3. API Cements and Additives

Section 3

API Cements and Additives Table of Contents Introduction................................................................................................................................................3-3 Topic Areas ............................................................................................................................................3-3 Learning Objectives ...............................................................................................................................3-3 Unit A: API Cements .................................................................................................................................3-3 API Classification ..................................................................................................................................3-3 Manufacturing and Composition............................................................................................................3-4 Unit A Quiz ............................................................................................................................................3-5 Unit B: Cement Properties .........................................................................................................................3-6 Factors Influencing Slurry Properties.....................................................................................................3-6 Thickening Times...................................................................................................................................3-7 Compressive Strength ............................................................................................................................3-7 Mix Water ..............................................................................................................................................3-8 Unit B Quiz ............................................................................................................................................3-9 Unit C: Cement Additives........................................................................................................................3-10 Introduction ..........................................................................................................................................3-10 Cement Accelerators ............................................................................................................................3-10 Lightweight Additives..........................................................................................................................3-10 Heavyweight Additives ........................................................................................................................3-10 Cement Retarders .................................................................................................................................3-11 Lost Circulation Additives ...................................................................................................................3-11 Fluid Loss Additives ............................................................................................................................3-11 Cement Dispersants or Friction Reducers ............................................................................................3-12 Gas Control Additives ..........................................................................................................................3-12 Salt as an Additive (Salt Cement) ........................................................................................................3-13 Unit C Quiz ..........................................................................................................................................3-14 Answers to Unit Quizzes .........................................................................................................................3-15

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Use for Section Notes…

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Introduction Cements are used universally in well completion operations to fill the annular space between the casing and the open hole. The principal functions of the primary cementing process are

B. Cement Properties

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to bond and support the casing

Learning Objectives

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to restrict fluid movement between formations

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to protect the casing from corrosion

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to protect the casing from shock loads when drilling deeper

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to seal off lost circulation (or thief) zones.

C. Cement Additives

Upon completion of the section, you should be familiar with: •

the API classification system and the ratings of the various cements based upon physical makeup

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the basic physical properties of commonly used Halliburton cements

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the functions of commonly used additives

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the basic steps in the cement manufacturing process.

The American Petroleum Institute (API) has identified nine types of cement according to chemical composition and physical properties. These types range from standard construction cements to cements designed for use thousands of feet below the surface.

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API Class A and B (Portland cement)

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API Class C (High early strength cement)

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API Class D, E and F (Retarded cement)

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API Class G and H (Basic cement)

This unit will cover

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API Class J (Special order only).

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API Classification

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Manufacturing and Composition

API Classes G and H cements are commonly used across a large geographical area. Classes A, B, and C are used in specific geographic locations where downhole conditions require special cement properties. Classes D, E and F are rarely used and only in special situations.

Topic Areas In this section, the following units are covered: A. API Cements

Unit A: API Cements

API Classification The nine types of cements classified by the API are

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Table 3.1 - API Classification And Properties Of Common Oil Well Cements Type

Range of Usage

Static Temp

Water Ratio

Slurry Weight

Volume

°F

gal/sk

lb/gal

ft3/sk

Remarks

Class A (Portland)

6000 ft

60°-170°

5.2

15.6

1.18

May be used when no special properties are desired and well conditions permit. No sulfate resistance.

Class B (Portland)

6000 ft

60°-170°

5.2

15.6

1.18

Moderate sulfate resistance.

Class C

6000 ft

60°-170°

6.3

14.8

1.32

Available in regular and high sulfate-resistant types.

Class G

8000 ft

200°

5.0

15.8

1.15

Basic cement, compatible with accelerators or retarders for use over the complete range of Classes A through E.

Class H

8000 ft

200°

4.3

16.4

1.06

200°

5.2

15.6

1.18

Basic cement, higher density, higher and lower water volume.

8000 ft

creating a hydraulic cement--one that hardens with the addition of water. Aspdin named the product portland cement because it resembled a stone quarried on the Isle of Portland off the British Coast. With this invention, Aspdin laid the foundation for today's portland cement industry.

You may also hear the terms Standard, Premium and Premium Plus when referring to oil field cements. •

Standard cement has characteristics similar to API Classes A&B. However; Standard Cement may not meet API specifications for Class A or B.

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Premium Cement has characteristics similar to API Classes G&H. However; Premium Cement may not meet API specifications for Class G or H.

