Comparative Study of the Mechanical Properties of Reduced Density Cements

Abstract

Cementing a casing string across weak formations or depleted reservoirs has the added challenge of tailoring the cement slurry to meet delivery criteria (i.e., density and rheology) while maintaining the mechanical properties of the set cement necessary to provide a dependable barrier. To help prevent fracturing the formation and inducing losses, cement density is often reduced, which strongly influences the mechanical properties of set cement. Common strategies for reducing cement density consist of adding water in the cement slurry; using additives such as hollow glass microspheres (HGS), synthetic latex, and elastomers; using foam cement; or adding resin. This paper discusses how cement slurries with reduced densities are designed using both traditional and alternative methods of making cement/resin composites and provides insight into the advantages and drawbacks of each. Stable cement slurries with a density of 13 lbm/gal were designed, and placement characteristics of thickening time and rheology were evaluated for the liquid cement slurry. Unconfined compressive strength (CS), Young's modulus (YM), tensile strength, permeability, and shear bond were investigated on the cured samples. Before taking mechanical and permeability measurements, slurry stability was verified using sedimentation testing. Any slurry that did not exhibit the necessary stability was redesigned and tested again. Only the final slurry designs exhibiting stability are discussed in this paper. Cement-resin composite cements exhibited similar performance to those containing HGS in terms of CS, YM, tensile strength, and shear bond but exhibited greater than two times the CS when compared to the synthetic latex modified, water-extended, and elastomeric slurry designs. The cement-resin composite provided almost twice the shear bond strength and increased tensile strength by 50% compared to other slurry compositions. In the current work, cement-resin composite, synthetic latex modified, microbead-based, water-extended, and elastomer-modified slurries are compared at 13 lbm/gal. Various parameters, such as mixability, ease of placement in the annulus, strength development, and long-term cement integrity, are evaluated. Traditional and newly introduced techniques for reducing cement slurry density and the resultant mechanical properties of the set solids are investigated. This information provides an alternate method of using cement-resin composites for designing and delivering dependable barriers tailored for low density applications.