Zonal isolation throughout the life of the well is important to help ensure that health, safety, and environmental (HSE) standards are not compromised and that the well operates economically. The life expectancy of a well is dependent on the protection that the wellbore can receive from primary cementing operations.
A primary cement job design that provides zonal isolation and preserves structural integrity over the life of the well can reduce or eliminate the need for remedial operations. Achieving successful zonal isolation comprises three key mechanisms:
Design and delivery of a reliable cement system that can withstand the effects of the operational loading and cyclical stresses exerted on the well by drilling, completion, and production operations.
An auto-seal feature in the cement sheath itself. In the event that the cement sheath fails and develops cracks and/or micro-annuli because of primary cementing job failure in the presence of a mud filter cake, the sealant sheath can then react and respond in an attempt to automatically seal itself if formation fluid enters the cemented annulus. This is a fail-safe technology mode that can be included in the design of the cement slurry.
Employing a packer element as part of the casing/liner string that can swell and seal the annulus if formation fluid comes in contact with the element. This additional contingency can help prevent flow of the hydrocarbon past the packer element by creating an effective mechanical seal.
By applying these features, it is possible to effectively and successfully perform primary cementing operations with additional protective options and place sealants that will last throughout the life of the well (Fig. 1).
Experience has shown that the mechanical properties of a sealant placed behind the casing is important to the success of a well throughout its productive life and ultimately minimizes its abandonment efforts and cost. This issue becomes more crucial when high-pressure, high-temperature (HPHT) wells are designed, drilled, and completed. In case of gas wells, the most common element of concern is the pressure encountered at different stages of the well's life, whereas for oil-producing wells, temperature may be of similar concern.
Events such as high-pressure fracturing/acidizing, pressure testing, and high-pressure production can damage the cement sheath behind the pipe in any well. This possibility will require more scrutiny of the sealant mechanical properties to help ensure good quality zonal isolation. Mechanical properties required of a cement system should be determined by studying formation competency, drilling/completion fluid densities, completion methods, production regime(s), and future abandonment necessities. In general, cement sheath damage can be classified into three general categories: debonding, radial cracking, or shattering. The failure can be caused when tangential forces exceed bonding forces, tensile strength, and plastic limits. This paper presents a three-step approach to design, placement, and completion of a tuned slurry system that can protect zonal isolation throughout the planned operations of a well.
As oilfield cementing evolves over time, more and more considerations should be brought into the design stage. These can include temperature cycling, high-pressure stimulations, and the method used to produce the well. Each operation performed on a well has an effect on the cement sheath behind the pipe. The effects should be considered before the primary cement job to minimize the costs necessary to maintain and abandon the well.