Cement Slurry Flow Properties and Mud Displacement

Learning Objectives

After completing this topic ” Cement job | Cement Slurry Flow Properties and Mud Displacement“, you will be able to:

• Summarize the role of fluid rheology in ensuring good mud and cement displacement.
• Describe the difference between laminar and turbulent flow and the relevance to cementing procedures.
• Describe the role of proper mud conditioning in ensuring good cement job quality.
• Describe the differences among washes, preflushes and spacers pumped during a cementing job.

Introduction

The objective of cementing is to place a homogeneous mixture of cement and additives between a casing string and wellbore (or between concentric casing strings) in a manner that:

• Ensures strong bonds between the pipe and the cement, and between the cement and formation
• Supports the development of a uniform, competent sheath of cement around the casing over the entire cemented interval

To achieve this objective, it is essential to remove any mud, drill cuttings and mud filter cake that could prevent cement from properly bonding with the pipe and formation, or even lead to uncemented voids behind the casing.

One way to aid this removal process is to include hardware such as centralizers, wipers, and scratchers as part of the casing string; another is to reciprocate and/or rotate the casing during cement displacement.

A more important consideration however, involves rheology – the study of flow and deformation of materials under applied forces. Of particular interest is the manner in which the displaced mud and the displacing cement behave under certain conditions of pressure, temperature and flow rate, and how this behavior affects mud, mud cake and cuttings removal.

This course discusses the rheological character of cement slurry, how slurry behavior affects mud removal, and provide general guidelines for optimizing mud displacement.

Application of Rheological Models

Having a rheological model that accurately explains and predicts fluid behavior is essential to the successful planning and design of a cement job. An appropriately applied fluid model enables engineers to:

• Determine the rate of increase in frictional pressure losses.
• Predict the annular pressure after slurry placement.
• Optimize mud and mud cake removal through slurry turbulence.
• Evaluate slurry pumpability through different downhole restrictions.
• Evaluate the slurry’s ability to transport lost circulation materials.

Before any cement is pumped, engineers can use such models to characterize the cementing process under various anticipated surface and downhole conditions. The models are incorporated into computer simulations using the following information:

• Data about the wellbore and casing geometry
• Drilling mud characteristics
• Number and placement of centralizers
• Volume, pump rate, and characteristics of the various fluids being pumped down the well

These simulations predict various aspects of the cementing process, such as the pressures along the path of the cement as it is pumped down the well. Engineers routinely use cement simulations to model the complex process of mud displacement from the annular space. Predicting mud displacement is important for at least two reasons:

• First, if the cement flow does not displace the fluids from the annular space, the uncemented void filled with those fluids may create a flow path for hydrocarbons behind the casing. Figure 1 illustrates a cross-section of wellbore and casing showing how decentralized casing can allow cement to bypass drilling mud in restricted areas and leave an uncemented void space.
• Secondly, poor mud displacement increases the potential for reservoir fluids, particularly natural gas, to flow into and up the cement column as it sets. This gas flow can itself cause channeling and further compromise zonal isolation.