HOLE CLEANINGBy

Capt. Janeshwar Prasad

Chief Chemist

IDT-ONGC-Dehradun

1

Hole cleaning is normally monitored by-

Continuous monitoring of standpipe

pressure / pump output ratio.

Cuttings’ volumes.

Torque and drag will help in detecting

cuttings bed formation.

2

`Volumetric Cutting concentration = Vol. of cuttings in annulus Total annular vol. Transport ratio =Vc / Va

Vc = VELOCITY OF CUTTING =va – vs

Va= ANNULAR VELOCITY

Vs =SLIP VELOCITY

3

CONTROLABLE VARIABLES1. Drilling fluid weight 2. Drilling fluid flow rate3. Drilling fluid rheology4. Drilling fluid regime5. Rate of penetration6. Hole angle 7. Hole size

4

UNCONTROLABLE VARIABLES

1. Drill pipe eccentricity

2. Cutting density

3. Cutting size

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6

Vs =

Slip velocity = 2 g dc ( ρc – ρf ) 1. 12 ρf g = Gravitational constant dc = Diameter of cutting ρc = Density of cutting ρf = Density of fluid

7

175 d (21- w) 0.667

(1) Vs = ----------------------- w0.333 m0.333

Vs = Slip velocity fpmd = average cutting

diameter, inw = mud weight, ppg, andm = annular viscosity, cps

(2) Vr = Va - Vs

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(3) LC = 100 Vr -----

Va

LC = lifting capacity %

Vr = cutting rising velocity, fpm (Vc)

, and

V a = annular velocity, fpm.

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10

MUD DENSITY

÷ CUTTING DENSITY

* 100 = % LIFT

8.33 ppg ÷ 21 ppg * 100 = 40 %

10.5 PPG ÷ 21 ppg * 100 = 50 %

16.0ppg ÷ 21 ppg * 100 = 76 %

A slight increase in mud weight Significantly effects cutting slip velocity improves the transport ratio .Change in momentum with change in mud weight M2 = M1 * ( ρ2 / ρ1 )

11

SOME OF THE LIFT COMES FROM :-

Friction of the mud passing by the cutting.

Friction also helps drag cuttings off the wall and back into flow.

Friction is influenced by mud weight.

12

Annular velocity is the second most influential factor affecting hole cleaning efficiency in a vertical well.

Fluid annular velocity is a key parameter in cuttings transport. If an hole cleaning problem exists cutting transport may be improved by increasing flow rate to its maximum value with flow regime and pump limitations. 13

Annular velocity - Average annular velocity

Flow profile – velocity of flow at various distances

Flow profile causes unequal distribution of forces on cutting causing the larger cuttings migrate to the wall and slip down the well

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Flow profile - depends on flow regimes

Flow regime=relationship between pressure and velocity

Turbulent Laminar Plug flow

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FLOW VERSUS PRESSURE

16

TURBULANT FLOW

LAMINAR FLOW

Psi

VELOCITY

Laminar flow is governed by the viscous properties of the fluid. The fluid flows smoothly, with all molecules moving in the same direction, but at different speeds. laminar flow profile is parabolic.

Turbulent flow is more chaotic. Molecules moving with different speeds in all directions .

17

The effect of annulus eccentricity on cuttings transport is rather slight under lower angles of hole inclinations either in laminar or turbulent flow .

The laminar flow provides better cuttings transport than turbulent flow in the range of lower angles of hole inclination.

18

Apparent viscosity Plastic viscosity Yield point Yield point – represents the force

required to initiate the flow or cause molecules to shear past each other

Plastic viscosity – additional force required to cause the mud to flow at a higher rate is represented by plastic viscosity . 19

Apparent viscosity – represents the total pressure required to cause a certain flow rate. It is a combination of both yield point and plastic viscosity.

Apparent viscosity decreases as shear rate increases.

20

In laminar flow , higher yield point / plastic viscosity ( Yp / Pv ) ratios improve cuttings transport .

in turbulent flow, mud rheology has little effect on cuttings transport.

21

Reduction of viscosity as the shear rate increases is known as the shear thinning and is desired quality of mud .

The higher the yield point and lower the plastic viscosity, the more shear thinning the mud will be.

