What is cement?
Cement is a mixture of several different types of rocks, crushed and heated to very high temperatures, and then ground to a superfine dust to use in the production of concrete.
Typically, cement combines rocks including limestone, shells, shale, iron ore, chalk, clay, and slate. These are first ground into little pieces before they are fed into a furnace—a cement kiln—and heated to some 2,700 degrees Fahrenheit, or more than 1,480 degrees Celsius.
The kiln rotates, and some of the elements in the rocks evaporate from the mixture. What remains is called clinker, which goes through several coolers before it can be handled. Handling is done by cement plants, which grind it into the fine powder we know as cement.
The American Society for Testing and Materials distinguishes between five basic types of cement:
Type I: Ordinary cement, used when no specific requirements regarding setting time or heat hydration are present. All cement generates heat when it comes into contact with water. This is called heat hydration, and in certain environments, low heat hydration is a must. In others, it is not a primary concern.
Type II: Modified cement, which is commonly used in foundation concrete. It sets more slowly than other cement types but is less prone to cracking.
Type III: Fast-setting cement that is often used in road construction and repairs because of this property, which allows for quicker construction/repairs times when time is critical.
Type IV: Low heat hydration cement, meaning that when water comes into contact with it, the cement produces less heat than it would otherwise.
Type V: High sulphate resistant cement. Sulphates are chemical compounds contained in underground water that can attack cement and erode it. For underground structures and structures in damp and offshore environments, this is the cement used to avoid fast erosion.
The cement used in oil wells, however, must be a special kind. It has to have very low permeability to seal the well from the surrounding rock and aquifers and it has to be strong enough to withstand the pressures it will be subjected to in the well, from the casing and from the surrounding rock. Related: Iraq’s 550,000 Bpd Oil Deal Is In Jeopardy
Because of these very specific requirements for oil-well cement, it is further classified into nine standard classes of cement by the American Petroleum Institute. The classification is made on the basis of well depth and hydration heat temperature.
Class A: Ordinary cement, corresponding to the ASTM’s Type I, applied at well depths of up to 6,000 feet, or 1,830 meters.
Class B: Also applied at depths of up to 6,000 feet, or 1,830 meters, this class of cement features higher sulphate resistance than Class A, but not as high as classes of cement used in deeper wells. Like Class A, Class B generates hydration heat temperatures of 111-200 degrees Fahrenheit or 44-94 degrees Celsius.
Class C: This class of cement shares the temperature and sulphate resistance properties of Class B, but with the added feature of high early strength for wells when the cement must set faster.
Class D: This is cement used in deeper wells, of between 6,000 and 10,000 feet, or 1,830 to 3,050 meters. This cement features moderate to high sulphate resistance, higher pressure resistance, and also higher hydration heat temperatures, at between 200 and 290 degrees Fahrenheit, or 94 and 144 degrees Celsius.
Class E: This cement is used in still deeper wells, of between 10,000 and 14,000 feet, or 2,050 to 4,270 meters. Like Class D, it has moderate to high resistance to sulphates and the same hydration heat temperature levels.
Class F: This class of cement is used for the deepest wells, at depths of between 10,000 and 16,000 feet, or 3,050 to 4,800 meters. It can withstand the highest pressure and temperatures of all cement classes.
Class G: This class of cement is used with additives, called accelerators and retarders, to either shorten or lengthen setting times depending on the purpose of the cementing job and the type of well that is being cemented. Class G cement is used for well depths of up to 8,000 feet or 2,440 meters.
Class H: Like Class G, this is a basic cement type that could take accelerators and retarders and is used for wells with a depth of up to 8,000 feet or 2,440 meters. The only difference is that it is coarser than Class G cement. Both these cement classes feature hydration heat temperatures of between 111 and 230 degrees Fahrenheit, or 44-111 degrees Celsius.
Class J: This cement, like Class F, can withstand the greatest pressures and temperatures underground but can also be mixed with accelerators or retarders to manipulate setting times.
Accelerators: These are chemicals that are mixed into the cement slurry in order to hasten setting times. This is necessary when cementing is not the final stage of the drilling process, but a point along it, typically in directional—horizontal—wells. Accelerators are used closer to the surface where temperatures are not very high.
Retarders: These have the opposite effect of accelerators and are used at greater depths. Slower setting times are better suited for the higher temperatures characteristic of greater depths and ensure the cement is distributed well before it sets, meaning that all of the cement can be pumped into the well and isolated before setting begins.
Extenders: Extenders make cement slurry less dense, which means that there are fewer molecules of cement per unit of water. On the one hand, this saves money as there is a greater volume of cement at the same cost. On the other hand, this makes it easier to seal off weak zones in the rock surrounding the well. Weak zones are areas in the formation where the rock is “looser”, which is a potential risk for the well.
Dispersants: These chemicals make cement less viscous. This means that there is lower friction between the cement molecules and water. This lower friction makes the cement flow more easily. Cement that contains a dispersant is better at displacing drilling mud from the well so it seals it off from its surroundings better. It also allows for a higher density of the cement slurry.
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 cementing, 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 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.
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. Related: US Energy Secretary: The Shale Boom Is Far From Over
The guide shoe, which is a short section of casing pipe, is then attached to the bottom end of the casing pipe. The guide shoe has a valve in it to make sure the cement only flows from the casing to the space between the pipes and the rock—called the annulus—but not back into the casing.
The floating collar is another valve sitting on top of the guide shoe that further ensures there will be no mixing between the cement and residual drilling mud. The guide shoe goes into the well first. Then the bottom plug is inserted into the casing and is caught by the floating collar. After that, the cement head is attached to the top of the last joint of the casing, at the top of the wellbore, and pumping the cement into the well begins.
Initially, pressure is moderate, but it is increased so that the slurry can break the bottom plug’s diaphragm and the cement can begin flowing through the guide shoe into the well and up the sides of the casing, filling the annulus.
After all the cement is pumped into the well, a second plug is inserted into the casing to push whatever cement remains inside the pipe down the wellbore and then up the sides. This is the end of primary cementing and the cement can begin to set.
In case a problem arises from the primary cementing stage after production begins, secondary or remedial cementing is carried out to fix the mistakes made during the primary cementing.
By Irina Slav for Oilprice.com
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