When ladened guck are used in largely swerved wells, there's the tendency for barite to settle towards the low side of the hole, creating a position of lighter slush on top and heavier slush on the bottom. The heavier subcaste also begins to resettle down along the length of the hole due to the gravitational pull. This nonstop movement of the slush prevents the development of more substantial gel strengths, compounding the settling problem. Variations in the hydrostatic head can beget conformation fracturing with coexisting loss of rotation, conceivably leading to an affluence of conformation fluids. Barite sag can be worrisome and time- consuming to correct, and thus, veritably precious. While slack is further of a problem in angled wells, it has also been observed in perpendicular wells.

  

How Barite Sag Is Initiated

The medium by which barite slack develops isn't easily defined but is believed to do under low shear rate conditions. In laminar inflow, shear is generated because fluid layers near the wall or pipes move at a slower rate to that further from the wall or pipe. The haste of a subcaste relative to the subcaste next to it's called the shear rate.


Shear rate is defined as the haste grade measured perpendicularly to the fluid inflow and is generally reported in complementary seconds( sec- 1).

The generally held belief is that in an inclined wellbore; low shear rates sufficiently intrude with the gel structure of slush to allow downcast movement of the coarser barite patches. The stir of these patches displaces lighter fluid up towards the high side of the hole. The perception is that the range of shear rates needed to set up this geste

is predominant underneath an eccentric pipe. Some dubieties have been raised regarding this area of the well’s figure being the only contributor to slack. It's delicate to understand how sometimes large slush weight reductions can be generated from such a fairly small volume of slush.

It seems more probable that a slack medium can live within the total volume of the slush, with the “ agreement ” of barite moving to the low side of the hole. Barite sag is boosted at the low side, by the veritably low haste present in that area of the well. Flow circle testing has proved that a slack medium can be set up at shear rates as high as 95 sec- 1.


In reaching these conclusions numerous wells were anatomized and only one prevalence was set up where barite slack was reported while drilling. This passed while sliding and was corrected by short ages of gyration. All other reported cases of barite slack coincided with the running covering, particularly after lacing the slush (mud)

indication of Sag

Barite sag won't be seen in the face tanks. It can be only detected by constantly checking the fluid viscosity at the inflow line.

The most critical time for Barite sag is during ages of relative inactivity when the fluid has had minimum agitation, times similar as

*First, When sliding( directional drilling)*

*Second, While running covering/ covering liner( One of Types Of covering)

*Third, While logging.

When rotation is proceeded after these types of conditioning, the viscosity at the inflow line must be checked continually and the data recorded. All results must be acclimated to compensate for oscillations in temperature, using a temperature map, located at the shale shakers.





A symptom of barite slack would be when viscosity checks at the shakers, see a drop in viscosity, followed by an increase, to be also followed ultimately, by slush weight of the correct viscosity. This idealized cycle of slush weights seen at the shale shakers may not be seen as it could be masked by new barite additions.



literal data has shown that weight measures indicating heavy slush, isn't always seen to follow light slush and that the correct viscosity is also only achieved when fresh barite is added. Heavy slush has been seen on some occasions.



Whether or not heavy slush is measured on any particular occasion may depend on carriage exertion. For case, the barite may have settled on the low side and circulating without gyration, may not introduce it back into the circulating slush, and not be seen at thesurface.However, if heavy slush is to be seen, it may be after passages with pipe( check also tripping pipe procedures), If this is the case. It's delicate to image a slack- related medium that could affect in heavy slush being present in the annulus, without the presence of light slush nearly in the circulating system. still, the absence of reported light slush may be explained by the use of heavy slugs, pumped previous to pulling out of the hole.

Delved cases of slack have shown reflective symptoms, with the before appearance of light slush at the shakers, generally as soon as the pumps were turned on, also followed by a gradational increase in viscosity until the correct slush weight, or on occasion, some heavy slush has been seen.

Barite Sag Causes

Incidents of barite slack have been reported on largely extended or swerved wells on multitudinous occasions since themid-1980’s. Sag can do in either dynamic or stationary conditions and may be indicated by variations in slush weight when circulating.

