Well Cementing Fundamentals

Overview of primary cementing and remedial cementing operations

Well cementing consists of two principal operations—primary cementing and remedial cementing. Primary cementing is the process of placing a cement sheath in the annulus between the casing and the formation. Remedial cementing occurs after primary cementing, when engineers inject cements into strategic well locations for various purposes, including well repair and well abandonment.

Basic two-plug primary cementing operation. After a well interval has been drilled to the desired depth, the drillpipe is removed and a casing string is lowered to the bottom of the borehole (top). The bottom of the casing string is usually fitted with a protective shoe, and centralizers keep the casing centered in the wellbore. Engineers pump chemical washes and spacer fluids down the casing interior, thereby displacing drilling fluid (middle left). They next insert a bottom plug, followed by a volume of cement slurry that is sufficient to fill the annulus (middle right). Continued pumping of cement slurry forces drilling fluid out of the casing interior, up the annulus and out of the wellbore. When the bottom plug lands at the bottom of the casing string, a membrane in the plug ruptures, opening a pathway for the cement slurry to enter the annulus. Engineers insert a top plug after the cement slurry, and the top plug is then followed by a displacement fluid (bottom left). Pumping the displacement fluid forces the top plug downward until it lands on the bottom plug, thereby isolating the casing interior and annulus and filling the annulus with cement slurry.

Primary cementing is a critical procedure in the well construction process. The cement sheath provides a hydraulic seal that establishes zonal isolation, 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.

Amber Zhang Ms.

Foreign trade 

Zhejiang Ruico Advanced Materials Co., Ltd. (Stock No.873233)

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