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Premium Plus Cement has characteristics similar to API Class C. However; Premium Plus Cement may not meet API specifications for Class C.

Cements consist of limestone (or other materials high in calcium carbonate), clay or shale, and some iron and aluminum oxides (if not found in quantity in the clay or shale). These dry materials are finely ground and mixed thoroughly in the correct proportions, either dry (dry process), or with water (wet process). The mixture is heated to very high temperatures causing the ingredients to react chemically, resulting in material called clinker. The clinker is ground with a controlled amount of gypsum to form cement.

Other materials having somewhat different cement properties are also commonly used in the primary cementing process. These materials do not fall in any specific API classification and are classified as “Special Cements.”

All cement classes are manufactured in essentially the same way and made of the same ingredients, only in different proportions. The water requirement of each cement varies with the fineness of grind or amount of surface area. High early strength cements (Class C) have a high surface area (fine grind); the retarded cements (Classes D, E, F) have less surface area, and the Portland cements (Classes A & B) have a surface area slightly higher than the retarded cements. Class G is a premium fine grind and class H is a premium coarse grind.

Manufacturing and Composition In 1824, Joseph Aspdin, a British stone mason, obtained a patent for a cement he produced in his kitchen. The inventor heated a mixture of finely ground limestone and clay in his kitchen stove and ground the mixture into a powder

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Unit A Quiz Fill in the blanks with one or more words to check you progress in Unit A. 1. The American Petroleum Institute has identified nine classes of basic cements. Five of those nine classes are commonly used in the oil field. They are _____________, _____________, _____________, _____________ and _____________. 2. The API cements most commonly used across a wide geographical area are classes _____ and _____. 3. Although both Class G and Class H cements have similar properties, Class H has a ___________ density. 4. Special cements are similar to basic cements in that they are commonly used in the _______________________ process. 5. Cement consists of ______________ that is ground with a controlled amount of _____________. 6. Although all classes of cements are manufactured in basically the same way, they differ in that different ______________ of ingredients are used. 7. The water requirements for each type of cement vary based upon _________________________.

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Unit B: Cement Properties The properties of cements used in the oil field vary based upon the following factors: •

Geographic location

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Conditions down-hole (temperature, depth, etc.)

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Type of cement job

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Type of mixing water.

Factors influencing slurry properties

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Thickening time

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Compressive strength.

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Flow properties

Too much water: Free water appears on top of the slurry in the sample cup and retards setting.

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Too little water: The slurry is thick and difficult to pump and accelerates setting.

Slurry density or cement slurry weight should, except for squeeze jobs, be great enough to maintain well control. As a result, slurry densities are always carefully monitored. In the field, this can be done either manually with a pressure balanced scale (Fig. 3.1) or automatically with a densometer. To avoid a poor primary cementing job, a slurry must be maintained at its proper density.

Cement properties can be changed to meet the needs of a particular job. This unit addresses the physical properties of oilwell cements and how these properties affect or are affected by conditions downhole. This unit includes •

•

Factors Influencing Slurry Properties The properties of cement slurry are influenced by factors such as: •

water ratio of cement slurries (gal/sk)

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slurry density (lb/gal)

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slurry yield (ft³/sk)

Figure 3.1 – Pressure Balanced Scale While on the job, be alert to slurry property changes that can be the result of improper slurry density. The slurry properties most affected by changes in density are

The water ratio or gallons of water required per sack of cement is important in determining the thickening time and compressive strength of the cement. Most slurries are mixed with an amount of water that provides a set volume equal to the slurry volume without freewater separation. Your Halliburton Cementing Tables (Red Book) contains a section entitled “Technical Data” in which you can find water requirements for various types of cements. The following rule of thumb can help:

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thickening time

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flow characteristics (pumpability)

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drilling fluid displacement efficiency

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free water

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settling

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compressive strength

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fluid loss.

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The “Technical Data” section of your Red Book contains temperature/thickening time information. This data represents average temperatures at various depths along the Gulf Coast and may not correspond to temperatures at the same depths in other areas.

The careful recording and monitoring of slurry density ensures a correct water to solids ratio is maintained. Slurry yield is the number of ft³/sk an API standard manufactured cement yields based upon a given amount of mixing water. In determining the volume of cement required to do a particular cement job, a caliper survey, volume requirements based on Red Book data, and/or regulatory requirements are used. Too much cement rather than too little is always advisable, especially where there is a possibility of mud contamination, dilution, or channeling.

