Recent advancements in completion technology have made tight-gas bearing formations more attractive for production. However, long horizontal sections with multiple fracture treatments can influence the effectiveness of the seal provided by the cemented annulus which, in turn, can affect the productivity of the completed well. Therefore, planning for cementing, followed with proper design and execution, is important to overcome the challenges associated with tight-gas well completions and production.
It is challenging to centralize casing strings placed in highly deviated wellbore sections. Poor centralization results in a non-uniform annular-flow profile as fluids tend to flow along the path of least resistance. If this phenomenon exists during cement-slurry placement, an uncemented channel can result on the narrow side of the annulus creating a flow path for fluids to migrate within the annulus.
Effective cement slurry placement resulting in an initial annular seal does not guarantee that the seal will be maintained through subsequent completion events. A cement sheath is, by nature, brittle when it is exposed to the stress environment that high pressure hydraulic stimulation produces. The cement sheath can crack, resulting in unwanted flow paths. In addition, fracture attempts early in the completion of a tight-gas well can weaken the hardened cement through fatigue, breaking the cements' sealing capability.
The lack of a cemented annular seal can result in the inability to control wellbore fluids affecting both the fracture treatments and subsequent production. If the cement sheath allows fracturing fluids an alternative path, then fracture initiation points might not occur at the desired location, minimizing the stimulation effectiveness. Production gases can also follow an alternative path, which allows for uncontrolled flow to thief zones and to the surface.
This paper discusses cement-slurry placement and enhanced sealing properties that a cement system should possess to help withstand the tight-gas completions.