3. pag-1002d hole clean in high-angle

BAROID

An Effective Approach to Keeping the Hole Clean in High-angle Wells

Applying weighted sweeps to keep the hole clean is poorly understood and often misapplied. Here is the best way to get good results

Mike Sewell, Newfield Exploration Co.; and Joe Billingsley, Baroid Drilling Fluids, Halliburton

Published in World Oil Oct. 2002 Vol. 223 No. 10

Drilling a high-angle well is almost always accompanied by hole-cleaning issues.

Traditional approaches such as high-viscosity sweeps, high rotary speeds, fast

pump rates, frequent short trips and lubricants may buy time if the well reaches

total depth (TD) quickly enough. However, in long, openhole intervals, hole

cleaning should be effective, or the operator may face sidetracking the well.

Among operators who are drilling high-angle wells, the focus is shifting from old

"cuttings removal" models to application of a sweep method that actually reaches

and removes the silt bed accumulating on the low side of the hole. As early as

1986, hole-cleaning research indicated that turbulent flow produced by relatively

thin drilling fluid is more effective at silt bed removal than a high-viscosity flow

profile.1 The silt bed, composed of formation sand and barite, can create the same

torque, drag and overpull conditions associated with differentially stuck pipe -

even in a cased hole.

Described here are: the basic application of weighted sweeps; limitations /

applications of design models; and effects of viscosity, rotation speed and

drillstring sliding. Basics of designing sweeps are presented, along with specific

application recommendations.

"Jet Stream" Sweeps

Consistent results in silt-bed removal have been achieved with fully-circulated,

low-viscosity, weighted sweeps which exceed drilling mud weight by 3 - 4 ppg and

provide a 200 - 400-ft column in the annulus. The weighted sweep is built from

Once the sweep reaches bottom and returns to surface, it can remove a significant quantity of silt.

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the mud in use; and the yield point remains at its normal drilling value. Viscosity

increase is minimal and results from barite additions, rather than from thickening

the mud. If pumped regularly at the normal circulating rate - while rotating pipe at

80 rpm (minimum) - once the sweep reaches bottom and is allowed to return to

surface, the weighted sweep can carry a significant, visible quantity of silt to the

shakers and mud cleaner. Operators routinely using this method observe radically

different "before and after" hole conditions.

For example, while planning a high-angle, West Cameron (Gulf of Mexico) well,

Newfield Exploration Co. received a "torque prediction analysis" from the

directional drilling service. For this well, where the angle would reach 55°+ with a

120° left-hand turn, torque at TD was expected to be 22,630 ft-lb. Instead, actual

torque peaked at 15,000 ft-lb, Fig. 1. Weighted sweeps, rather than high-vis

sweeps, were circulated throughout the drilling operation. No beads or lubricants

were used. Hole cleaning issues were minimal.

Fig. 1 - Weighted Sweep application on Gulf of Mexico well.

The end-of-well report included this directional driller's description of conditions in

the tortuous, final 1,300 ft of the 9-7/8-in. hole, from 9,900 to 11,200 ft (MD):

"The assembly slid without any trouble, which I felt was unusual for the profile of

this well."

Old Models Fall Short

Flow rate and fluid viscosity are usually evaluated first when a hole cleaning

problem is suspected. While the flow profile is the basis for designing the right



hole-cleaning program, many misapplications rely on inappropriate or incomplete

modeling.

Many traditional models are based on an "average annular velocity," which implies

a uniform flow regime. This applies to straight holes and vertical sections of

directional holes, but does not apply from the kick-off point (KOP) down. Field

evidence now demonstrates that flow profile deviation due to pipe eccentricity may

be more important than other parameters typically considered for hole cleaning.

Pipe eccentricity can markedly influence the calculated flow profile for a given

point in the borehole, as shown in Fig. 2.

Fig. 2 - Flow visualization for Herschel-Bulkley model (rotating) for pipe eccentricity of 0.05 (where 0.0 = Centered pipe; 1.0 = Drill pipe touching wellbore).

Power Law and Bingham Plastic models were necessary for oilfield applications

when only slide rules and hand calculators were available. Computers now make it

possible to use the more sophisticated Herschel-Bulkley model. Although more

exact in its calculations, the latter does not yet account for drill-pipe rotation,

making it the better choice for depicting the dead areas while slide drilling, Fig. 3.

