What is cementing?

Cementing is the process of mixing cement with water and additives into slurry that is then poured into a cavity to isolate it from its surroundings and prevent the entry of water and other liquids into the cavity and the flow of liquids from the cavity into its environment.

In oil and gas drilling, well cementing refers to the process that isolates the wellbore from the rock and underground aquifers around it and keeps the wellbore whole, along with the steel casing. It is part of the broader process of casing and cementing.

Purposes of cementing

Well cementing has several purposes in the oil and gas industry:

• Zonal isolation: blocking the access of underground water into the wellbore.

• Water protection: making sure not just that water does not run into the well, but also that oil and gas from the well do not end up in underground aquifers.

• Casing corrosion protection: the cement that is poured around the steel casing of a wellbore protects the metal from the corrosive effects of water and elements in the surrounding rock.

• Structural support for the casing: Casing pipes are fixed in place with the help of the cement layer around them, which makes the well safer by preventing their dislocation.

Types of cementing

There are two types of cementing in the oil and gas industry: primary and secondary. The first one is much more common and is part of the casing and cementing stage in oil well drilling.

Primary cementing: Once the wellbore is drilled, steel pipes that are collectively called a well casing are lowered into it. To keep them firmly in place, cement is poured through the drill string into the well and up the sides of the casing, forced up by the pressure of its own weight.

Some wells have more than one layer of casing and, respectively, more than one layer of cement between the casings. The cement layers reduce the risk of a blowout, along with the blowout preventers.

In addition to fixing the casing in place and cutting off the access of water to the wellbore, cementing is also used to plug so-called lost circulation zones. These are cavities in the formation that can divert a lot of drilling mud and compromise drilling. This is why they need to be plugged.

In shale oil wells, primary cementing also involves plugging the vertical well before directional drilling begins.

Secondary cementing: Also called remedial cimenting, secondary cementing is used to fix problems created during the primary cementing stage or problems that have emerged during the lifetime of the well.

Secondary cementing is applied by one of two methods: squeeze cementing and plug cementing.

Squeeze cementing involves pumping a certain amount of cement slurry down the wellbore at a high pressure to fill all cavities that have been identified as problematic for the normal operation of the well. It is usually used for casing cracks or cracks and holes in the rock.

Plug cementing, as the name suggests, involves making a plug out of cement to seal off a hole, normally to prevent the flow of water and other fluids into and from the wellbore.

Plug cementing is also used in well abandonment, after the casing is cut at a certain depth. The well then must be sealed with a cement plug to make sure there will be no random release of whatever hydrocarbons remain in the ground.

Cementing equipment

Cementing equipment consists of:

- cement mixers

- pump for the slurry

- cementing head

- wall scratchers

- guide shoe

- floating collar

- two plugs

In addition to all these tools, cementing equipment includes a device called a centralizer, whose purpose is to keep the casing straight in the middle of the wellbore so the distribution of the cement is even all around.

The centralizer is essential in the whole extended casing and cementing process because if the casing is cemented off center, there is a significant risk for drilling mud retaining access to the cement seal around the casing and compromising its quality as an isolator between the well and the rock around it.

Cementing process

Casing and cementing is the final stage in the drilling of an oil well before it begins producing. Before the casing is installed and cementing begins, most of the drilling mud remaining in the well is flushed out—circulated—along with any rock cuttings that have remained at the bottom of the well.

Then, the scratchers, made from wire, looped or sticking out like brush bristles, are attached at equal intervals to the casing pipes, which are then rotated or pushed up and down, so the scratchers can clean the walls of the well from stuck drilling mud and make the cement adhere to the walls better.

Once the walls are clean and the casing is fixed in the oil well, a plug is sent down the wellbore to prevent the cement from mixing with the drilling fluid that is left at the bottom of the well. This plug is called the bottom plug. It has a hollow core and something called a rupture disk or a diaphragm inside.

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