagged with #AskCabot all day to our Twitter account or to our #AskCabot webpage. If you have a question we haven’t answered before, you could be featured in next week’s recap.

#AskCabot Recap: Cabot’s Water Use
#AskCabot Recap: How old is the Marcellus Shale?
Bill desRosiers

Raised in Highland Falls, New York, William desRosiers learned about responsible resource development, firsthand, as a part of his family's mining business. William received his B.S. in Management, B.A.in History and MBA from Misericordia University. He currently serves in External Affairs for Cabot Oil & Gas Corporation. His primary responsibilities include strengthening media relationships, managing company-run fundraising programs, building better community relations and representing Cabot every chance he has.

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