Plug Choices and Techniques
When a loss zone cannot be stabilized with sealing materials it may be desirable to try a plug. Several choices and techniques are available; however, in all cases a reasonably accurate estimate of depth of the loss is required prior to setting the plug. Spinner surveys, radioactive traces, and temperature surveys are most commonly used for this purpose.
The following is a typical list of plug choices. These are generally classified as soft, medium or hard plugs:
a. Soft Plugs
- DIASEAL M squeeze *
- Attapulgite squeeze *
b. Medium Plugs
- Bentonite-diesel oil squeeze (gunk)
- Cement-bentonite-diesel oil squeeze
- Invert, bentonite-diesel oil squeeze c. Hard Plugs
- Cement
- Barite
- Depends on filtration to produce a firm plug.
The following are examples of material combinations for several of the above plugs:
a. Bentonite-diesel oil (10 bbl - 11.0 lb/gal)
- 7.5 bbl diesel oil
- 21 sx bentonite
- 50 lb coarse mica
- 50 lb fine mica
- 10 lb fiber
- The density of the mixture can be in-creased with barite.)
b. Cement-bentonite-diesel oil (10 bbl - 11.2 lb/gal)
- 6.75 bbl diesel oil
- 13.5 sx bentonite
- 13.5 sx cement
- sealing material (as desired)
- The density of the mixture can be in-creased with barite.)
c. Invert bentonite-diesel oil (10 bbl - 16.4 lb/gal)
- 5.9 bbl water
- 10 lb caustic soda
- 5 lb chrome-lignosulfonate
- 32.5 sx organic, amine-treated bentonite (oleophilic)
- 30 sx barite
- Mix the caustic and lignosulfonate into the water, then add the oleophilic bentonite, and finally, the barite.)
Note: The invert bentonite-diesel oil (BDO) system reacts with oil or oil-based mud just as the normal BDO system reacts with water.
Squeezes
Formulation of Soft Squeezes - DIASEAL "M" and Attapulgite - DIACEL "M" and attapulgite squeezes depend on lack of filtration control to produce a firm plug. They are sometimes called high water loss squeezes.
An attapulgite or DIACEL "M" squeeze is formulated as follows: 1. Clean mixing pit thoroughly.
- 6/94 11-18
- Add desired volume of water and mix 50 lb/bbl of DIASEAL "M" or attapulgite.
- Pump down drill pipe and place in suspected loss zone. If annulus is not full, pump mud down annulus while pumping squeeze down drill pipe. When annulus fills and squeeze is in place, apply 150-200 psi on annulus with annular BOP closed. This will "soft squeeze" the material into the loss zone.
Formulation of Diesel Oil-Bentonite and Diesel Oil-Bentonite-Cement Squeeze: The principal underlying diesel oil-bentonite and diesel oil-bentonite cement slurries is that large amounts of bentonite and cement can be readily mixed with diesel oil. When the plug encounters mud or water, the bentonite and cement hydrates, forming a dense plastic plug which can be squeezed into the loss zone.
The following is a formulation procedure for a diesel oil-bentonite or diesel oil-bentonite-cement squeeze:
- Add desired volume of diesel to a clean pit. Make sure no water is in the pit.
- Add bentonite or bentonite and cement. Make sure no water is encountered by the mixture.
- Pump 5 barrels of diesel down drill string ahead and behind the slurry. Displace to the loss zone with mud. Pump and displace at a rate of 3 bbls/min.
- If the annulus is not full, pump mud down the annulus at a rate of 4 bbls/min while pumping the squeeze.
- After the plug is placed in the loss zone and if the annulus is full, squeeze with 300-400 psi with the annular preventer closed.
- Pull the drill string well clear of the squeeze. (Mix and place the squeeze with a cementing unit, if possible.)
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