Upstream Oil and Gas – Drilling the Well

In Upstream Oil and Gas Drilling the Well lesson, we’ll give a quick introduction to the characteristics of an oil or gas well. We also present the initial steps in drilling – well site preparation, spudding in, and rigging up. We’ll also discuss some of the differences between onshore and offshore drilling operations.

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What is a Well?

A well is the term for any perforation through the earth’s surface designed to find and release oil and gas hydrocarbons. In summary, a well is created by drilling a hole, called a borehole, into the earth with a drilling rig that rotates a drill string with a bit attached.

As the wellbore is drilled, at prescribed depths, sections of steel tubing known as casing are set in the hole (slightly smaller than the borehole). The casing program is defined by the drilling engineer when the well is designed, and depends on the ultimate or target depth of the well and the geology of the drilled formations.

Casing is cemented into place to provide structural integrity to the newly drilled wellbore to first protect the water table. As the well is drilled deeper, casing strings are set and cemented to isolate any potentially dangerous high pressure zones from each other as well as preventing pressure from reaching the surface.

With this zone safely isolated and the formation protected by the casing, the well can then be drilled deeper, often into potentially more unstable formations. Here, a smaller bit and smaller size casing is used. Modern wells often have 2-5 sets of subsequently smaller hole sizes drilled, each inside one another and each with a different size of cemented casing.

A cased wellbore is like a collapsible telescope, with the largest lens at the top of the well.

Of course, before you can start drilling a well, there are some preliminary steps taken to prepare the well site, and these differ whether the well is onshore or offshore.

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Onshore Drilling Preparation

First, let’s briefly talk about onshore drilling preparation.

A drilling rig is a machine which creates the boreholes and/or open shafts in the ground. The term “rig,“ generally refers to all of the complex of equipment that is used to make a well.

Before a land-based well can be drilled the site must be prepared, including leveling the land on which the derrick will be assembled. Access roads must often be created so workers and equipment can get to and from the rig. This is quite a project in the jungle or areas that are mountainous or wooded.

Reserve pits are then dug and lined so that cuttings, material and used mud can be properly contained. In some onshore locations, environmental laws prohibit drillers from digging mud pits. Instead, large metal tanks are used at the rig site to catch all cuttings and waste materials which are then hauled offsite for disposal. The same collection process is used in offshore well drilling.

The final step in site preparation is to dig what is called a cellar for the equipment needed as the well drilling operation gets started. The cellar can be dug and formed in place with concrete, or it can be a pre-fabricated, four-walled steel structure that is driven into the ground to the desired depth. The cellar is set at the exact place where the wellbore will be centered.

Spudding In

The start of drilling is very commonly referred to as “spudding in.”

Once the cellar is set, spudding in begins with drilling a large-diameter but shallow hole. This is referred to as a conductor hole and is typically 100-200 ft deep. Once drilled, it is lined with conductor casing or ‘pipe’ and cemented into place. Conductor casing is used to keep the top of the well-bore from collapsing in on itself.

Onshore, conductor casing can be drilled and often set by a third-party mobile casing crew contractor.

Offshore, the drilling rig is used for this process.

Rigging Up

Another onshore process known as “rigging-up” begins as the rig mast is hoisted into position and the equipment substructure is centered over the conductor pipe.

The mast or derrick is raised over the substructure and other equipment such as engines, pumps, and rotating and hoisting equipment are aligned and connected.

Water and fuel tanks are filled. Additives for the drilling fluids, called drilling mud, are stored on location. When this work is completed, the rig is then ready to drill another large diameter hole, approximately 100 to 150 meters deep. This second segment has three important functions:

seal off surface water aquifers
stabilize the top of the well
provide an attachment for the blowout preventers (BOPs)

It is very important to make this hole right because all subsequent drilling operations will be based on this segment.

Once the BOPs are activated, the true well drilling begins towards the target depth and reservoir.

Offshore Drilling Preparation

Offshore drillingrefers to the act of extracting underground resources which lie off the sea coasts. The term is often applied to drilling in lakes, inland seas, bays or channels.

The actual process of drilling the offshore well is similar to an onshore, land-based well, but the offshore location makes it much more complex. Often, new offshore wells are drilled while others are being produced from the same production platform.

An offshore rig or platform is like its own small city. They need to generate the power needed to run the equipment and have fresh water and sewage infrastructure to be self sufficient.

It takes hundreds of staff to drill an offshore well. All rig personnel are transported to and from the rig by helicopter or crew boats. Since staff are generally required to be onsite for rotating two-week shifts, they have their own cafeteria, sleeping quarters and entertainment halls.

Shift rotation can vary depending on location and covers the E&P operator, drilling contractor and oilfield technical staff and service providers. In foreign locations the shift rotation is often 28 days on and 28 days off.

All equipment, food, fuel, supplies and waste must be constantly transported to and from the rig by work boat. Movements need to be delicately choreographed because of the limited floor space on the drilling platform.