Whether you're setting foundations or constructing roads, good cementing practices are the key to structural integrity and controlling costs. Well construction is no exception.

Getting a cement plug right, first time, is important because subsequent plugs are expensive. In the North Sea, a second 600 ft (183 m) plug in 81/2-in. vertical hole at 8,000 ft (2,440 m) will generally exceed US $210,000. Using a semisubmersible rig ($120,000 per day), costs breakdown as:

• Round trip to tag the failed plug $60,000
• Round trip to set second plug $60,000
• Waiting on cement $60,000
• Tripping in to tag second plug $30,000

Design and Execution

Experience shows that effective cement plugs are founded on correct design and execution. During design, consideration must be given to well evaluation, written procedures, laboratory testing, spacer and slurry design. An allowance for cement contamination at the top of the plug (between 100 ft and 200 ft [30 m and 61 m]) should be made also. This will lower the kick-off point 200 ft (61 m) below the 'top' of the cement.

During execution, attention must be paid to contamination, mud and hole conditioning, slurry volume and coordinating rig operations. Often an impatience to set and tag the plug can lead to insufficiently hardened cement. Preventing mud contamination is important - this can reduce compressive strength by 70% when using oil based mud systems. Operational problems can be avoided by using the actual dimensions of drill-pipe to calculate displacement volumes.

General Best Practices

Cement plug volume is critical and pumping insufficient slurry is a common cause of plug failure. If openhole size is not known, plug volumes should be based on the height of the cement plus 50% over hole-gauge to account for washouts. An additional ten barrels of slurry will help compensate for mud contamination. Other considerations are to 'spot' a base and 'wash' the interval. As the plug is not being set on a firm base, a viscous reactive pill (up to 300 ft [91.5 m]) should be spotted below the proposed plug. This serves as a base. In order to remove as much mud as possible before placing cement, the plug placement interval is washed when tripping in with the work string. Reciprocating and slow rotating (up to 40 RPM). Circulation should be done at maximum annular velocity circulation (240 ft to 400 ft/min [73 m to 122 m/min] dependent on well conditions) is a proven method of ensuring mud removal. This process ought to be repeated for each stand across the entire plug interval while shutdowns should be minimized to keep the mud fluid.

Plug height should not exceed 700 ft (213 m), since the extra time taken to pull slowly out of the plug increases the risk of cementing-in the cementing assembly. For 6-in. and 8-in. hole respectively, use of 27/8-in. or 31/2-in. stinger attached to the end of drillpipe will reduce plug 'stripping' when pulling the assembly. The recommended length of this assembly is 1.5 x plug length.

Other tips include pumping a minimum of 40 barrels of spacer (or 500 linear ft [125 linear m]) in the annulus ahead of the cement plug to separate the cement from the mud. The spacer, cement and displacement should be pumped at maximum possible rates with the cement unit. However to avoid over displacement this rate should be slowed to 1 to 2 barrels per minute before the end of calculated displacement. The plug should be under displaced. The U tube effect is best employed to find cement balance point. Here well conditions must confirm a vacuum.

After cement plug is in place, pull out of the hole slowly (30 ft to 50 ft/min) until 500 to 1,000 ft (152 m to 305 m) above the cement plug and careful breaking of connections avoids damaging or stripping the plug. Any delays must be minimized as the slurry is designed with a short pump time. Circulating on top of the plug is best avoided and circulation should be broken slowly to leave the plug undisturbed.
Waiting on cement time is based on the time that cement requires to reach 1,000 psi for an isolation plug or 3,000 psi. for kick-off purposes. These times are defined by laboratory data.

Required Tools

Using specialized tools can also help. A side-port diverter tool will change flow direction from downwards to outwards. This minimizes cement intermixing and improves hole cleaning. Experience with mule shoes shows that narrow slots are not recommended as they do not improve hole-cleaning. A cementing swivel head will help enhance mud displacement and prevent cement channeling. The use of this tool is recommended when tripping in and during the entire cement job. The recommended rpm is between 20 to 50 rpm. During pumping cement and displacement, reciprocating is not recommended.
Additionally, the use of a ball is essential to clean the drillpipe and stinger out of cement. The ball generally is dropped when 1,000 ft (305 m) clear of the plug and is slowly circulated out to the shakers.