L15-Cementing Lecture 1

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Transcript of L15-Cementing Lecture 1

Cementing: Lecture 1 Fundamentals of Cementing Operations

Functions of Cement in Wells Cementing is the process by which cement slurry is placed in the annulus, bonding the casing to the formation The conventional method of doing this is to pump cement down the casing and displace it around the casing shoe into the annulus A good cement job is essential to allow further drilling and production operations to proceed

Cementing Operation

Components of Cementing OperationFloat shoe A float shoe prevents cement from flowing back into the casing once the cement is displaced behind the casing. Shoes have either inner parts made of aluminum or cement; both being easily drillable, with the advantage that cement is more resistant to impact. Float Collar A float collar is a one way valve placed at one or two joints above the shoe. The float collar provides the same functions as a float shoe by preventing fluid back flow into the casing: mud backflow during running in hole and cement slurry backflow after cement displacement. The distance between the shoe and float collar is called Shoe Track. Wiper Plugs Both top and bottom plugs are used during cementing operations. They are used to separate the various fluids from one another.

Bottom Plug The red bottom plug has a shallow top, is made of rubber, and has a hollow core. It is used ahead of the cement slurry to prevent cement/drilling fluid contamination and to clean the casing wall of filter cake. After the bottom plug comes into contact with the float valve, sufficient pressure (150 to 350 psi) causes the top diaphragm to rupture, allowing the cement slurry to flow through it. Top Plug The black top plug has a deep cup on its top and has a solid, molded rubber core. It is dropped after the cement slurry has been pumped, to prevent contamination with the displacement fluid. The top plug also signals the end of displacement by forming a seal on top of the bottom plug, causing a pressure increase. The main functions of cement plugs are: Separate mud from cement Wipe the casing from mud before cement is pumped and then wipe casing from th cement film after the complete volume of cement is pumped. Prevent over-displacement of cement Give surface indication that cement placement is complete Allow the casing to be pressure tested

Bumping the PlugThe bottom plug is first released and is followed by cement. When the bottom plug lands on the float collar a pressure increase on surface is indicated. A small increase in pressure will rupture the bottom plug and allow cement to flow through it, through float collar, shoe track, casing shoe and then around the casing. The top plug is released from surface immediately after the total volume of cement is pumped. The top plug is displaced by the drilling fluid and it, in turn, pushes the cement slurry into the annulus. When the top plug lands on the bottom plug a pressure increase is observed at surface. This is called bumping the plug. Bumping indicates that the total volume of cement is now displaced behind the casing. Usually, at this time, the casing is pressure tested to a precalculated design value to check its integrity. Pressure testing casing while the cement is still wet is recommended as this reduces the chances of breaking the set cement or creating microchannels if the test is carried out a few hours later when the cement sets.

Washes & SpacersPre- Flush In any successful primary cementing operation the cement slurry must displace the fluid surrounding the casing. Mud and cement are often incompatible and contact between them can lead to severe channelling or the formation of an un-pumpable viscous mass. To avoid this problem an intermediate fluid is used as a pre-flush to clean the drilling mud from the annulus. The simplest form of pre-flush is a 'wash' - usually water, with the possible addition of a surfactant. Such a pre-flush is very effective in removing mud from the annulus as turbulence can be achieved around the complete annulus. Spacers Spacers are difficult fluids to design. They must be compatible with both mud and cement and have the correct rheological properties to minimise mixing and channelling. Although weighted spacers can theoretically achieve turbulence, care must be exercised in assuming that turbulence will occur beneath eccentric casing, in the narrow section of the annulus. If laminar flow exists beneath the casing and turbulent flow above, channelling will result.

The most important functions of the initial or primary cement job are: To support the casing string; To prevent the movement of fluids from one formation to another through the annulus; To protect the casing from corrosive fluids in the formations The cement slurry is able to meet these requirements by providing adequate compressive strength and low permeability when the cement hardens. The critical factor in obtaining a satisfactory cement job is to place the cement completely around the casing to prevent channelling

A secondary or squeeze cement job ..may have to be done at a later stage to carry out some remedial work on the well (eg, sealing off certain zones, repairing casing leaks). This involves forcing cement through holes or perforations in the casing into the annulus and formation. Like this ..

