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Mixing and pumping barite slugs to dry the drill pipe before tripping out of a well is one of the most common of all rig operations, yet when the pipe fails to pull dry, it can be frustrating for the crew and expensive for the operator. Slugging procedures on many rigs are a haphazard affair and often, if the slug attempt fails, the crew is not really sure what went wrong.

Slugging is often ignored in well planning because in comparison to other operations, it is a rather small detail. Some operators that are aware of potential waist, do have slugging policies. Some only allow the use of sack barite for slugs: others set limits for slug densities. Such policies can be difficult to enforce and sometimes result in false information appearing on reports.

Pulling the pipe dry is important for several reasons. More well control incidents, leading to blowouts, have occurred on trips than during any other routine rig operation. When tripping out of a well, it is essential that the volume of steel removed from the well be replaced with an equal volume of the working liquid (mud) in order to maintain bottom hole pressure. It is difficult to measure hole fill-up accurately when pulling a wet string.

Personnel safety is also a primary concern. When mud is spraying around at every broken connection the floor and tools are quickly covered. It is difficult for the crew to work safely while trying to maintain their balance on the slippery floor. Mud lost on wet trips also represents money spent unnecessarily. Not only is whole mud lost during the trip, but there is also the cost of time and material required to re-build volume. Finally, drillfloor morale is an important consideration. No crew enjoys working in a constant shower of drilling mud. Spirits sag, and so does efficiency. The fluid technician (mud engineer) who tries to keep accurate records of the barite used for slugs may be shocked at the cost over a 30-day period, especially if bulk material is used.

The discussion below takes a look at the slugging process in order to develop some procedures that, when applied to most rigs, should enable the drill crew to pull dry pipe consistently.

If the drilled borehole with the drillstem inside is thought of as a U-tube, and both sides of the tube are filled with the same density fluid, the hydrostatic pressure exerted by the fluid is equal on both sides of the tube. When a fluid of different density is added to one side of the tube, the side with the denser, (heavier) fluid will displace the lighter side until the tube returns to a balanced condition. This phenomenon can be seen on the rig when drilling fast, in tophole formations. The annulus becomes charged with cuttings if the drilling rate exceeds the circulating rate. The cuttings increase the effective density of the fluid in the annulus, and when the kelly or topdrive is broken off to make a connection, mud flies in all directions. The mud on the annulus side of the U-tube is displacing the cleaner, lighter mud on the dillpipe side.

When slugging the pipe, the goal is to deliberately unbalance the system so that the drillpipe side of the U-tube will slightly displace the annulus side. In other words, the hydrostatic pressure in the drilpipe must be greater than that in the annulus for the slug to be effective.

Since hydrostatic pressure depends upon the height and the density of a column of fluid, the pipe can be slugged with a small (short) heavy slug, or a large (long) lighter slug, provided that the mud system was balanced before the slug was pumped. So long as the hydrostatic pressure is greater in the drillpipe, the system will become unbalanced and the fluid level inside the drillpipe will fall to the point of balance.

There are no established standards for slug fall. Rig equipment, experience, present operations, and company preferences are all variables that must be considered. However, given certain conditions, it is possible to make some estimates in order to determine the density and length required for a good slug. In other words, what slug density and volume would create enough imbalance to ensure that the drillpipe would pull dry throughout the trip?

The first step in calculating slug densities is to decide what length of empty drillpipe would the slug create, or how far should the slug drop? The slug density required to achieve the drop in a balanced system can be estimated once the desired slug drop is determined.

Assume the following:

5 inch, 19.5 lbs/ft drillpipe. Capacity 0.01776 bbls/ft (0.0093 m3/m)
Slugging pit with 25 barrels (4 m3) available to the pump
Mud weight of 12.0 ppg (1.4 sg)

1. Solve for the approximate length of a 25 bbl slug in the 5-inch drillpipe.

Bbls / bbls/ft = ft
25 / 0.01776 = 1408 ft. (429 m)

One joint of Range II drillpipe is about 31 ft (9.43 m) therefore, if the rig pulls triples, that is, three joints to a stand, one stand will be about 93 ft (28.3 m) long. Assume that a slug drop of about a stand and a half, that is, 150 ft. (45.7 m), is desired.

Solve for the slug density required to cause a drop of 150 ft (45.7 m).
Fluid density x [(slug drop / slug length)] + 1 = slug density
12 x [(150 / 1408) + 1)] = 13.27 or 13.3 ppg (1.6 sg)

As long as the desired slug drop and the pipe diameter remain constant, the ratio of the slug drop to its length can be treated as a constant value and used for various system densities as shown in the examples below.

The constant for the example well is:

(150 / 1408) + 1 = 1.106534

Assuming 150 feet of drop, determine the slug densities required in 9.0 ppg, 14.0 ppg, and 16.0 ppg systems respectively.

