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. Introduction 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.
Technical and product enhancements in well cementing have allowed the introduction of non-foamed cement systems mixed at densities as low as 7.5 ppg that attain ultimate compressive strengths as high as 1,000 psi. These non-foamed ultra low-density slurries are simplifying blending and mixing operations. In California oilfields and oilfields throughout the world, drilling wells into weak and low-pressure formations, has always been an industry challenge. Successful cement placement in these types of well environments is also a challenge. Lost circulation and partial fluid returns is major culprit to poor cement bonding in addition to higher well costs. Operators in many California fields are forced to drill wells "blind", for example, drilling without mud returns. Drilling "blind" does not preclude the challenging need to properly cement these wells to obtain zonal isolation. The process of foaming cement systems to reduce slurry densities has been applied successfully over the years in wells throughout the world. Though foamed-cementing has been successful, the design and execution add components of operational and technical challenges. These would include accurate metering of nitrogen, uncertainty of actual cement density within the wellbore, additional equipment and personnel requirements, and special drilling rig components. The need for simplicity has allowed the introduction of non-foamed ultra low-density cement systems. In California, successful cement jobs were pumped using non-foamed systems at densities as low as 10.0 ppg. In Colorado, 8.9 ppg slurry was successfully pumped that had ultimate compressive strength of over 1,400 psi. In both cases, these systems provided simpler and an economic alternative choice. These ultra low-density slurries can be designed to meet wide range well temperatures and depth applications. The paper will discuss these systems along with field applications and case histories from California and other parts of the world. Introduction Foamed cement has been successfully applied as a well cementing technique since the late 1970's. The solution of foaming cement slurry to reduce its density has solved well-cementing challenges throughout the world. The need for lower density slurry is necessitated by the need to cement across weak zones, low reservoir pressure formations, naturally fractured rocks, and highly permeable sands. The foam cementing process requires adding nitrogen (or air in some circumstances) to cement slurry to generate foam. Typically the cement slurry density is kept constant and pumped at a constant rate and nitrogen rates are increased during the job. Increasing the nitrogen rates during the job is done to compensate for the compression of the nitrogen bubble. Nitrogen gas is mixed with cement slurry at pre-designed ratios and the foam is stabilized by the addition of surfactants. Although adding nitrogen to reduce cement density has been successful well cementing process; it requires additional pumping equipment, an intricate slurry design and a complicated pumping schedule. Another method of reduccing cement density is to incorporate low density materials, such as low-density spheres, into the system. Even though low-density spheres have been available to the industry for some time, their use has not been widely applied. Recent applications, and current understanding of the pozzolanic (also known as ceramic spheres) and the borosilicate type spheres (also known as glass spheres), makes it possible to reduce the density of cement slurry; maintain desired properties, yet simplify design and field execution. Applications of the low-density sphere cement slurries have also proven to be very competitive to foamed systems. Low-density, high compressive strength cement slurry produces high performance system, allowing for the design application of a one-slurry cement job instead of multiple systems.
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