1. Multiple Stage Cementing Tools

Multiple stage cementing (MSC) tools can be run in the casing string for any of the following reasons:

To improve the chance of retrieving casing from a specific depth eg., to allow for the possibility of sidetracking later. Using such a collar, the cement slurry can be circulated out from above a chosen depth, just after the first stage cement slurry has been placed.

A multistage cementing collar in combination with a packer can be used to provide a mechanical seal to increase the chance of placement of good cement above it.

Extremely long cement columns will sometimes require cementation in two or more stages due to the high hydrostatic pressures involved ie., losses are expected with a cement hydrostatic pressure induced at the minimum TOC required.

Opening and Closing The Stage Cementing Collar

There are two types of plug which may be used for opening the stage cementing collar (hydraulically):

a free-fall (or chased with slow spm) opening plug which is dropped from surface after the first stage is complete, or:

a displacement type opening plug which is pumped down the casing at the end of the displacement of the first stage cement slurry. The stage collar is thus opened immediately after the first stage cement slurry is in place. In this way the best possible chance of circulating out the excess cement slurry above the ports is obtained.

The use of free fall opening plugs is preferred since displacement type opening plugs do not permit observation of the bumping of the first - stage plugs and therefore calculated displacement volumes have to be relied upon. Moreover, the lower section of casing cannot be pressure tested until after drill out of the upper multiple stage collar (MSC).

All multiple stage cementing collars are closed using similar plugs independent of the method of opening.

For highly deviated and/or high pressure wells the use of displacement type plugs are recommended.

Centralisers

Centralisers should be run immediately above and below the stage cementing collar to provide the best possible chance of obtaining a concentric annulus and optimum cement distribution. Rigid centralisers should be used inside casing. A cement basket shall be situated just below the stage collar if losses are expected.

Rate of Fall of Free-Fall Plugs

The rate at which a plug will free fall depends on such factors as gradient and viscosity of the mud, specific gravity and shape of plug and relationship between cross‑sectional areas of the plug and casing. Suppliers can give information for their particular plugs.

A fairly conservative rule of thumb is that a plug will take 1 sec/m, falling in normal drilling fluid in a vertical well.

Warning : It must not be attempted to surge with the rig pumps.

Pumping

Pressure surges must be minimised by breaking circulation carefully (2-3 bpm), particularly on opening the stage cementing collar, to avoid weakening or shearing the closing sleeve shear pins by shock loading when the opening plug lands.

Stage Cementing Collar Failures

The most common failure is leakage after closing the ports. This is usually the result of damage sustained by seals during movement of the sleeves. The only remedy is to squeeze cement.

To reduce the potential for failures, the following is advised:

Store the stage cementing collar in a cool place and do not expose it to extreme temperatures for prolonged periods of time.

Clean with air and re-grease the small ball check valves in the closing sleeve.

If the stage cementing collar leaks after bumping the plug, do not attempt to re-pressurise the casing.

Displace the closing plug with some cement followed by some water (for water based muds) or low toxicity oil (for oil based muds) to prevent contamination with the mud used for displacement.

Stage cementing collars should not be located where there is a sharp change in hole deviation since bending stresses can make it difficult to open and close.

Ensure that a check is made of the part numbers of supplied multistage equipment to ensure compatibility.

Common failures include the:

• improper movement of the first stage closing plug
• failure to close the MSC sleeve
• failure to seat the MSC opening plug
• failure to seat the closing plug
• under displacement
• error in pump efficiency
• foam mud

Check the volume pumped during opening of the MSC as the efficiency seen is likely to be repeated during displacement of the first stage cementing plugs. Never exceed the shoe track volume when opening MSC ports.

2. Multiple Stage Cementation Procedure

This technique shall be used if losses are expected or large volumes or cement are required.

Different Techniques

There are two widely used multi-stage cementing techniques, the free fall and the continuous displacement type. The comparison in application between the two techniques are tabulated below:

Procedure

Stage 1

1. CIRCULATE 150 % casing contents. CONDITION mud. Optimum YP (8-10).

Note: Yield points are greater in high angle holes.

IF deviated hole condition permit, THEN reciprocate casing over 20 ft during circulating and cementing. Space out casing to have collars away from pipe rams.

2. DROP first bottom plug and PUMP pre-flush. DROP second bottom plug.

3. PUMP first stage cement with lead and tail slurries. For lead slurry, PRE-HYDRATE all Bentonite in fresh water (50 % of the required volume). CALCULATE cement volume accurately from calliper log plus 15 % excess. TOC of lead for first stage to be 200 ft above MSC.

4. RELEASE first stage top plug.

5. PUMP 2 bbls of slurry. Slow down displacement when top plug passes MSC. CONTINUE to displace cement. SLOW DOWN displacement 10 bbls prior to bump.

6. BUMP plug.

IF… plug does not bump, THEN… CHECK volume pumped form displacement tasks and OVERDISPLACE by a maximum of half the shoe track.

7. PRESSURE TEST casing to 3 500 psi 15 minutes for 9 5/8” csg.

8. BLEED OFF and CHECK for backflow.

9. DROP MSPC opening plug and allow to fall for 30 minutes at 40 spm.

10. Slowly INCREASE pressure to 500 psi for five minutes. INCREASE pressure to 750 psi for two minutes. INCREASE pressure to 1 000 psi.

Note:   A sudden drop in pressure and circulation indicates that the   packer element has opened and the circulation ports are open.

11. CIRCULATE 9.5/8in x 13.5/8in annulus clean and CHECK returns

Stage 2

1. PUMP 20 bbls mixwater.

2. CEMENT second stage as per programme.

3. DROP MSC closing plug.

4. DISPLACE with mud.

5. BUMP plug.

6. PRESSURISE to 1 500 psi (depends of shear pin) differential pressure to close MSC ports and lock closing sleeve. HOLD pressure for five minutes and slowly RELEASE to close check valve.

7. INCREASE pressure to 3 500 psi and test casing for 15 minutes.

8. BLEED OFF pressure and CHECK for backflow.