Mud that has high shear thinning properties has a flatter flow profile and is more likely to exhibit plug flow . 22

SLIP VELOCITY-

INCREASES WITH THE SIZE AND DENSITY OF THE CUTTING .

ALSO INCREASES AS THE CUTTINGS BECOME MORE SPHERICAL.

23

DESCRIPTION PV YP INITIAL

GEL

10 MM. GEL

Thick mud

Intermediate mud

Thin mud

Water

16

14

8

1

37

21

8

0

13

13

2

0

29

22

3

0

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Controls both the size and amount of cuttings generated.

High ROP Production of large and more cuttings.

Increase in torque with higher ROP Bit is digging deeper. Large cuttings. Torque continues to build Poor hole cleaning.

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Pipe rotation improves the cutting transport ratio.

Pipe eccentricity in the annulus reduces the cutting transport ratio.

Flow profile more elongated on the side with no pipe, and velocity is very low around pipe

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It takes time To circulate cuttings away from

the bit and BHA before making a connection

To circulate the hole clean before tripping out of the hole.

A large number of stuck pipe incidents can be traced to not allowing enough circulating time before connection or trip

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BOYCOTT SETTLING

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In a directional well.

There may be substantial cuttings beds even though the shakers are clean and torque and drag is moderate.

The reduction of slip velocity has a diminishing effect as hole angle increases , the axial component of slip velocity decreases 29

Angle of inclination Mud properties and flow regime Rate of penetration Time Flow rate Cuttings beds Pipe rotation and eccentricity

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0 –30 DEG.( VERTICAL )

30 – 65 DEG. ( TRANSITIONAL )

65 DEG. + ( HORIZONTAL )

DIRECTIONAL WELLS

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10 20 30 40 50 60 70 80 90 100 ANGLE OF INCLINATION-DEG.

VOL. CUTTING CONC.

LARGE DIAMETER HOLE

SMALL DIAMETER HOLE

Experience has shown that deviated holes with hole angles in the 40 to 65 degree range are the most difficult to clean.

This is due to the tendency of cuttings to form beds and to slide back down the hole.

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Cutting concentration increases dramatically and so cutting bed formation between 30-45 deg.

Cuttings tend to settle out of the 40 to 550. Section more rapidly than in other sections, due to a phenomenon known as boycott settling.

Cutting beds remain constant at higher angles, > 65 deg. They are more packed and harder to disturb .

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Homogeneous suspension is the most efficient transport mechanism.

Cutting rolling or bed transport is the least efficient transport mechanism .

Formation of cuttings beds can be suppressed to a degree by using good suspension characteristics.

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Mud weight

cutting concentration increased drastically

between 35 –45 deg. At low mud weights.

Cutting bed heights was substantially

reduced with small increase in mud weight,

at any angle. The sliding and avalanching of cuttings beds

occurred less frequently with heavier muds.

36

Cutting beds are more fluedalized in

heavier mud and are thus more easily

disturbed.

The minimum velocity needed to

initiate cutting rolling is less with

heavier mud.

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Viscous mud cannot penetrate the cutting beds as easily as less viscous mud.

The eccentric drill pipe pushes the flow profile away from the cuttings beds.

Less viscous fluids promote higher fluid velocities under the eccentric drill pipe.

Hole cleaning in high angle hole sections improves as the flow behavior index (n) Increases and as yield point and plastic viscosity decrease.

38

When annular fluid is laminar, thin, turbulent

sweeps followed by thick sweeps may aid

hole cleaning by first stirring the cuttings,

then sweeping them out of the hole .

A change in rheology has less effect when

the pipe is rotating. This is because pipe

rotation adequately disturbs cutting beds .

39

Hydraulic hole cleaning may be

supplemented by mechanical means.

Wiper trips and drill string

rotation( when possible ) disturb the

cuttings bed and encourage

transport . top drives are beneficial by

allowing pipe rotation and circulation

while tripping .

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Pipe rotation is required more with

high viscosity muds than with low

viscosity muds .The viscosity effect is

pronounced with water base muds

than OBM .

Cutting beds tend to slide more with

oil base muds than with WBM .41

Rate of penetration : - The rate of penetration influences

the size and amount of cuttings. The rate of penetration may have

an effect on hole cleaning in the lower angle and vertical sections of the well, however .

