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Drilling Engineering > Unit 8 - Cement > Flashcards

Unit 8 - Cement Flashcards

1
Q

What is cement?

A

Seal material → between casing and borehole →provides barrier to the flow of fluids from, or into the formation behind the casing and from, and into, the subsequent hole section.

  • also used for repair work on well
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2
Q

What is a primary cement job?

A
  • pump cement down the inside of the casing → through casing shoe into annulus
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3
Q

What are secondary or squeeze cement jobs?

A
  • generally part of remedial work on a well ( e.g. sealing off water producting zones or repairing casing leaks)
  • called squeeze cement jobs, because they involve cement being force through holes or perforations in the casing into the annulus and/or the formation
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4
Q

Cement slurry is usually made up from ?

A
  • cement powder
  • water
  • chemical additives
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5
Q

Name some major cement additives!

A
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6
Q

What are pilot tests?

A
  • tests of cement slurry in the lab before use → help to assess the effect of different amounts of additives on the properties of the cement
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7
Q

What are the most important functions of a cement sheat between casing and the wellbore?

A
  • prevent fluid movement from one formation to another or from the formations to surface through the annulus between the casing and borehole
  • support the casing string (specifically surface casing)
  • to protect the casing from corrosive fluids in the formations.
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8
Q

What is a retarded cement?

A
  • takes much longer time to set hard than the other classes of cement powder (Classes D,E,F)
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9
Q

Which class is the cement most commonly used?

A
  • class G
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10
Q

Retardation is due to what?

A
  • due to a coarser grind
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11
Q

What is a diesel oil cement? And when are they used?

A
  • mix of one of the basic cement classes (A,B,G,H), diesel oil or kerosene and a surfactant → these cements have unlimited setting times and will only set in presence of water

→ often used to seal off water producing zones → they absorb and set to form a dense hard cement

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12
Q

What does the amount of mixwater depend on?

A
  • slurry needs to be easily pumpable
  • all of the cement powder needs to be hydrated so that high quality hardened cement is produced
  • needs to be ensured, that all of the free water is used to hydrate the cement powder and that no free water is present in the hardened cement
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13
Q

What happens if you use too much mixwater?

A
  • cement wil not set into a strong impermeable cement barrier
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14
Q

What happens if you don’t use enough mixwater for the cement?

A
  • slurry density and viscosity will increase
  • pumpability will decrease
  • less volume of slurry will be obtained from each sack of cement

→ sometimes the amount of mixwater used will be changed to meet specific temperature and pressure conditions which will be experienced during the cement job

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15
Q

What are the fundamental properties, that must be considered when designing any cement slurry?

A
  • compressive strength
  • thickening time (pumpability)
  • slurry density
  • water loss
  • corrosion resistance
  • permeability
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16
Q

What do you call “Waiting on cement” (WOC)?

A
  • if drilling operation is delayed, because you have to wait on the cement setting
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17
Q

the development of compressive strength is a function of ?

A
  • temperature
  • pressure
  • amount of mixwater
  • elapsed time since mixing
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18
Q

What is the thickening time of a cement slurry?

A

time, during which the cement slurry can be pumped and displaced into the annulus ( meaning: slurry is pumpable during this time)

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19
Q

How can you change the slurry density?

A
  • by changing amount of mixwater or using additives
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20
Q

Why do you need a low water loss when you want to do a squeeze job?

A
  • because the cement must be squeezed, before the filter cake builds up and blocks the perforations
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21
Q

For what do you use additives in cement?

A
  • vary slurry density
  • change compressive strength
  • accelerate or retard the setting time
  • control filtration and fluid loss
  • reduce slurry viscosity
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22
Q

What are accelerators?

A
  • added to shorten the time taken for cement to set
  • especially important in shallow wells (low.temp) → because slurry may take a long time to set
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23
Q

What are retarders?

A
  • in deep wells higher temp. will reduce cement slurry’s thickening time → retarders are used to prolong this thickening time to avoid the risk of cement setting prematurely in casing
24
Q

What are lightweight additives (Extenders)?

A
  • used to reduce slurry density for jobs where hydrostatic head of cement slurry might exceed the fracture strength of certain formations
25
Q

What do lightweight additives allow?

A
  • to add more mixwater → increases the amount of slurry which is produced by each sack of cement powder
26
Q

what is the most common lightweight additive?

A
  • bentonite → absorbs water and therefore allows more mixwater to be added but will also however reduce the compressive strength and sulphate resistance
27
Q

When do you need heavyweight additives? And what are the most common ones?

A
  • when you cement through an overpressured zone
  • most common ones :

Barite - can be used to attain slurry densities up to 18ppg but causes a reduction in strength and pumpability

Hematite

  • has a high specific gravity can raise slurry densities up to 22 ppg

but reduces pumpability and therefore friction reducing additives may be required

28
Q

What are friction reducing additives (dispersants)?

A
  • added to improve flow properties of slurry → they lower the viscosity of the slurry so that turbulance will occur at lower circulating pressure → thereby reducing risk of breaking down formations
29
Q

What is the effect of diesel oil as an additive in drilling mud?

A
  • it decreases the density
30
Q

What happens if drilling mud and cement start to mix and why is that a problem?

A
  • sharp increase in viscosity

→ if you have a highly viscous fluid in the annulus then it starts to form channels which are not easily displaced → these channels prevent a good cement bond all around the casing

31
Q

How do we prevent mud contamination of the cement?

A
  • we pump a spacer fluid ahead of the cement slurry
32
Q

Why do you sometimes have to do a cement job in 2 stages?