Hole conditions which may impact slack tendency are

*Temperature – Advanced temperatures increase slack tendency.
*Hole angle – slack tendency is increased in diversions> 30 °.
*stationary time – Although slack can do under dynamic conditions, its goods are generally not apparent until the slush system has been stationary for a considerable period of time – after tripping, logging or running covering.
*Semi-Static conditions – Minor movements which break gels, similar as tripping pipe or running wireline logs increase slack tendency. Slow rotation rates can produce conditions likely conducive to slack.
*Hydrocarbon fluxes can affect slush rheological parcels

mud parcels which impact slack tendency are

*Rheology, Surfacevs. Downhole – Sag occurs indeed when traditional rheology measures taken under face conditions( high PV, YP and Gels) indicate that it should not. When measured under downhole temperature and pressure conditions on a FANN 70 viscometer, guck which parade sagging geste
in the well-conditioned generally demonstrate different rheology and suspense characteristics than their normal face measures. The degree of friction between face and downhole rheology is to some extent a measure of the stability of the slush system. The lower friction the more stable the slush system. A crucial factor that effects the friction in rheological geste
in an invert conflation slush system is the type of base oil painting used. The base oil painting’s density versus temperature geste is critical.
*mud weight – Variations in viscosity will be more pronounced at advanced slush weights.

Ways to help and Cure Barite Sag

1 Rheology

The Hershel- Bulkley/ Yield- Power Law model more correlates with lab measured slack portions, since it more directly describes fluid geste
at low shear rate.

*A fluid Tau0( yield stress) of 7 to 8 lbs/ 100 sq ft will typically be enough to reduce the static slack in field guck to respectable situations.

*Check the slush rheology at elevated temperatures(e.g. 120 °F) to gain an suggestion of downhole rheology.

*Use FANN 70 testing before the well to optimize slush product attention for stable downhole slush rheological parcels.

*Testing with special outfit called the High Angle Sag Tester( HAST) simulates downhole conditions and shows whether a fluid requires special cumulative treatments to ameliorate suspense parcels.

*Additions of DURATONE have been shown to reduce slack tendency.

2 Mud Weight

Maximum and minimal slush weights should be recorded when circulating bottoms up after passages in swerved wells, especially after logging. It's important to maintain invariant slush weight throughout the circulating system. sweats should be made to treat and equate any imbalance as snappily as possible.

still, this could bear circulating until the viscosity is homogeneous previous to continuing drilling operations, If the original circulating viscosity( ECD in Drilling) is close to the fracture grade.
When using invert conflation guck in high- temperature wells, it's important to measure the temperature at which the slush weight is recorded to avoid misconstructions between barite slack and thermal expansion/ compression goods.

3 oil painting/ Water rate

HAST tests have shown that dwindling the oil painting/ water rate decreases slack tendency.

4 Accoutrements and Systems

The stylish treatment to help barite slack is to insure sufficient gels and low- end rheology.

*In Water Grounded slush this can be achieved with several products, including AQUAGEL and AQUAGEL GOLD SEAL, and polymers similar as BARAZAN PLUS.
*In oil painting Grounded slush and synthetic guck the use of low end rheological modifiers similar as RM- 63 in confluence with GELTONE and SUSPENTONE( a suspense agent for invert mixes) have been used successfully to help barite slack.

5 Drilling Practices

*Modify operations to maintain sufficient annular disturbance through acceptable rotation and gyration rates, to limit ages of low shear. Any operation that induces low shear( slow circulating rates, running covering or running logs) has the implicit to initiate slack. Laboratory work has verified that the shear rates generated when running covering are of the right magnitude to induce barite slack in an inclined borehole.

*At the planning stage of wells linked as having a high slack eventuality, consideration should be given to rotary drilling rather than sliding. Sag is most likely to do when the drill pipe isn't being rotated while low shear conditions are being generated. This is particularly important for 60- 75 degrees wells where indeed high annular rapidity may not exclude slack. Minimize sliding in these intervals. Pull back one stage and rotate at high rpm after extended ages of sliding to introduce heavy slush from the low side of the hole into the main body of the slush.

*Avoid pumping base oil painting slugs to “ wake up ” the MWD tool particularly incontinently before a trip. The oil painting can thin a significant portion of the slush and promote conditions conducive to barite slack.

*In situations where slush weight is close to fracture grade it's important to minimize the implicit increase in nethermost hole pressure due to sagged slush. After a trip, the implicit threat of circulating bottoms up in the digression section is that some light slush can be circulated out of the hole while being replaced by slush from the suction hole at the correct viscosity. The heavier, sagged, slush in the deeper part of the digression section coupled with the slush of correct viscosity now in the upper part of the hole could beget increased nethermost hole pressures initiating downhole losses.

*Again, in situations where the severance pressure is close to slush weight and barite slack is suspected minimize the effect of low viscosity slush on hydrostatic by carrying the drilling assembly into the hole, breaking rotation and circulating bottoms up at several points while running backin.However, the effect on the annular hydrostatic pressure will be reduced if the eschewal of balance column isn't circulated out all at formerly, If the fluid has sagged.

*still, stop and condition the slush, If inordinate changes in slush viscosity are observed at face. Allow a minimum of two total slush gyrations to enable chemical treatments to be effective and to insure all viscosity oscillations are excluded.

 
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