9.0 ppg system x 1.106534 = 9.95 or 10.0 ppg (1.2 sg) slug
14.0 ppg system x 1.106534 = 15.5 ppg (1.9 sg) slug
16.0 ppg system x 1.106534 = 17.7 ppg (2.1 sg) slug

Notice that as the system density increases, a heavier slug is required to realize the same 150 ft slug drop. In order to maintain the same slug drop in systems with higher mud weights, either a greater slug volume, or a higher slug density is required. Suppose the slug volume is increased from 25 to 30 barrels (4.7 m3).

Length of slug in drillpipe is 30 / 0.01776 = 1689 ft (515 m)

Ratio from the formula with the longer slug is (150/1689) + 1 = 1.108881

9.0 ppg system x 1.108881 = 9.8 ppg (1.2 sg) slug

14.0 ppg system x 1.108881 = 15.2 ppg (1.8 sg) slug

16 ppg system x 1.108881 = 17.7 ppg (2.1 sg) slug

It can be seen that the additional 5 barrels (0.79 m3) makes little difference in density requirements, even though the slug occupies an additional 281 ft (85.6 m) of drillpipe (1689 - 1408 = 281).

It is not necessary to work the calculations often. The drillpipe size does not change frequently, and the slugging pit has a limited capacity. The decision between pumping a long, lighter slug or a short, heavier slug is a matter of operational preference. If the U-tube is unbalanced, the slug should fall. If the slugs are mixed according to these guidelines, then there is little else that the worker who mixes the slug can do. There are, however, some other factors that will influence the effectiveness of slugs.

The U-tube should be balanced before the pipe is slugged. A difference of 0.2 or 0.3 ppg between the mud weight at the flowline and the mud weight going down the hole may make a considerable difference in slug fall. If the system is known to be out off balance and cannot be circulated, then a longer or heavier slug than normal will be required. The best solution is to circulate the annulus clean before slugging the pipe.

The slug must be displaced properly. If the driller leaves the slug above the rotary table, the fluid will not fall and the connection will be wet. Likewise, if the slug is greatly over-displaced, it will be ineffective. Slug displacement errors are fairly common. That is why some crews seem to have more trouble slugging the pipe than others. The blame, if there is any, should not always be borne by the mud hand. He is not responsible for circulating the annulus clean, or displacing the slug. Below are some suggestions for getting a good slug.

Learn, or calculate the volume in the lines from the slugging pit to the rotary table. Do not accept some number that has been passed on for years. Either look at the rig plans, or trace and measure the lines. This may be time consuming, but it only has to be done once. When an accurate volume has been determined, post it in barrels (m3) and pump strokes so all drillers will chase their slugs in the same manner. Using time rather than strokes for guide is not accurate.

When the pump efficiency is checked (as when bumping a cementing plug), apply the revised pump efficiency to the slugging process.

Calculate and post the information needed to build a good slug under several different conditions. For example, slug densities and volumes for various mud system densities, various drillstring diameters, volume per inch (cm) of the slugging pit, and the height of the suction off the bottom of the slugging pit. This should encourage all rig personnel to follow the same procedures.

In most cases it is not a good practice to mix a slug far in advance of its use. If it must be mixed in advance, good agitation is a must.

Be certain that the mud weight returning from the annulus is as accurate as possible before building the slug and make certain that the slug is weighed carefully a few minutes after the barite is mixed.

Agitate or stir the slug constantly.

Mix, rather than dump the barite into the hopper. Dumping causes settling and inaccurate weights. It may also plug the bit nozzles.

If the hopper discharge into the slugging pit is directly above the suction, rig up some sort of splash shield. Discharging on top of the suction will cause aeration, resulting the inaccurate weights and inefficient pumping.

If the slug is very thick, pump efficiency and slug fall will be affected. The best viscosity for a slug is about the same as that of the mud system. If slug viscosity is a problem, have the fluid technician (mud engineer) recommend the best type and amount of thinner required to adjust the viscosity of the slug.

In order to reduce waste; it is a good idea to estimate the amount of barite that will be required for the slug. Mix the estimated amount, or a little less; let it mix well, then weigh the slug.

If barite is packaged in 100 lbs sacks, then about 60 sacks (6000 lbs) will increase the density of 100 bbls 1.0 ppg. A 25 barrel slug should require about ¼ that much, or about 15 sacks (1500 pounds). Estimate the surge tank, mix about 15 sacks and weigh the slug. If barite is packaged in 50 kg sacks, mix about 12 sacks to 4 m3. Mixing in this way is faster, less work, and more economical.

There are several operations and conditions that can make slugging the pipe difficult or impractical, no matter how careful and conscientious the crew. Fishing jobs, long tapered strings, very small or partially plugged nozzles and extremely viscous mud fall into this category. In these cases, the driller must be patient and try to vary his techniques, seeking the right combination. Most of the time, with a little planning and good communication between crews, slugging problems can be all but eliminated. One thing is certain, good slugs save time, money and hard work.

To request the full 40 page document from PHD Precise in Russian or English, please email: support-ru@phdprecise.com

Labels: Drill pipe, dry pipe slugs, oil gas, slugging

posted by The Rogtec Team @ 11:33 

1 Comments:

 Anonymous said...

This is a very useful article--a lesson worth learning especially if you are involved in drilling operations

24 May 2009 20:14  

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