Cement Additives

Generally, additives are used to alter setting time, change slurry density, lower the water-loss characteristic, improve flow properties, or improve the strength of the bond with the pipe.

10.4.2.8.1 Retarders

In high-temperature formations, retarders are used to prolong the setting time so that the cement can be pumped into place. Commercial retarders include chemicals similar to mud thinners such as lignosulfonates.

10.4.2.8.2 Accelerators

For low-temperature formations, an accelerator is used to speed up the setting time. It reduces the waiting-on-cement (WOC) time. Some of the most frequently used accelerators are calcium chloride, sodium chloride, (table salt), and certain forms of gypsum. They cause the cement to absorb or react with water more quickly causing the cement to set and develop strength faster.

Another way to make cement set faster is to use less water when mixing the slurry. But reducing the water makes the slurry thicker and harder to pump into place. So if less water is used, a substance called a dispersant may be used. The dispersant chemically wets the cement particles in the slurry so that they slip past each other and easily flow into place.

10.4.2.8.3 Dispersants

Fluid loss is the water lost from the slurry to the formation during slurry placement operations causing a subsequent increase in slurry density and changes in slurry characteristics. Filtration, or water-control, additives such as dispersants and organic cellulose are used to prevent excessive fluid loss. If the volume of water lost is large, the slurry becomes too viscous to pump and a filter cake is formed. This filter cake can bridge the space between the casing and the formation and prevent the rest of the cement from circulating to its intended placement. Water filtering into the formation may also cause problems, particularly in the producing zone preventing oil flow.

Dispersants may function as different types of additives, since they allow the slurry to flow easily without much water. They may be used instead of accelerators, heavyweight additives, and filtration-control additives.

10.4.2.8.4 Heavyweight Additives

In wells where heavy drilling muds were used, the cement slurry that will be used has to be at least as heavy as the drilling mud. Weighting agents, like barite, sand, and hematite, are added to increase the slurry weight.

Dispersants reduce the amount of water needed to mix the slurry. They maybe used to produce a heavy cement slurry. The maximum cement slurry density that may be obtained using dispersants is about 17.5 ppg. By carefully selecting the dispersant ratio and weighting materials, slurry weights up to 22 ppg may be mixed and still be thin enough to be pumped downhole.

10.4.2.8.5 Lightweight Additives

In low-pressure zones, the high hydrostatic pressure of long columns of neat cement may fracture the formation and result in lost circulation. Lightweight additives are added to reduce the weight of the slurry.

One way to reduce the weight of the slurry is to add more water to it, because water is lighter than cement. However, adding too much water to the slurry will permanently reduce the strength of the cement. Extra amounts of water may also settle out of the slurry and form undesirable channels or water pockets in the set cement. To prevent the water from settling out, bentonite may be added to the slurry. Bentonite acts chemically to hold the water and to keep it from settling out. Bentonite also produces increased volume. This combination of bentonite, cement, and extra amounts of water produces lighter, more versatile slurry.

10.4.2.8.6 Extenders

The volume of slurry per sack of dry cement that can be mixed is called the yield of the cement. Any additive that will aid in getting a greater yield from a sack of cement is called an extender. Once an extender is added mixing water will be required in order to mix neat cement. Since water is lighter in weight than cement and is less

10.4.2.8.7 Bridging Materials

Zones of lost circulation are sometimes encountered when drilling a well. Bridging agents to control excessive cement loss into the zone are required. The most common materials used for controlling this loss of cement are kolite, gilsonite, and clinton flake. Kolite and gilsonite are coarsely ground hydrocarbon materials, while clinton flake is finely shredded cellophane.

10.4.2.8.8 Other additives

Other special additives are used to solve various problems encountered in cementing a well. Examples of these are:

Fine sand helps to control the strength loss encountered in set cement at temperatures above 250°F.

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Antifoam agents are used to control the foaming tendencies of some cements.

Silica flour helps stabilize the cement so that it remains strong in high- temperature formations.

Sodium chloride, or salt, is a very versatile additive. In small amounts, it accelerates the setting time of cement. In large amounts, it retards the setting time. When used with other additives, such as bentonite, it works as a mild dispersant. When cementing through salt zones, saturated salt water should be used to mix the slurry.