Primary cementing is a critical procedure in the well construction process. The cement sheath provides a hydraulic seal that establishes zonal iso-lation, preventing fluid communication between producing zones in the borehole and blocking the escape of fluids to the surface. The cement sheath also anchors and supports the casing string and protects the steel casing against corrosion by formation fluids. Failure to achieve these objectives may severely limit the well's ability to reach its full producing potential.
Most primary cementing operations employ a two-plug cement placement method (right). After drilling through an interval to a desired depth, a drilling crew removes the drillpipe, leaving the borehole filled with drilling fluid. The crew then lowers a casing string to the bottom of the borehole. The bottom end of the casing string is protected by a guide shoe or float shoe. Both shoes are tapered, commonly bullet-nosed devices that guide the casing toward the center of the hole to minimize contact with rough edges or washouts during installation. The guide shoe differs from the float shoe in that the former lacks a check valve. The check valve can prevent reverse flow, or U-tubing, of fluids from the annulus into the casing. Centralizers are placed along critical casing sections to help prevent the casing from sticking while it is lowered into the well. In addition, centralizers keep the casing in the center of the borehole to help ensure placement of a uniform cement sheath in the annulus between the casing and the borehole wall.
As the casing is lowered into the well, the casing interior may fill with drilling fluid. The objectives of the primary cementing operation are to remove drilling fluid from the casing interior and borehole, place a cement slurry in the annulus and fill the casing interior with a displacement fluid such as drill-ing fluid, brine or water.
Cement slurries and drilling fluids are usually chemically incompatible. Commingling them may result in a thickened or gelled mass at the interface that would be difficult to remove from the wellbore, possibly preventing place-ment of a uniform cement sheath throughout the annulus. Therefore, engi-neers employ chemical and physical means to maintain fluid separation. Chemical washes and spacer fluids may be pumped after the drilling fluid and before the cement slurry. These fluids have the added benefit of cleaning the casing and formation surfaces, which helps achieve good cement bonding.
Wiper plugs are elastomeric devices that provide a physical barrier between fluids pumped inside the casing. A bottom plug separates the cement slurry from the drilling fluid, and a top plug separates the cement slurry from the displacement fluid. The bottom plug has a membrane that ruptures when it lands at the bottom of the casing string, creating a pathway through which the cement slurry may flow into the annulus. The top plug does not have a membrane; therefore, when it lands on top of the bottom plug, hydraulic communication is severed between the casing interior and the annulus. After the cementing operation, engineers wait for the cement to cure, set and develop strength - known as waiting on cement (WOC). After the WOC period, usually less than 24 hours, additional drilling, perfo-rating or other operations may commence.
Well construction typically consists of installing several casing strings, each requiring a primary cementing operation (Figure 2). As the well deepens, the diameter of each casing string is usually smaller than the preceding one.
Nearly all well cementing operations use portland cement, which consists mainly of anhydrous calcium silicate and calcium aluminate compounds that hydrate when added to water. The hydration products, principally calcium silicate hydrates, provide the strength and low permeability required to achieve zonal isolation.
The conditions to which portland cements are exposed in a well differ significantly from those encountered at ambient surface conditions associ-ated with buildings, roads and bridges. Well cements must perform over awide temperature range - from below freezing in permafrost zones to temperatures exceeding 400°C [752°F] in geothermal wells. Consequently, cement manufacturers produce special versions of portland cement for use in wells. In addition, more than 100 cement additives are available to adjust cement performance, allowing engineers to customize a cement formulation for a particular well environment. The principal objective is to formulate a cement that is pumpable for a time sufficient for placement in the annulus, develops strength within a few hours after placement and remains durable throughout the well's lifetime.
Additives may be classified according to the functions they perform. Accelerators reduce the cement setting time and increase the rate of compressive strength development. Retarders delay the setting time and extend the time during which a cement slurry is pumpable. Extenders lower the cement slurry density, reduce the amount of cement per unit volume of set product, or both. Weighting agentsincrease the density of the cement. Fluid loss control agents control leakage of water from the cement slurry into porous formations, thereby preserving the designed cement slurry properties. Lost circulation control agents limit flow of the entire cement slurry out of the wellbore into weak, cracked or vugular formations and help ensure that the cement slurry is able to fill the entire annular space. Dispersants reduce the viscosity of the cement slurry, which allows a lower pumping pressure during placement. Specialty additives include antifoam agents, fibers and flexible particles. Cement additives are an active domain of research and development, and the industry regularly introduces new and improved products.