The Power Law model also does not account for rotation but, because it also does

not address gel strengths or low-shear-rate stresses, it becomes more appropriate

for analyzing rotating pipe.



Fig. 3 - Herschel Bulkley vs Power Law for rotating and sliding scenarios.

Other Effects, Increased Viscosity

In conventional wells, barite "sag" began to occur when deviated wells exceeded

30°. Mud weight out was lighter than mud weight in, despite the absence of gas or

water flow, which might account for density reduction. The corrective mud

treatment required raising the low-shear-rate rheology (three- and six-rpm Fann

viscometer readings), making the entire mud system thicker. The yield point also

increased from the 6 to 10 lb/ft 2 range to as much as 30 lb/ft 2 . Thicker fluids

were perceived as cleaning the hole better; but exaggerated flow on the top side

of the hole meant more sluggish fluid movement on the low side, and even faster

silt / barite buildup.

While rotating, mud under the drill pipe is not quite stagnant, but is very sluggish,

Fig. 4. The calculated flow profile in this figure, for 0.5 eccentricity, indicates a

problem area under the drill pipe. Since fluid follows the path of least resistance,

the thicker the fluid, the more pronounced the flow deformity. There is an

optimum low-end rheological value - based on 6-rpm Fann readings - beyond

which, hole cleaning becomes more difficult.



Fig. 4 - How viscosity increase affects flow profile.

Weighted sweeps have an entirely different effect. The 2-D and 3-D flow profiles

depicted here reveal an abnormal "jet-stream" flow in the annulus which is

capable of removing fines from the low side of the hole. When drag from pipe

rotation combines with gravity due to weight variance, i.e., a weighted sweep, this

jet stream appears to flow through the narrow part of the annulus and scour the

bottom side. Further, the increased mud weight provides more buoyancy, which

helps lift fines and silts out of the hole. The success of the above mentioned case

well supports this conclusion.

Rotation speed. Pipe rotation speed contributes to cuttings delivery. For a

weighted sweep to be effective, the pipe should always be turning while the sweep

circulates up the annulus. Increased rotation speed causes significant step

increases at 100 - 120 rpm, and again at 150 - 180 rpm (Fig. 5); but there is a

down side to relying on high pipe-rotation speeds.2



Fig. 5 - Effects of pipe rotation speed.

Directional tools required for aggressive directional programs often limit the rotary

to 60 - 80 rpm. The tools can malfunction at higher speeds and defeat efforts to

achieve efficient hole cleaning. Further, increased pipe rotation speed may

improve delivery of drilled cuttings to surface, but not necessarily remove or

prevent fines accumulation on the low side of the hole. Pumping a weighted sweep

achieves fines removal at the normal rpm range.

Sliding and the dead zone. Slide drilling can create "dead spots" where sludge

and silt may build up. Since sliding is unavoidable in most high-angle wells, it is

critical to understand the dynamics of silt-bed formation.

Slide drilling puts a large length of the drillstring in contact with the low side of the

borehole. Even the most liberal flow profiles show small stagnant areas adjacent to

the drill pipe / wellbore contact. During connections and trips, the stagnant mud

pockets allow some barite or fines accumulation on the low side of the hole.

The revised Cuttings Export model - which provides optimized rheological and

pipe-rotation parameters - calculates the ability of different sweeps to transport

normal drilled cuttings, and rates the sweeps accordingly, Fig. 6. However, the

ability to transport cuttings to surface does not, by itself, assure a clean borehole.



Fig. 6 - Cuttings transport efficiency (Zero = Normal drilling).

On another Gulf well, the "drill ahead hydraulics" model predicted hole cleaning

problems would occur while drilling 5,800 ft of 7-in. hole at a 40°+ angle.

Weighted sweeps were pumped while sliding, but were not circulated out

completely prior to surveys and short trips. When a short trip was attempted in

5,600 ft of open hole, it was necessary to backream. Multiple tight spots and

packed-off areas were encountered. Some of these packoffs were attributed to a

balled bottomhole assembly, causing operators to ignore the silt-bed buildup.