A
  • because sometimes you have weak formations which may not be able to support the hydrostatic pressure generated by a very long column of cement slurry
33
Q

How do you cement the second stage of a 2 stage cement job?

A
  • you include a multi-stage cementing tool into the casing string at the point at which the bottom of the second stage is required
  • when the second stage slurry is ready to be pumped, the multistage tool is opened and second stage slurry is pumped down the casing throught the stage cementing tool into the annulus
34
Q

Why do you cement the whole annulus of the conductor and surface casing?

A
  • to prevent it from buckling under very high axial loads produced by weight of the wellhead and BOP
35
Q

Where is a float collar positioned?

A
  • 1 or 2 joints above the guide shoe→ it acts as a seat for the cement plugs used in the pumping and displacement of the cement slurry
36
Q

The correct usage of centralisers will help to do what?

A
  • improve displacement efficiency ( means to place the cement all the way around the casing)
  • prevent differential sticking
  • keep casing out of keyseats
37
Q

Why are centralisers particularly necessary in deviated holes?

A
  • Casing tends to lie on the low side of hole

→ on the high side is less resistance to the flow and the cement will therefore tend to flow up the high side of the annular space

→ mud channels will start to form on low side of hole preventing a good cementing job

38
Q

How do you prevent a centraliser from moving up and down ?

A
  • by positioning the centraliser across a casing coupling or a stop collar
39
Q

What can you use a cement head for?

A
  • release bottom plug
  • mix and pump down cement slurry
  • release top plug
  • displace cement without making or breaking the connection to the top of the casing
40
Q

Why do you want a turbulent flow in the spacer fluid/

A
  • because it will result in a more efficient displacement of the mud
41
Q

What is the task of a wiper plug?

A
  • to wipe the inside of the casing clean

-

42
Q

How can i know if the formation did break down during the cementing process?

A
  • because the mud returns slow down or they stop during the displacement operation
43
Q

Reasons for multistage operation are to reduce:?

A
  • long pumping times
  • high pump pressures
  • excessive hydrostatic pressure on weak formations due to relatively high density of cement slurries
44
Q

Why do you need to replace the conventional shut- off plug by a plug with flexible blades during a 2 stage cementing job?

A
  • because it has to pass through the stage cementing collar
45
Q

Why do you need to drop a special dart after the first stage is completed in a 2 stage cement job?

A
  • because the dart lands in the inner sleeve of the stage collar→ then pressure is applied to casing above the dart and therefore to dart → retaining pins on inner sleeve are sheared and sleeve moves down ⇒ ports in the outer mandrel are now uncovered and circulation is established thourgh stage collar before second stage slurry is pumped
46
Q

How do you close the ports in the stage collar ?

A

by pressuring up the closing plug

47
Q

What is a disadvantage of stage cementing?

A
  • casing can not be moved after the first stage cement has set in the lower part of the annulus → this increases the risk of channeling and a poor cement bond
48
Q

What is a stinger cement job?

A
  • you cement the casing through a tubing or drillpipe string, which is known as a cement stinger, rather than through the casing
49
Q

Why do you not use plugs in a stinger cement operation?

A

Because the diameter of the stinger is usually so small that contamination of the cement is unlikely if a large enough liquid spacer is used

50
Q

what is the main disadvantage of a stinger cement job?

A
  • for long casing strings → rig time is lost in running and retrieving the inner string
51
Q

what is the main cause for poor isolation during cementing?

A
  • mud channels in the cement, which exist because the mud in annulus has not been displaced

Reasons:

  • poor centralisation of casing during cementing operation→ because cement will follow least path of resistance to the flow→ so the highest resistance to flow occurs where the clearance is least → this is where mud channels are most likely to occur

-

52
Q

How can you improve the drilling mud displacement?

A
  • use centralisers → especially at critical points in casing string
  • move casing string druing the cement job → rotation is preferred to reciprocation because this could cause surging against the formation
  • condition the mud ( low yield point, low plastic viscosity)
  • try to displace the spacer in a turbulent flow → this may not be practicable in a large diameter casing ( high pump rates/pressures may cause erosion or a formation break down)
  • use spacers to prevent mud contamination in the annulus
53
Q

How do you perform a negative pressure test ( inflow test) ?

A
  • by reducing the hydrostatic pressure inside the casing → by using a DST tool or displacing the well with diesel
54
Q

What can you do to evaluate a cement job?

A
  • Detect the top of cement (TOC)
  • Measure the quality of the cement bond
55
Q

What are methods to detect the Top of the cement (TOC)?

A
  • temperature surveys → you run a thermometer inside the casing just after the cement job → the thermometer responds to the heat which is generated by the cement hydration→ helps to detect the top of the cement collum in the annulus
  • radioactive surveys → radioactive tracers are added to cement slurry before it is pumped → after cement job is complete, logging tool is run and detects the top of the cement in the annulus by identifying where the radioactivity decreases to the background natural radioactivity of the formation
56
Q

How do you measure the quality of the cement with a cement bond log (CBL) ?

A
  • CBL tool is a sonic tool → run on a wireline
  • distance between transmitter and receiver is 3 ft.
  • time take for signal to reach the receiver as well as the amplitude of the returning signal give an indication of the cement bond
  • the speed of sound is greater in casing than in formation or mud → first signals which are received at the receiver ar those with travelled through the casing → if the amplitude is large (strong signal) it indicates, that the pipe is free (poor bond)
  • if the cement is firmly bonded to the casing and formation, the signal is attenuated and is characteristic of the formation behind the casing
57
Q
A

Drilling Engineering (13 decks)

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