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.
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.