Weighted Cementing

Weighted cementing is used to secure and support the casing or liner within a wellbore.  This process involves the use of a cement slurry that is heavier than the usual cement used in standard cementing operations.  The increased weight (density) of the cement slurry is achieved by adding weighting materials such as barite or hematite.  By using a heavier cement slurry, it provides enhanced wellbore stability, effective zonal isolation, and robust support for the casing, which are essential for the safe and efficient operation of oil and gas wells.

Purpose and Benefits

• Wellbore Stability  -  The added weight helps maintain wellbore stability, particularly in high-pressure zones where lighter cement slurries might not provide adequate support.
• Preventing Fluid Influx  -  Heavier cement can counterbalance formation pressures, preventing the influx of formation fluids (gas, oil, or water) into the wellbore during the cementing process.
• Effective Zonal Isolation  -  Ensures effective zonal isolation by filling and sealing the annular space between the casing and the wellbore, which prevents cross-flow between different geological formations.
• Supporting Casing  -  Provides structural support for the casing, helping to prevent collapse or deformation.

Process

• Preparation
  • Cement Slurry Design  -  Engineers design the cement slurry to achieve the desired density, typically between 15 to 20 pounds per gallon (ppg), depending on the specific well conditions.
  • Additives  -  Weighting materials like barite, hematite, or other dense minerals are added to the cement slurry to increase its weight.

• Cementing Operation
  • Pumping  -  The weighted cement slurry is pumped down the casing and up into the annular space between the casing and the wellbore.
  • Displacement  -  The slurry is displaced into place by pumping drilling mud or another fluid behind it to ensure it fills the intended space completely.
  • Setting  -  The cement slurry is allowed to set and harden, providing a strong and stable barrier.

• Quality Control
  • Density Monitoring  -  Continuous monitoring of the slurry density during pumping to ensure it meets the design specifications.
  • Pressure Testing  -  Conducting pressure tests after the cement sets to ensure the integrity of the seal and the stability of the wellbore.

Challenges

• Mixing and Handling  -  The heavier slurry can be more challenging to mix and handle, requiring specialized equipment and procedures.
• Pumping Pressure  -  Higher pumping pressures may be needed to move the denser slurry, which can put additional stress on equipment.
• Compatibility  -  Ensuring the compatibility of the weighted cement with the formation and the wellbore fluids to prevent chemical reactions that could compromise the cement's performance.

Applications

• High-Pressure Wells  -  Particularly useful in high-pressure and high-temperature (HPHT) wells where standard cementing might not be sufficient.
• Deepwater Drilling  -  Used in deepwater drilling operations where the wellbore pressures are significantly higher.
• Remedial Cementing  -  Can be applied in remedial operations to repair or improve existing wells where standard cement has failed.

   

Weighted Cementing Formula
\(\large{ x \;=\;  \frac{  (\; \frac{SD \; 11.207983 }{ SG_c} \;) \;+\; (\; SD \; CW \;) \;-\; 94 \;-\; (\; 8.33 \; CW\;)  }{  (\;1 \;+\; \frac{ AW }{ 100} \;) \;-\; (\;\frac{SD }{ SG_a \; 8.33} \;) \;-\;  (\; SD \;+\; \frac{ AW }{ 100} \;)  }  }\)
Symbol	English	Metric
\( x \) = additive req. pounds per sack of cement	\(lbf\;/\;stroke\)	-
\( SD \) = req. slurry density	\(lbm\;/\;gal\)	-
\( SG_c \) = specific gravity of cement	\(dimensionless\)	-
\( CW \) = water req. of cement	\(gal\;/\;stroke\)	-
\( AW \) = water req. of additive	\(gal\;/\;stroke\)	-
\( SG_a \)  = specific gravity of additive	\(dimensionless\)	-