Annular displacement flows of non-Newtonian fluids in Primary Cementing

 

 

These flows are at the heart of the primary cementing process. In this process a sequence of non-Newtonian fluids are pumped in sequence along a narrow eccentric annulus. Typically the cement slurry is preceded by a spacer fluid and/or wash, the purpose of which is to effectively displace the drilling mud from the well and to act as a buffer between drilling mud and cement.

The fluid stages pumped are typically very long, relative to the annular radii, and variations in the annular geometry occur relatively slowly along the length of the well. Therefore, the basic flow to understand is the displacement of one non-Newtonian fluid by another, along a uniform inclined eccentric annulus.

Figure 1: Snapshots from a typical displacement experiment in the 6m UBC annulus. With poor choice of fluids and operating conditions there is a tendency to finger on the wide side.

 

 

 

Contributors:

-         M. Carrasco-Teja

-         I. Frigaard

-         M. Martinez

-         M. Moyers-Gonzalez

-         S. Pelipenko

-         S. Sood

-         S. Storey

 

 

 

In an eccentric annular geometry fluid velocities on the wide side of the annulus are faster than those on the narrow side. Unless the fluid properties are carefully chosen there is a tendency for the displacing fluid to finger through the displaced fluid on the wide side of the annulus, leaving behind fluid on the narrow side, (see Figure 1, right).

In the worst case, the fluid on the narrow side of the annulus may even remain static, due to having a relatively large yield stress. This situation corresponds industrially to the formation of a static mud channel. After the displacement is complete these mud channels remain in place while the cement slurry hardens and eventually form porous conduits connecting subsurface strata. These channels may often be observed on logs of poorly cemented wells.

Via careful modeling and experimental study, it is possible to understand quantitatively the situations when unsteady displacements (such as in Figure 1) occur, when static mud channels form and when we have a steady displacement. This latter case is the ideal situation for primary cementing, in which the entire displacement front advances along the annulus at the same speed at each azimuthal position.

The basis of our modeling work on laminar displacements is the recognition that the annulus is relatively narrow and may consequently be treated as a Hele-Shaw cell, which reduces the problem from 3 spatial directions to 2D, see Figure 2.

 

 

Figure 2: Schematic of the Hele-Shaw approach when we “unwrap” the eccentric annulus and average across the annular gap.

 

Our 2D modeling work allows fast simulation of complete primary cementing jobs and evaluation of the job design. It is also possible to evaluate job designs without simulation. Extensions of this work include:

-         Study of interfacial instabilities during displacement

-         Special treatments necessary for horizontal displacements

-         Effects of cement pulsation

-         Simulation of the effects of casing reciprocation and rotation

 

Relevant publications:

 

1.       S. Bittleston, J. Ferguson & I.A. Frigaard, “Mud removal and cement placement in primary cementing of an oil well.”, J. Engineering Mathematics, 43, pp. 229-253 (2002).

2.       S. Pelipenko, MSc thesis, University of British Columbia, (2002).

3.       S. Pelipenko and I.A.Frigaard, “On steady state displacements in primary cementing of an oil well.” Journal of Engineering Mathematics, 48(1), pp. 1-26, (2004).

4.       S. Pelipenko and I.A.Frigaard, “Effective and Ineffective Strategies for Mud Removal and Cement Slurry Design.” Society of Petroleum Engineers paper number: SPE 80999, April (2003).

5.       S. Sood, MASc thesis, University of British Columbia, (2004).

6.       S. Pelipenko and I.A.Frigaard, “Two-dimensional computational simulation of eccentric annular cementing displacements.” IMA Journal of Applied Mathematics, 69: pp. 557-583, (2004).

7.       S. Pelipenko and I.A.Frigaard, "Visco-plastic fluid displacements in near-vertical eccentric annuli: lubrication modelling." J. Fluid Mech. 520, pp. 343-377, (2004).

8.       M. Moyers-Gonzalez, PhD thesis, University of British Columbia, (2005).

 

Contact: Ian Frigaard for more details