Drilling & Well Completion

Excess Cement

Excess Cement: A Crucial Factor in Oil & Gas Well Cementing

In the oil and gas industry, well cementing is a critical process that ensures the integrity of the wellbore. It involves filling the annulus (the space between the wellbore wall and the casing) with cement to prevent fluid flow between different formations and to secure the casing in place. Excess cement refers to the amount of cement used beyond the theoretical volume required to fully fill the annulus. This seemingly simple concept is crucial in ensuring successful well cementing and can significantly impact the overall project outcome.

Why is excess cement used?

The use of excess cement is driven by several factors, aiming to mitigate risks and ensure a secure well:

  • Uncertainties in Hole Diameter: The exact diameter of the wellbore can vary due to factors like geological formations and drilling techniques. Using excess cement accounts for these uncertainties, ensuring the entire annulus is filled.
  • Contamination Risk: The cement slurry might mix with drilling mud or formation fluids, diluting its properties. Excess cement compensates for this potential dilution, ensuring the required cement density and strength.
  • Cement Slurry Loss: The cement slurry might leak into porous formations, reducing the available volume. Excess cement accounts for this loss, preventing incomplete filling of the annulus.
  • Improving Bond Strength: A thicker layer of cement improves the bond between the casing and the formation, providing better stability and reducing the risk of casing movement.
  • Ensuring Proper Coverage: Excess cement allows for a wider distribution of cement slurry, ensuring that the entire annulus is completely covered and eliminates potential voids.

Typical Excess Cement Ratios:

The amount of excess cement used typically varies between 30% and 100% of the theoretical volume, depending on the specific well conditions. Factors like well depth, hole diameter, formation characteristics, and the complexity of the well design all influence this decision.

Impact of Excess Cement on Well Performance:

Excess cement, while crucial for well integrity, can also introduce challenges:

  • Increased Cost: Using excess cement increases the material costs and potentially the cementing operation time.
  • Potential for Cement Squeeze: Excessive cement volume can exert significant pressure on the surrounding formations, potentially causing fractures or compromising wellbore stability.
  • Waste Management: Disposing of excess cement slurry can be a logistical and environmental challenge.

Optimizing Excess Cement:

To strike a balance between safety and efficiency, optimizing the amount of excess cement is crucial. This involves careful planning, considering factors like:

  • Wellbore geometry and formation properties: Understanding the wellbore's size, shape, and the characteristics of the surrounding formations is essential.
  • Cement slurry properties: Selecting the right type of cement slurry and its rheological properties can minimize loss and ensure optimal flow.
  • Cementing equipment and techniques: Choosing the appropriate equipment and cementing techniques can enhance the placement and ensure the desired cement distribution.

Conclusion:

Excess cement is a crucial aspect of well cementing, contributing to a secure and reliable well. Understanding the factors influencing its use and optimizing its volume through careful planning and advanced techniques is key to achieving successful cementing operations while minimizing associated risks and costs.


Test Your Knowledge

Excess Cement Quiz

Instructions: Choose the best answer for each question.

1. What is the primary reason for using excess cement in well cementing?

a) To increase the weight of the casing. b) To ensure complete filling of the annulus, accounting for uncertainties and potential losses. c) To reduce the cost of cementing operations. d) To make the cement slurry more fluid.

Answer

b) To ensure complete filling of the annulus, accounting for uncertainties and potential losses.

2. What is the typical range for excess cement ratios used in well cementing?

a) 5% to 15% b) 10% to 30% c) 30% to 100% d) 100% to 200%

Answer

c) 30% to 100%

3. Which of the following is NOT a potential challenge associated with using excess cement?

a) Increased cost of cementing operations. b) Improved bond strength between the casing and formation. c) Potential for cement squeeze and wellbore instability. d) Waste management challenges.

Answer

b) Improved bond strength between the casing and formation.

4. What is a key factor in optimizing the amount of excess cement used?

a) The type of drilling mud used. b) The depth of the well. c) The diameter of the casing. d) All of the above.

Answer

d) All of the above.

