Evaluating a cement system for zonal isolation and its ability to support the casing have always, among other parameters, used compressive strength as an important set cement slurry requirement. Recent studies have concluded that compressive strength is not the most important characteristic of the cement to provide zonal isolation and casing support. Elasticity and tensile strength of the cement system is more important for casing support and zonal isolation. In Kern county California, cyclic steam wells are a common completion method for heavy oil recovery. These wells may have temperature variations between production and steam injection cycles of up to 300°F. These extreme wellbore temperature swings create a large amount of induced stress on the cement sheath. Currently, the industry approach is to develop the highest compressive strength cement system possible to maximize cement bonding. However, advanced modeling now available indicates that such systems might not be optimized for such applications. A lightweight cement system has been developed and laboratory tested utilizing fit-for-purpose additives to give the set cement enhanced elastic and tensile properties. This paper will discuss the design and development, along with the potential application of an enhanced set-cement mechanical property system. The results of a Cement Stress Model, and the impact on cementing cyclic steam wells will also be presented.
The main purpose of a cement system is to provide zonal isolation for the life of the well. Portland cement is commonly used with a mixture of additives to produce cement slurry designed to meet well requirements. Such requirements would include well depth, bottom hole static and circulating temperatures, fracture and/or leakoff pressures, pore pressure, and any other special well or operational requirements. It is typically these conditions that dictate the slurry density, fluid loss, free water, and thickening time requirements in addition to set slurry compressive strength requirements.
The compressive strength requirements of the set cement is usually a value chosen by the operator, the service company, or the local oil and gas regulatory agency. It is a given that at a minimum, the compressive strength should at least be high enough to support the pipe weight at a given cement slurry height. However, it is our opinion that there is no direct relationship between the ability for a cement to bond to the pipe and the formation and the compressive strength of the set cement. A compressive strength value is chosen as important parameter of the slurry design. The strength, however, should be high enough to support the pipe as mentioned above and the slurry should not lose such strength due to retrogression. In high temperature wells, or wells that will be exposed to temperatures above 230°F, silica flour and/or silica sand is added to prevent cement strength retrogression.
The above slurry requirements are designed to meet well conditions while the drilling rig is on the hole. However, it is imperative that the stress conditions on the cement sheath also be considered and quantified for the life of the well while under production and/or injection cycles. These stress conditions are mainly created due to the heating and cooling of the wellbore and the pressure cycles of injection and production. It is therefore the ability a set-cement system to provide good zonal isolation is not only dependent on the condition of the well while the cement is being placed, but also during the well production/ injection. Even though this paper is focused on cement slurry design considerations, it is important to note that there are factors that will effect the quality of even the most perfectly designed cement slurry. These factors would include poor pre-job practices, poor spacer design, poor dry blending procedures, mixing problems, and post job problems.