Always remember temperature, rather than depth, has the greatest effect on cement thickening times. Thickening time is also affected by conditions that cannot always be controlled during laboratory tests, such as •

water invasion - causes failure to set

Thickening Times

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loss of water to the formation - causes accelerated set

The thickening time, based upon laboratory testing, is the time required for a cement to become unpumpable. Thickening times are established in response to

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shutdown during cement slurry placement - speeds up thickening and slurry set faster than reported by laboratory conditions

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Laboratory test results

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contamination - causes setting failure.

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Actual well conditions

These factors affect the thickening time of cement to various degrees. It is important to remember that the slurry thickening time changes if one of these situations is encountered.

Laboratory thickening time tests (Fig. 3.2) are conducted using a sample of cement (with additives, if used) and mixing water. The cement slurry is tested to determine the amount of time for thickening to take place. The following well conditions are controlled during these tests: •

bottomhole circulating temperature (BHT)

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well depth

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well pressure.

Finally, when considering thickening times, remember that moving cement is less likely to setup; therefore, once you stop moving the cement slurry, the cement starts to build gel strength. A basic rule should always be followed: never stop moving the cement until it is in place.

Compressive Strength Compressive strength is the amount of strength required to support a string of casing. Compressive strength provides the basis for most WOC (waiting-on-cement) regulations. It is a generally accepted rule that a compressive strength of 500 psi is the minimally acceptable standard for most cement operations. However, consult the regulatory guidelines to determine the minimum strength requirements for the state in which you work.

Figure 3.2 – Pressure-temperature thickening time tester.

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Deciding how long to wait for your cement to build up enough compressive strength for drilling out is a function of cement type, additives, and wellbore temperature.

Mix Water Cement slurry contamination is a major concern during the cementing process. It would be ideal if the water supply for mixing cement was completely free from contaminants. This is not always practical, you must consider the most readily available source of water. Additives in the drilling fluid can also contaminate cement slurry and affect its setting properties. Table 3.2 lists some common sources of contaminants and the effect each has on cement slurry properties. Halliburton has definitive guidelines for allowable levels of contaminants in cement mix water.

Other factors that affect cement curing or WOC times (field variables, completion procedures, and curing conditions) do not allow a foolproof WOC time to be set. Thus, the rule of thumb is to achieve a minimum compressive strength of 500 psi before drilling out.

Table 3.2 - Mixing Water or Mud Additive Contaminants Source of Contaminant Mixing Water

Figure 3.3 – Machine Used to Test Compressive Strength

Mud Additives

Figure 3.4 – Testing Compressive Strength of Sample

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Type

Effect on Cement Slurry

Salt (1% to 8% per weight of water)

Accelerates set

Organic Material (decomposed plant life, waste effluents)

Retards set

Agricultural Products (farm fertilizer

Accelerates set

Sea Water

Accelerates set

Barium sulfate (BaSo4) (Barite)

Increases density, reduces strength

Caustics (NaOH, Na2COH3,etc)

Accelerates set

Calcium compounds (CaO, Ca(OH)2, CaCl2, CaSo, 2H20)

Accelerates set

Thinners (tannins, lignosulfonates, quebracho, lignins, etc.)

Retards set

Fluid-loss control additives (CMC, starch, guar, polyachrylamides, lignosulfonates)

Retards set

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Unit B Quiz Fill in the blanks with one or more words to check your progress in Unit B. 1. An important factor in determining the thickening time and compressive strength of the cement is the ______________________ or gallons of water required per sack of cement. 2. In order to determine the water requirements for the cement you are using, refer to your _____________. 3. Difficulty in pumping cement slurry downhole can be the result of ________________________. 4. The required cement thickening time is usually determined under _________________________ conditions. 5. Contaminants which can affect the cement slurry usually come from two sources: mixing water and ______________________. 6. An important thing to remember when dealing with thickening times in the field is that the cement starts to build __________________ once you’ve stopped _____________. 7. Generally, a compressive strength of 500 psi is a _____________________________ standard for most cement operations. 8. The wellbore temperature and the cement type, density, and additives are factors to consider when determining ____________________________________time.

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Unit C: Cement Additives information on cement accelerators can be found in Halliburton’s Cementing Technology Manual.