Planning the cement jobEach cement job must be carefully planned to ensure that the correct cement and additives are being used, and that a suitable placement technique is being employed for that particular application: The cement can be placed correctly using the equipment available; The cement will achieve adequate compressive strength soon after it is placed; The cement will thereafter isolate zones and support the casing throughout the life of the well

Classification of CementThere are several classes of cement API Class C 3S C 2S C 3A C4AF CaSO Fineness approved by the % API. % The differences Sq cm/Gram % % % between the cements lie in the distribution A 53 24 8 8 3.5 1600-1900 of the five basic compounds, which are 1500-1900 B 44 32 5 12 2.9 used to make cement: 8C3S, 8 C2S, 4.1 C3A, 2000-2400 C 53 16 C4AF,E CaSO4. 26 D& 50 5 13 3.0 1200-1500G H 52 52 27 25 3 5 12 12 3.2 3.3 1400-1600 1400-1600Compounds (a)

Classes A and B: These cements are generally cheaper than other classes of cement and can only be used at shallow depths where there are no special requirements. Note: Class B has a higher resistance to sulphate than Class A Class C: This cement has a high C3S content and so produces a high early strength Classes D, E and F: These are known as retarded cements due to a coarser grind, or the inclusion of organic retarders (lignosulphonates). Their increased cost must be justified by their ability to work satisfactorily in deep wells at higher temperatures and pressures Class G and H: These are general purpose cements which are compatible with most additives and can be used over a wide range of temperature and pressure. Class G is the most common type of cement used in most areas. Class H has a coarser grind than Class G and gives better retarding properties in deeper wells

Other types of cement not covered by the API specification include: Pozmix cement - formed by mixing Portland cement with pozzolan (ground volcanic ash) and 2% bentonite. Very durable & less expensive than most other types; Gypsum cement - formed by mixing Portland cement with gypsum, giving a high early strength and can be used for remedial work. They expand on setting and deteriorate in the presence of water; Diesel oil cement - a mixture of one of the basic cement classes (A, B, G, H) with diesel oil or kerosene with a surfactant. They have unlimited setting times and will only set in the presence of water. Consequently they are often used to seal off water producing zones where they absorb and set to form a dense, hard cement

Mixwater RequirementsAPI Cement Classification

Following tabulated figures are based on: API Class Slurry Well Depth The need to have a Mixing that Wt. easily pumped; slurry is Water (a) A minimum amount of free water Gals/Sk Lbs/Gal FtA (Portland) 5.2 15.6 0-6000

Static Temp deg F 80-170

Effects of reducing the amount of mixwater: B (Portland) 5.2 15.6 0-6000 80-170 Slurry(High Early) compressive strength, and viscosity will all C density, 6.3 14.8 0-6000 80-170 increase; D (Retarded) 4.3 16.4 6-10000 170-230 Pumpability will decrease; E (Retarded) 4.3 16.4 6-14000 170-290 Less F (Retarded) of slurry will be obtained from each sack of volume 4.3 16.4 10-16000 230-320 cementG (Basic Calif) H (Basic Gulf Coast) 5.0 4.3 15.8 16.4 0-8000 0-8000 80-200 80-200

Properties Compressive StrengthTo support the casing string a compressive strength of 500 psi is generally thought to be adequate (includes a generous safety factor). The casing shoe should not be drilled out until this strength has been attained - referred to as waiting on cement (or WOC) Development of compressive strength is a function of several variables: temperature pressure amount of mixwater elapsed time since mixing With proper accelerators added - the WOC time may be reduced to 3-6 hours. Following table shows some typical compressive strengths for different cements under varying conditions

Com pressive Strength Tem perature deg F Pressure (psi) Class A & B Portland High early streng th class C 60 80 95 110 140 170 200 0 0 800 1,600 3,000 3,000 3,000 615 1,470 2,085 2,925 5,050 5,920 780 1,870 2,015 2,705 3,560 3,710 440 1,185 2,540 2,915 4,200 4,830 5,110 325 1,065 2,110 2,525 3,160 4,485 4,575 3,045 4,150 4,775 API clas sG API clas sH Retarded class D,E,F Typical compressive strength (psi) at 24 hours

Compressive Strength Temperature deg F 60 80 95 110 140 170 200 Pressure (psi) 0 0 800 1,600 3,000 3,000 3,000 2,870 4,130 4,670 5,840 6,550 6,210 Typical compressive strength