Another series of weighted sweeps was pumped, this time while rotating pipe,

based on the model's effectiveness indications. The sweeps were circulated out

completely. The hole was cleaned, and drilling continued to TD with no further

problems.

Designing a Weighted Sweep

A weighted sweep is usually 3 - 4-ppg higher than the mud weight and spans 200

- 400 ft in the annulus, contributing about 50 psi of hydrostatic pressure. The drill

fluid must have good rheological and fluid-loss properties before the sweep is

mixed and pumped, i.e., a loose mud with a high filtration rate will not sweep as

well and may exacerbate any existing problem. The sweep must make it all the

way to surface.

Normal to slightly decreased pump rates (200 gpm or less) are recommended for

pumping weighted sweeps. As a rule of thumb, if the end of the sweep is less than



0.5-ppg over original mud weight (at the flowline), the sweep was probably too

short or too light, or both. When building a weighted sweep with higher viscosity

mud, a greater weight difference may be necessary to achieve results.

Weighted sweeps are often pumped as preventive maintenance, particularly in

larger hole diameters, where annular velocity and flow profile are less than ideal

for cuttings and silt-bed removal. The directional driller and drilling fluids engineer

can coordinate timing of the sweep with sliding and rotating to ensure optimal

results.

Shaker-screen mesh size influences how much of the removed silt bed is visible as

the sweep crosses the shaker. A 210-mesh screen usually shows convincing

results. On one of the first wells to experiment with weighted sweeps in an oil-

based mud system, the first sweep returned 16 bbl of silt. Subsequent sweeps

yielded 4 - 12 bbl per run. Drilling resumed with no further problems on the

formerly high-torque, packed-off hole.

The following recommendations will assist in design and application of an effective

weighted sweep. However, weighted sweeps will not always eliminate torque and

drag in complicated wells. Proper well design and planning are the first defenses

against a sidetrack scenario.

Recommendations

The following procedures are recommended for most-effective, weighted-sweep

application:

l Calculate sweep height in the vertical part of the annulus, and added hydrostatic head in the annulus.

l Rotate at 80 rpm+ while pumping the sweep, 180 rpm+ is preferred. It is not recommended to pump a weighted sweep while sliding.

l Keep pumping until the sweep is out of the hole. If pumping stops while the sweep is in the deviated hole, it falls out of the "jet stream" and redeposits captured fines.

l Avoid increasing viscosity of the weighted sweep. l Plan to run 3 - 4 sweeps to achieve desired results when hole-cleaning

problems are indicated. One sweep will probably not be sufficient to remedy an existing problem.

l Avoid checking for flow or closing the annular with the weighted sweep in the drill pipe.

l If desired for preventive maintenance: 1) schedule sweeps every 6 - 8 hr or 500 - 1,000 ft (whichever comes first, unless the hole dictates greater frequency); barite / silt build-ups are dependent on time and footage; and 2) pump a sweep while circulating bottoms up before tripping, and another



once back on bottom.

In summary, this innovative and effective practice challenges long-held beliefs

about sweep viscosity and pipe rotation. Nonetheless, objections to weighted

sweeps - that they will "knock the bottom out" or "take too much time"- will

quickly subside as more high-angle wells are drilled with fewer sidetracks.

Literature Cited

1 Azar, J., A. Pilehvari and S. Shizari, "State of the art cuttings transport in

horizontal wellbores,"SPE Drilling and Completion, Sept. 1999, 14:3, pp. 196 -

199.

2 Krepp, A., "Hole cleaning in ER wells: It's not the mud man's job,"Drilling & Well

Performance Technology Newsletter, 4th Quarter 1999, pp. 6 - 7.

The authors

Michael Ray Sewell is a Drilling Engineer for Newfield Exploration

Co. One of the founders of the company, he has over 26 years'

experience in the industry, including 12 years with Tenneco as a

drilling / production / reservoir engineer. He holds a BS in PE from the

University of Missouri, 1975, and is a member of SPE and SPWLA.

Joe Billingsley is a Senior Technical Advisor for Baroid, a division of

Halliburton. He holds a BS in physics from Texas A&M University and

has over 22 years' experience in the industry. Mainly, he focuses on

trouble-shooting problem wells, as well as international assignments.

He is a member of SPE, API and AADE.

9/18/2003

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