Introduction

The common accelerators are

Wells are cemented in a variety of temperature conditions:

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Calcium Chloride (most widely used)

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Sodium Chloride

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Seawater

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below freezing (permafrost zones in Alaska and Canada)

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450° to 500°F (deep gas wells)

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Cal-Seal

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500° to 1,500°F (geothermal steam wells)

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ECONOLITE

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1,500° to 2,000°F (fire flood wells)

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VersaSet

By using basic cements (API Classes G or H) and additives, you can tailor cement slurries to fit any specific well requirement. Most additives are available as liquids or free-flowing powders. Liquid additives are added directly to the mixing water. Powders are usually dry-blended with cement before they are transported to the well. When necessary, they can also be dispersed in the mixing water at the job site.

Lightweight Additives Sometimes, a slurry weight needs to be reduced to protect formations that have a low fracture gradient or for economics. To reduce the weight of cement slurries, you can add water, low specific-gravity solids, or foam cement.

•

accelerators

Bentonite is one of the most commonly used lightweight additives. You can use it to formulate these different lightweight cements:

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lightweight additives

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Blended gel cement (1 to 16%)

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heavyweight additives

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Premixed bentonite (prehydrated)

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retarders

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Modified cement (Humble patent)

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lost circulation additives

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High gel salt cement (Gulf patent)

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fluid loss additives

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dispersants

Foam cement using nitrogen (N2) produces 4- to 19-lb/gal slurries, which have excellent strength to density ratio (low permeability).

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gas control additives

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specialty materials

There are nine categories of cement additives:

Heavyweight Additives The main purpose of heavy weight additives is to restrain high formation pressures. Heavy weight additives are used to obtain slurry densities up to 20 lb/gal. They have a low water requirement and have a uniform particle size range from batch to batch. Heavyweight

Cement Accelerators Cement accelerators shorten thickening time and reduce WOC. They also increase early strength. Accelerators are widely used on surface pipe, shallow wells, and cement plugs. Additional

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additives are chemically inert and compatible with other additives. The most common materials used for weighting cements are •

Hematite (iron ore)

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Barite (more common in drilling fluids)

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Ottawa sand

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Salt

Cement Retarders Cement retarders slow cement setting time (hydration) to allow for safe cement placement. The high temperatures in deep wells will cause cement to set prematurely. The higher the temperature, the faster the cement sets. Retarders usually have a limited effective temperature range. For example, a particular retarder may be only effective from 115 to 225°F. The chemicals widely in use as retarders include those listed below: Lignins (sodium lignosulfonate, calcium lignosulfonate)

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Carboxymethyl hydroxyethyl cellulose (CMHEC)

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Saturated salt water

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Organic acids

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Synthetic polymers

Low fracture gradient

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Unconsolidate formations

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Fractured formations

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Cavernous formations

Actually, lost circulation control during cementing involves adding material that will prevent losses to the cement slurry while you are placing it. Some of the more commonly used lost circulation materials include

Of the materials listed, hematite is most widely used, because it most closely fits physical requirements and achieves the highest effective specific gravity.

•

•

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Granular (gilsonite, coal, perlite, walnut hulls, mica)

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Fibrous (nylon, polypropylene)

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Laminated (cellophane)

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Specialized flash setting or gelling materials

Fluid Loss Additives The purpose of fluid loss additives is to help prevent water loss from cement slurry. This allows displacing the maximum amount of mud, compressive strength development, and bonding between the formation and the casing. Fluid loss additives counteract the tendency of cement slurries to lose the water that the slurries need to help achieve a good cement job. Permeable zones can rob the cement slurry of its water, thus creating a filter cake of cement. The filter cake increases frictional pressures and increases the potential for a number of problems. Fluid loss additives reduce the permeability of the cement filter cake. They are especially useful in squeeze cementing. Fluid loss additives include

Lost Circulation Additives

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Organic polymers

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Dispersants

“Lost circulation” or “lost returns” refers to whole fluid loss or cement slurry loss to formation voids during drilling or completing a well. You should not confuse it with volume decrease because of the filtration or volume needed to fill a new hole. Circulation can be lost due to

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Cement Dispersants or Friction Reducers

Gas Control Additives

Dispersing agents are added to cement slurries to improve their flow properties. Since they can be pumped in turbulent flow at lower pressures, you can reduce the horsepower required. As a result, you can also reduce the chances of lost circulation and premature dehydration. Additives classified as dispersants include the following: •

