Summary
Cement-sheath shrinkage after placing the cement slurry in the annulus has
been of concern in oilwell cementing because of the potential detrimental
effects that shrinkage can have on long-term zonal isolation.
This paper compares several methods for measuring cement shrinkage under a
variety of conditions including those downhole. Using the various methods,
cement shrinkage in environments where the cement slurry is either closed to or
open to external water sources was measured, and the values were compared. The
methods used for measurement under atmospheric conditions were based on
American Petroleum Institute (API) test apparatus (balloon and ring mold) and
flask methods currently used in laboratories. Measurements under pressure were
performed using the balloon and ring-mold methods, in-house designed equipment,
and commercial equipment.
The values obtained by the different methods are discussed. Procedure
modifications are proposed for measuring volume changes to cement slurries on
the basis of placement time, shear history, and gel-strength development.
The results presented in this work can be used to test shrinkage
characteristics of cement slurries in the laboratory under downhole conditions
and to select a cement system that is fit for purpose.
Introduction
The main purpose of a primary cementing job is to provide effective zonal
isolation for the life of the well so that oil and gas can be produced safely
and economically. In recent years, the number of wells with annular pressure
worldwide has increased significantly. The extent of the problem related to
annular pressure buildup in Gulf of Mexico wells has been reported recently
(Bourgoyne et al. 1998).
From a cementing perspective, a path for fluid migration could be created if
drilling fluid is not effectively displaced; if cement slurry is not placed in
the entire annulus; and/or if the cement sheath fails, either because of
shrinkage and/or because of loss of structural integrity from its lack of
capacity to withstand stresses from well operations. The current discussion
focuses on the contribution to near-term cement sheath failure from shrinkage.
A number of papers have been written about cement shrinkage, particularly its
measurement methods for oilwell cement slurries (Root and Calvert 1971; Beirute
and Tragesser 1973; Parcevaux and Sault 1984; Chenevert and Shrestha 1991;
Keating et al. 1989; Moran et al. 1991; Ghofrani and Plack 1993; Justnes et al.
1995; API TR 10TR2 1997; Baumgarte et al. 1999; Backe et al. 1999;
Cementing Solutions 2002; Goboncan and Dillenbeck 2003; Jennings 2005; Ravi et
al. 2006). Cement-sheath failure to provide zonal isolation because of
shrinkage may be a result of volumetric shrinkage leading to debonding and
microannulus formation between the cement sheath and casing or formation,
tensile cracks and subsequent increased permeability resulting from shrinkage
stresses, or both causes.
The phenomenon of cement shrinkage in oilwell cement slurries during and
after the cementing operation has two component: the theoretical volume changes
based on the volume of products and volume of reactants, and the bulk-volume
change. The absolute volume of the products formed is less than the volume
occupied by the reactants (i.e., cement and water). This phenomenon is referred
to as hydration-volume shrinkage, total (internal) shrinkage, chemical
shrinkage, or simply hydration shrinkage. A small percentage of the
theoretically possible total shrinkage manifests itself as volume reduction,
referred to as bulk shrinkage or autogenous shrinkage.
The current discussion addresses issues related to quantification of
dimensional changes to the cement formulation during the early stages of cement
hydration when the rate of shrinkage is high.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
28 February 2007
- Meeting paper published:
16 April 2007
- Revised manuscript received:
9 June 2008
- Manuscript approved:
15 June 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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