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Centralization. Effective mud removal is essential to a good cement job. In an eccentric casing annulus, mud will preferentially flow through the large side instead of in the narrow part of the annu- lus. Good centralization is needed at the shoe and anywhere that a good bond is important (such as for zonal isolation).
The restoring force of a spring centralizer is quoted by the manu- facturer. Given this, the correct centralizer placement can be calculat- ed for a particular casing string, mud weight, and hole deviation. Service companies can recommend centralizer placement, but these
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recommendations should be examined closely to ensure that they meet your requirements.
In between centralizers in a deviated well, the casing will sag down towards the low side. The amount of this sag can be calculated as follows:
where
Dmp = Deflection at midpoint, in inches w = Weight of casing, in lbs/ft
L = Length between centralizers, in feet
f = Inclination, in degrees
D = Casing OD, in inches d = Casing ID, in inches
Of course, the amount of sag will have to be added to the distance from the wellbore center to the casing center at the centralizers if the total amount of eccentricity is needed where the centralizer OD is less than hole diameter, such as in a washed out hole.
In a mudline suspension system the manufacturer sometimes sug- gests that centralizers not be used, to avoid wear on the inner profile. An uncentralized casing will give you little chance of successful cementing. Minimize the number to those run over critical sections (especially the shoetrack), but not running any at all will compromise the cement job.
Placing a centralizer over a stop collar will reduce wear on the wellhead, casing, and centralizer, and will also reduce drags. Some small diameter centralizers do not have enough clearance for this and so must be run between two collars. Rigid centralizers are usually placed between two collars.
Centralizer placement. Rigid centralizers must not be run in open hole. Following is a general guide to use when the centralization pro- gram is not calculated. It may be modified as necessary. Where a sub- sea wellhead or suspension system is in use and the manufacturer rec-
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ommends no centralization, this will normally be to reduce wear on the wellhead inner profile and sealing surfaces. However, centralization is so important to getting a satisfactory cement job that it may be thought necessary to run some centralizers, even if it is the minimum number at critical depths and around the shoe only.
Casings 185/8in or larger:
■ Two springs on the bottom two joints ■ One spring per joint in the cemented section
■ One rigid inside the conductor shoe and one at 10-20 m below the
top of the previous casing or conductor
■ For subsea wellheads run one rigid in the joint below seabed
Casings smaller than 185/8in and larger than 95/8in:
■ Two springs on each of the shoetrack joints
■ One spring per two joints over the cemented section, straight hole.
Use one per joint if the calculated centralization is close to or below the API recommended minimum of 67%.
■ One per joint in the buildup section only if cemented ■ One rigid inside the previous casing shoe
■ One rigid at 10-20 m below the hanger
The number of spring centralizers in the open hole may be reduced if wear is likely in a subsea wellhead inner profile.
Casings 95/8in or smaller (springs between stop collars if small annu-
lar clearance):
■ Two springs on each of the first two joints
■ One spring per two joints over the cemented section, straight hole.
Use one per joint if the calculated centralization is close to or below the API recommended minimum of 67%.
■ One rigid inside the previous casing shoe ■ One rigid at 10-20m below the hanger
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The number of spring centralizers in the open hole may be reduced if wear is likely in a subsea wellhead inner profile.
Liners 7 in or smaller (springs between stop collars due to the small annular clearance):
■ Two springs on each of the first two joints ■ One spring per two joints in the open hole ■ One rigid inside the previous casing shoe
Float equipment and accessories. For a normal one-stage cement job, the usual setup is a floatshoe with a float collar at two joints up the string and bottom and top plugs. Other equipment will be needed for multistage jobs, subsea operations, etc.
Correctly sized top and bottom plugs are both vital. If no bottom plug is run, the top plug will wipe a film of mud ahead off the casing, which may be enough to contaminate the cement around the shoe. For deep, critical jobs two bottom plugs may be used to separate mud, spacers, and slurry. This may take longer to drill out but an hour of extra drilling is worth paying for a better cement job.
If a mixed casing string of more than one inside diameter is used, special plugs may be needed. Generally the plugs will fit one casing OD for a range of casing weights, but it is possible that use of extremes of weights in the same size casing or different casing sizes cannot be cov- ered with the standard plugs, and you may have to arrange special order plugs. Do not accept standard plugs modified by sticking on rub- ber strips around the top fins; the strips can become detached and plug the floats.
Types of liner hangers. A liner hanger generally uses a set of slip elements to grip the wall of the previous casing. Mechanical and hydraulic setting systems are used, though hydraulics are used more often.
In addition to the basic liner hanger, some systems contain integral packers that may be set after cementing. These allow reverse circulating out of excess cement without imposing circulating pressures on the for- mations if they are set immediately after finishing cement displacement.
These integral packers are quite reliable and can be routinely used. Liners may also be set on bottom and cemented without hanging off on the previous casing, though this is uncommon. A cementing shoe with side passages is needed instead of the more conventional type.
Hangers (surface wellhead). The choice of hangers will be limited to those compatible with the wellhead system. On a nonfloating rig, a solid (mandrel) hanger may be used to allow the BOP to be safely nip- pled down without having to wait on cement, if other circumstances allow. Setting these hangers is simpler and safer than lifting the stack and setting a slip and seal assembly, trimming the casing, and running a seal bushing for the next spool. More care is needed to space out, and a landing joint of appropriate length to reach from the top of the hang- er to the drillfloor is necessary.
If a solid hanger is to be run, a slip and seal backup will still be needed to land the casing if, for some reason, the casing is not run all the way to planned depth. Therefore, a slip and seal assembly and a seal bushing should be kept handy for emergency landing. A solid hanger cannot be used to land the casing under extra tension, which is some- times done to prevent buckling.
Other accessories. If casing is to be reciprocated while cementing, scratchers may help remove wall cake and induce turbulence.
External casing packers (ECPs) are designed to seal off the annu- lus to stop gas or fluid migration through the setting cement. The dan- ger here is that below the ECP, hydrostatic pressure will drop very rapidly as the cement sets unless special additives are used, so that the cement below the ECP could become porous due to gas flow from the formation into the setting cement. Consult the cement contractor and consider all the factors carefully before choosing to use an ECP.
Special cement heads are available to allow you to rotate the cas- ing during displacement. However, experience shows that reciproca- tion during displacement is more effective in displacing mud. Reciprocation must be used with caution in sticky hole conditions, especially if using a fixed hanger that may be in the BOP if the casing gets stuck while reciprocating.