Polymers

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Salt

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Retarders

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Fluid Loss Additives

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Organic acids

Natural (methane) gas migration through unset cement is a major concern (Fig. 3.8). Cement slurries are capable of transmitting hydrostatic pressure, and maintaining overbalance pressure control, while in the fluid state. However, cements naturally want to develop gel strength (gelation) when left static. The time period of gelation, before cement sets may be several hours. In this state the unset slurry will not fully transmit fluid pressure from wellbore fluids. While in a gel state, and before hardening, the cement will also lose a small amount of filtrate to porous zones resulting in a volume reduction. Since pressure may not be transferred though the gelled cement, a pressure reduction occurs at the point(s) where filtrate has leaked off. Gas from nearby porous zones is then free to percolate through the unset cement to other zones or the surface. The path of gas migration is permanent and exists even after the cement has fully hardened. To combat this problem several additives may be used in the cement slurry. A powdered additive may be used to minimize the length of time the cement is in the gel state. Gas, such as nitrogen, is often used to foam the cement so that volume reduction, and thus pressure reduction, is minimized at the point of filtrate loss.

Figure 3.7 – Dispersants improve the flow properties

Figure 3.8 – Channel through cement caused by gas migration.

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Salt as an Additive (Salt Cement) Salt is also used in cement slurries to improve bonding of set cement to salt sections and shales. It also causes the set cement to expand. Cement slurries containing salt have been very effective in protecting hole washing in salt zones. They also prevent shale sections from sloughing or heaving during cementing. When drilling through salt zones (Fig. 3.9) or troublesome shales (Fig. 3.10) with fresh water mud systems, severe hole enlargements may occur.

Figure 3.10 - Hole washout due to sloughing of water sensitive shales. Cementing with fresh water cement slurries can cause similar hole enlargements. You can prevent annular bridging and the resulting lost circulation by using cement slurries containing salt. Shales that are sensitive to cement filtrate can actually be softened by it before the cement sets. If softened enough, the shale will flow. This creates channels behind the cement sheath from one perforated zone to another. Blending dry granulated salt with cement at the bulk plant makes salt-saturated cement much easier to use. It allows you to eliminate waste, and save time and labor; it can also reduce the possibility of foaming.

Figure 3.9 - Hole washing in a salt zone.

While sodium chloride is the salt generally used with cement, potassium chloride is used also. In some cases, potassium chloride may be effective at lower concentrations; it does not significantly affect cement slurries any differently than sodium chloride, except at higher concentrations.

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Unit C Quiz Fill in the blanks with one or more words to check your progress in Unit C. 1. Most additives are available as _______________ or ________________________________. 2. Cement accelerators shorten __________________________ and reduce WOC. 3. _________________ is one of the most commonly used lightweight additives. 4. The main purpose of heavy weight additives is to restrain _________________________________. 5. The ________________________________ in deep wells will cause cement to set prematurely. 6. “Lost circulation” or “lost returns” refers to ______________________ loss or _______________________loss to formation voids during drilling or completing a well. 7. __________________________________ counteract the tendency of cement slurries to lose the water that the slurries need to help achieve a good cement job. 8. Because cements with dispersers added can be pumped in turbulent flow at lower pressures, you can reduce the __________________ required. 9. The path of gas migration is __________________ and exists even after the cement has fully hardened. 10. _________________________ can help prevent shale sections from sloughing or heaving during cementing.

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Answers to Unit Quizzes Items from Unit A Quiz

Refer to Page

1. A, B, C, G, H

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2. G, H

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3. higher

3-4

4. primary cementing

3-4

5. clinker, gypsum

3-4

6. proportions

3-4

7. fineness of grind

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Items from Unit B Quiz

Refer to Page

1. water ratio

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2. Red Book

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3. too little water

3-6

4. actual well

3-7

5. drilling fluid (or mud additives)

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6. gel strength, pumping

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7. rule of thumb (or generally accepted rule)

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8. WOC

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Items from Unit C Quiz

Refer to Page

1. liquids, free-flowing powders.

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2. thickening time

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3. Bentonite

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4. high formation pressures

3-10

5. high temperatures

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6. whole fluid , cement slurry

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7. Fluid loss additives

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8. horsepower

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9. permanent

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10. Salt cements

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