42

Flow rate is the primary hole-

cleaning parameter and maximum

pump rate, within the constraints

of maximum ECD and the potential

for downhole losses, will provide

optimal maximum pump rate, will

provide optimal hole cleaning. 43

Most influential factor At lower mud weights , more annular

velocity is required . Annular velocity =flow rate / cross

sectional area of annulus

44

the influence of pipe rotation

increases as the angle increases

Without pipe rotation , cuttings

beds are almost certain to exist

in high angle wells.

45

Cutting beds do not usually cause

problems while rotating . it is when

the pipe is moved axially that it

may become stuck.

Pipe rotation has significant effect

at high angle wells by tearing up

cutting beds .46

Pipe eccentricity has significant effect in high

angle wells because of its effect on flow

profile. At higher angles of inclination, the pipe is

laying on the low side of the hole and the reduction of velocity greatly hampers the cutting transport .The effect is more pronounced with laminar flow than turbulent flow.

47

has less effect on cutting beds size

than other factors, such as mud

weight , pipe rotation, and flow

rate .Cutting beds become more

packed and thicker as angle increases,

mud weight and flow rate decreases.

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CSF – Number of bottoms up to clean hole

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Angle 27 ½” 17 ½” 12 ¼” 8 ½”

0-30 deg.

2.25 1.75 1.5 1.25

30-65deg

2.75 2.5 1.75 1.5

65+ 3+ 3 2 1.75

Angle Depth (ft) CSF CSF* Depth(ft)

Total Depth

0 -30 0 -4000 1.5 6000

30-65 4000-6000 1.75 3500

65+ 6000-12000

2 12000 = 6000+3500 +12000 = 21500Ft

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Maintain adequate flow rates- especially

in directional wells.

A simple rule of thumb for vertical wells :

the annular velocity should be twice the

cutting settling rate . Another rule of thumb : 1000 gpm for

17.5 inch hole , 750 gpm or more for 12.25 inch hole and 500 gpm or more in 8.5 inch hole 51

Control of ROP Stop drilling when HOLE condition

dictate, plan wiper trips Circulate hole clean before pooh.

Use pipe rotation to disturb cutting beds in deviated wells.

Maintain adequate mud properties. Proper Yp /Pv ratio is desired .

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0 –30 DEG.( VERTICAL )

30 – 65 DEG. ( TRANSITIONAL )

65 DEG. + ( HORIZONTAL )

DIRECTIONAL WELLS

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0 – 45 DEG.

45– 55 DEG.

55 DEG. +

DIRECTIONAL WELLS

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•Highest possible flow rate should be applied in all ranges of inclinations , but particularly in the range of higher angles (region III: 55-90 degrees).Assuming that the effect of a slide down of the cuttings bed takes place also in an actual drilling , then above recommendation becomes critical for the angles of inclination 40-45 degrees.•For the range of lower inclinations the laminar flow inside an annulus with highest possible mud yield value should be used.

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Assuming that the range of mud yield values within 0-20 lbs/100 sq. ft. is the range commonly used in the field , then the mud yield value 20 lbs/100 sq. ft. is recommended.

The highest possible YP/PV ratio is recommended for this region. Assuming that the range of YP/PV ratio within 0-2 is the range commonly used in the field , then the YP/PV ratio = 2 is recommended .

In the range of intermediate inclinations (region II: 45-55 degrees), either turbulent or laminar flow may be used . 56

Usually, when a high flow rate is in use, a larger amount of mud materials ( bentonite , chemicals ) are necessary to mix the mud providing laminar flow.

Therefore, turbulent flow is rather preferable .

For range of higher inclinations (region III : 55-90 degrees ) turbulent flow is definitely preferable . At same time , YP/PV ratio still important in this region & should be maintained as high as possible . 57

Application of high – viscosity slugs in order to clean annulus after drilling is stopped makes sense only in region I ( 0-45 degrees ) . These slugs will not help in region II ( 45-55 degrees) and may even worsen the cleaning rate in the region III ( 55-90 degrees ) .

When annular fluid is laminar, thin, turbulent sweeps followed by thick sweeps may aid hole cleaning by first stirring the cuttings, then sweeping them out of the hole . 58