Stability of Cement Plugs Placed Off-Bottom

 

 

 

Cement plugs are placed in the well either during abandonment or construction. In both situations it can be necessary to place the plug above the bottom of the well. As the cement slurry has a density that is significantly larger than that of the fluids below it, this situation is mechanically unstable. There is a tendency for the heavier fluid to fall through the lighter fluids below.

Solutions:

This situation may be prevented only by one of 3 methods:

-         Use a mechanical device to support the heavier fluids, (as practiced by some service companies)

-         Carefully design a mixing process during placement, so that a mild enough density gradient exists under the cement plug for there to be no destabilization, (not attempted, but could be interesting).

-         Design the fluid properties, so that when placed the fluids will remain static even if there is a sharp change in fluid properties at an interface, (no mixing).

   

 

 

Fluid properties design:

-         With a sharp interface, static stability can be achieved only by having a high enough yield stress relative to the density difference (buoyancy forces).

-         Experimental observations indicate that when close to static stability, the principal failure mode is a slumping exchange flow, in which the heavy fluid slides downwards along the bottom of the inclined well displacing lighter fluids upwards.

-         This exchange flow can be studied analytically and conditions found under which the velocity is zero. Generalisation of this analysis allows us to specify static stability conditions.

-         Lab-scale experiments are used to validate the results.

 

Relevant publications:

 

1.       I.A. Frigaard, “Stratified Exchange Flows of Two Bingham Fluids in an Inclined Slot.” J. Non-Newtonian Fluid Mech., 78, pp. 61-87, (1998).

2.       I.A. Frigaard & O. Scherzer, “Uniaxial Flows of Two Bingham Fluids in a Cylindrical Duct.” IMA J. Appl. Math., 61, pp. 237-266, (1998).

3.       I.A. Frigaard & J.P. Crawshaw, “Preventing buoyancy driven flows of two Bingham fluids in a closed pipe: Fluid rheology design for oilfield plug.” J. Engng. Math., 36(4), pp. 327-348, (1999). 

4.       H. Fenie & I.A. Frigaard, “Transient Fluid Motions in a Simplified Model for Oilfield Plug Cementing.” Mathematical and Computer Modelling, 30(7-8), pp. 71-91, (1999).

5.       J.P. Crawshaw & I.A. Frigaard, “Cement Plugs; Stability and Failure by a Buoyancy-driven Mechanism.” Society of Petroleum Engineers paper number: SPE 56959, (1999).

6.       S.W. Fosso, M. Tina, I.A. Frigaard & J.P. Crawshaw, “Viscous-pill Design Methodology leads to Increased Cement Plugs Success Rates: Application and Case Studies from Southern Algeria.” Society of Petroleum Engineers paper number: SPE 62752, September 2000.

7.       I.A. Frigaard & O. Scherzer, “The Effects of Yield Stress Variation on Uniaxial Exchange Flows of Two Bingham Fluids in a Cylindrical Duct.” SIAM J. Appl. Math., 60(6), pp. 1950-1976, (2000).

 

 

Contributors:

-         J. Crawshaw

-         H. Fenie

-         I. Frigaard

-         O. Scherzer

 

Contact: Ian Frigaard for more details