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Well Cementing

Primary Cementing


The placement of cement slurries into the annulus between casing and open hole or previous casing or both to provide zonal isolation, support axial load of casing string, provide casing support and protection and support the borehole. The primary cementing jobs are categorized by cementing operation techniques. These are some primary cementing jobs:

  • Inner-string or Stab-in cementing (for large-diameter pipe)
  • Cementing through pipe and casing
  • Stage cementing
  • Outside cementing through tubing or pipe
  • Special slurry cementing (Salt-saturated, Delayed-set, HP-HT)

2

Remedial Cementing

Remedial cementing is usually undertaken to correct issues associated with the primary cementing job. The first and the best approach is to avoid remedial cementing by improving the quality of designing and executing the operations from drilling to completion. The decision must be taken by considering the severity of the problem, cost, risk factors and effectiveness of remedial cementing job. Remedial cementing operations consist of two broad categories:

Squeeze cementing which includes:

  • Running squeeze
  • Hesitation squeeze
  • High-pressure squeeze
  • Low-pressure squeeze
  • Packer/retainer squeeze
  • Bradenhead squeeze

Plug cementing which consists of:

  • Abandonment
  • Directional drilling/sidetracking
  • Well control
  • Lost-circulation control
  • Formation testing
  • Zonal isolation/conformance
  • Wellbore stability

3


Designing a Cement Job


1-Compute fluid volumes (Slurry, Wash, Spacer and Disp. Volumes) based on:

  •  Hole capacity (Caliper data if available)
  •  Casing capacity
  •  Annular length

2-Check the well security by computing the hydrostatic and dynamic pressure and compare to:

  • Formation pore pressure
  • Formation fracture pressure
  • Tubular burst pressure
  • Tubular collapse pressure

3-Checking the temperature and thickening time
4-Check for an efficient mud removal to prevent mud channeling and to ensure good zonal isolation:

  • Optimize fluid properties
  • Optimize pumping rate
  • Optimize casing centralization

5-Ensure good well cleaning:

  •  Optimize pre-flushes and flow rate

Designing the cement job is based on the information obtained from the well which is called well parameters. Well parameters are:

1- Depth

The depth of the well influences slurry design by:

  • Volume of wellbore fluids
  • Friction pressures
  • Hydrostatic pressures
  • Temperature

2-Wellbore geometry

The hole shape and geometry determine the casing/open-hole clearance and deviation angle respectively. These two factors influence the designing parameters such as, well conditioning, friction pressure and fluid displacement. Running the caliper logs provides the data about the actual size of the hole. Adding centralizers are installed on the casing to provide the clearance with side of the hole.

3-Temperature

Bottomhole circulating temperature (BHCT) which affects:

  • Thickening time
  • Slurry Rheology
  • Fluid loss
  • Stability (settling)
  • Setting time

Bottomhole static temperature (BHST) which affects CS development and cement integrity for the life of the well
Temperature differential which is the temperature difference between the top and bottom of cement placement

4-Formation pressures

The formation pressures consist of:

  • Pore pressure
  • Fracture pressure

To maintain the integrity of the wellbore, the hydrostatic pressure and the dynamic pressure (hydrostatic pressure plus friction pressure) exerted by the cement slurry must be more than pore pressure and less than fracture pressure with safety margin. The burst and collapse pressure of the casing should be considered to prevent failure and damage to casing.
5-Formation characteristics
The composition of formations is another factor in designing the cement slurry. Shale formations are sensitive to fresh water. The swelling clays, high-pH fluids, weak zones and corrosive gas-contained are the examples of formation which special slurry design should be considered.

4


Cement Additives


The net cement slurry which is the mixture of fixed portion of cement and water is not suitable to use for different well conditions. To modify the slurry properties, additives are added to the cement. Additives are classified into following categories:

Antifoams/Defomers

Antifoams/Defomers are the additives which reduce and prevent the formation of foam in the slurry. Foam changes the density of the slurry on the surface which leads to wrong slurry density monitoring. Furthermore, the air bubbles are trapped in the cement slurry and weaken the set cement.

Accelerators

Accelerators are used to accelerate the cement setting and shorten the slurry setting time.

Retarders

Retarders are additives used to control and increase the setting time of the cement slurry.

Extenders

Extenders are minerals or chemicals added to the cement in order to decrease the slurry density and lower the hydrostatic pressure during and after the placement.

Weighting Agents

Weighting agents are the material added to the cement to increase the slurry density for controlling the high pressure formations.

Dispersant

Dispersants are additives used to improve the rheological properties of the slurry, help the slurry enters the narrow side of the hole and decrease the friction pressure.

Fluid-Loss-Control Additives

Fluid-Loss-Control Additive (FLCA) is an additive used to control the fluid loss to the formation to ensure the performance of the slurry.

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