5. Which of the following is NOT a factor to consider when optimizing excess cement?

a) Wellbore geometry and formation properties. b) Cement slurry properties. c) Cementing equipment and techniques. d) The type of drilling fluid used.

Answer

d) The type of drilling fluid used.

Excess Cement Exercise

Scenario: You are working on a well cementing project. The well is 2,000 meters deep with a 12-inch casing. The theoretical volume of cement required to fill the annulus is 500 cubic meters.

Task:

  1. Determine a suitable excess cement ratio for this well, considering the following factors:

    • The well is in a complex geological formation with potential for formation fluid influx.
    • The cement slurry to be used has a high viscosity and a potential for slurry loss.
    • The cementing equipment is capable of handling a significant volume of cement.
  2. Calculate the total volume of cement needed, considering the chosen excess cement ratio.

  3. Briefly explain your reasoning for selecting the excess cement ratio and the potential risks associated with using this volume.

Solution:

Exercice Correction

**1. Excess Cement Ratio:** Considering the complex geological formation, potential for fluid influx, and slurry loss, a higher excess cement ratio is recommended to ensure complete annulus filling and prevent cement squeeze. A ratio of 70% is a reasonable choice in this case. **2. Total Cement Volume:** Total cement volume = Theoretical volume + (Excess ratio * Theoretical volume) Total cement volume = 500 m³ + (0.70 * 500 m³) Total cement volume = 850 m³ **3. Reasoning and Risks:** The chosen 70% excess cement ratio accounts for the uncertainties in the wellbore diameter, potential fluid influx, and potential slurry loss, ensuring complete annulus filling and adequate bond strength. However, using this volume of cement could lead to: * **Increased costs:** Higher cement volume requires more material and potentially longer cementing operation time. * **Potential for cement squeeze:** The significant pressure exerted by the cement slurry might fracture surrounding formations or compromise wellbore stability, requiring mitigation strategies. * **Waste management challenges:** Disposing of the excess cement slurry requires proper environmental management practices and potentially additional costs.


Books

  • Fundamentals of Well Cementing by R.E. King & J.A.K. Scott (This book provides comprehensive coverage of well cementing practices, including discussions on excess cement.)
  • Well Cementing: Chemistry, Design, and Operations by S.A. Holditch, R.J. Warpinski, and W.L. Teufel (This book delves into the technical aspects of cementing, including the impact of excess cement on well performance.)
  • Oil and Gas Well Cementing: A Practical Guide by J.L. Williams and C.M. Evans (This book offers a practical approach to well cementing, with sections on excess cement considerations.)

Articles

  • "Excess Cement in Oil & Gas Well Cementing: A Critical Review" by [Author Name] (Search for articles on scholarly databases like ScienceDirect, Scopus, and Google Scholar for recent research on excess cement.)
  • "Optimizing Excess Cement in Well Cementing: A Case Study" by [Author Name] (Search for articles focusing on case studies and practical examples related to excess cement management.)
  • "The Impact of Excess Cement on Wellbore Stability and Productivity" by [Author Name] (Find articles exploring the potential drawbacks of excessive cement usage and its effects on well performance.)

Online Resources

  • Society of Petroleum Engineers (SPE): www.spe.org (SPE website offers a vast library of technical papers, journals, and presentations on well cementing and related topics.)
  • International Association of Drilling Contractors (IADC): www.iadc.org (IADC provides resources and information on various drilling and cementing practices, including excess cement considerations.)
  • Schlumberger: www.slb.com (Schlumberger, a leading oilfield services company, offers online resources and technical publications related to cementing and well construction.)
  • Halliburton: www.halliburton.com (Halliburton, another major oilfield services provider, has a wealth of information on cementing technologies and practices.)

Search Tips

  • Specific keywords: Use keywords like "excess cement," "well cementing," "cement slurry," "cementing design," "cement squeeze," and "cement optimization" to narrow down your search.
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  • Filter by date: Specify a date range to focus on recent research and industry updates related to excess cement.
  • Scholarly databases: Limit your search to reputable academic databases like ScienceDirect, Scopus, and Google Scholar to find peer-reviewed articles and publications.

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