3-Consideration on Cement Plugs

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PLUG CEMENTINGDate: Sep 99Module: CF110Page: 1

PLUG CEMENTING CONSIDERATIONS

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• Job Design Considerations:- The design of the job starts with the definition of the objective. Setting a plug for lost circulation is quite different from setting a plug to abandon a depleted zone or plug back a well.

- Key questions to consider in design:- At what depth will the plug be set?- Across which formation is the plug going to be set?- At what density should the slurry be mixed?- What is the BHT?- What volume should be pumped?- What are the appropriate slurry properties?- What is the appropriate thickening time?- How does one insure that the cement will not be contaminated with the mud?- Are pipe centralization and rotation necessary?- Waiting on cement time?

• Cement Volume:- Abandonment plugs' lengths and depths are usually stated by government regulations.

- Whipstock plugs must be of considerable length to provide for a gradual deviation of the bit. Cement excess and reversing might be necessary in the case of exact depth for whipstocking. Care must be taken to avoid excessive hydrostatic pressure on lower, depleted or weak zones, otherwise the plug will not be placed at the desired depth.

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Job Design ConsiderationsWhy is the cement plug being set?

At what depth will the plug be set?

Across which formations is the plug going to be set?

At what density should the slurry be mixed?

What is the BHT?

What volume should be pumped?

What is the required thickening time?

How to insure the cement will not be contaminated by mud?

Are pipe centralization and rotation necessary?

Waiting on cement time?

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•Cement Slurry , Density, and Compressive Strength:-Lighter slurries are usually used for lost circulation control to avoid the loss of the cement into the formation.

-For sidetracking purposes, the ideal is to get a set cement with a compressive strength higher than that of the formation. This can often be achieved by having a reduced water or higher density slurry. On average, a 15.8 ppg slurry develops a final compressive strength of at least 5,000 psi. A reduced water slurry of 17.5 ppg develops a final compressive strength of at least 8,500 psi. When the difference between cement density and hole fluid density is high, the cement will tend to fall through the lighter fluid. Addition of sand or any other weighting agent will not improve the compressive strength of a lower water content slurry.

-For better control of the slurry density, the batch-mixing technique is preferable. Slurry densities usually range from 15.6 ppg to 17.5 ppg to insure good compressive strength development.

-Viscous slurries with high gel strength are needed for lost circulation plugs to restrict flow into voids or fractures.

-When placing the cement using coiled tubing, the slurry rheology needs to be lower to avoid high friction pressures.

• Thickening Time and WOC Time:-Early compressive strength depends heavily on the thickening time. Rig time can be saved with a proper slurry design. The slurries must be designed for a thickening time in accordance with well conditions and job procedures, plus a reasonable safety factor. Recommended time for WOC should be 12 to 24 hours. Since the well temperature for a cement plug job is difficult to know accurately, a common practice is to allow for longer WOC times. A minimum of 500 psi compressive strength is normally recommended for drilling out cement.

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Slurry PropertiesDensity - lighter for Lost CirculationDensity - heavier for SidetrackingDensity - homogeneous - batch mixingRheology - higher for Lost CirculationRheology - lower for placement with Coiled TubingCompressive Strength - higher for SidetrackingCompressive Strength - less important for Lost CirculationCompressvie Strength - minimum 500 psi for drill outThickening Time - enough for placement - 1.5 to 2 hours

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• Evaluation of the Job, Reason for Failures:-After the WOC time has elapsed, the job results are evaluated. This is normally done by tagging the cement. Depth of top of the plug and hardness of the cement are the key indicators to measure the success of failure. Whenever a cement plug has failed to meet the objectives of the job, the reason(s) for the failure should be carefully investigated to modify and improve the design of the repeated attempt and to be successful the next time.

-Cement plug failures can occur for the following reasons:-Lack of hardness (sidetracking)-Poor isolation (plugback, abandonment)-Wrong depth (all plugs)-Not in place due to sinking to the bottom (all plugs)-Not in place due to loss to thief zone (lost circulation)

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Reasons for Cement Plug Failures

Lack of hardness (sidetracking).

Poor isolation (plugback, abandonment).

Wrong Depth (all plugs).

Not in place due to sinking to the bottom (all plugs).

Not in place due to loss to thief zone (lost circulation).

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• Reasons for failure can be traced to the following:1. Slurry not designed for enough compressive strength.2. Not enough WOC time.3. Inaccurate BHST.4. Cement contamination during displacement and POH.

-Contamination in annulus-Overbalanced displacement

5. Slurry not designed for the specific problem (lost circulation).6. Not enough cement volume.7. Difference between cement and hole fluids' densities too high, causing the

plug to sink

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Reasons for Failure Can be Traced to the FollowingSlurry not designed for enough compressive strength.

Not enough WOC time.

Inaccurate BHST.

Cement contamination during displacement and POH.

Slurry not designed for the specific problem (lost circulation).

Not enough cement volume.

Difference between cement and hole fluids' densities too high, causing the plug to sink.

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•Plug Cementing - Conclusions

1. Place the plug in a competent formation (i.e., a hard formation) to avoid losses.

2. Use ample cement, especially for sidetracking purposes.

3. Use a tailpipe through plugback intervals.

4. Use centralizers on the tailpipe where the hole is not excessively washed out.

5. Use a drill pipe plug and a plug catcher for positive indication of slurry depths.

6. Circulate the hole sufficiently before running the job. Use a mud of low yield point and low plastic viscosity, but of sufficient weight to control the well.

7. Ahead of the cement, run a high vis pill that is compatible with the mud and will prevent the cement from sliding down from the hole.

8. Use spacers and washes to combat the effects of mud contamination during placement of the slurry. Densified cement with a dispersant are also useful.

9. Allow ample time for the cement to set.

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Plug Cementing - ConclusionsPlace the plug in a competent formation (i.e., a hard formation).Use ample cement.Use a tailpipe through plugback intervals.

Use centralizers on the tailpipe where the hole is not excessively washed out.Use a drill pipe plug and a plug catcher.

Condition the well before running the job, using low YP and PV mud, but of sufficient weight to control the well.

Ahead of the cement, run a high-vis pill that is compatible with the mud and will prevent the cement from sliding down the hole.

Use spacers and washes to combat the effects of mud contamination.Densified cements with a dispersant are also useful.Allow ample time for the cement to set.

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•DP Centralization to ensure homogeneous positive mud removal•Viscous pill placed below cement slurry to avoid “sinking” cement slurry through light 9.0 lb/gal mud.

•Diverter tool used, to make sure that there is no downwards jetting action that might break/penetrate the pill

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Drill Pipe Centralization & Diverter Tool

DRILLPIPECENTRALIZED

9.0 lb/gal MUD

9.0 lb/gal MUD

SPACER

16.0 lb/gal CEMENT

DIVERTER TOOL

9.1 lb/gal VISCOUS BENTONITE PILL

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•No diverter tool is being used - jetting action of cement breaks the viscous pill and cement slurry “Falls” down to the bottom leaving no cement.

•Bad centralization/standoff causes bad mud removal, some mud behind DP is not removed, resulting in bad cement, mud contamination up to POH workstring.

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Bad Cement Placement Technique

BENTONITE PILL

9.0 lb/gal MUD

9.0 lb/gal MUD

SPACER

Contaminated CEMENT

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Horizontal WellBad Placement Technique can have dramatic effects as there are just few inches of travel between the bottom and the top of the pipe.

CEMENT SLURRY

MUD

3-Consideration on Cement Plugs

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