Hole-cleaning.pdf

Hole Cleaning

Objectives On completion of this module you will be able to:  Describe the different Flow regimes and their effect on cuttings removal  Explain the effect of inclination on hole cleaning  Understand the effect of pipe rotation for hole cleaning  Describe best practices for hole cleaning  Understand the impact of mud properties on hole cleaning

Hole Cleaning Problems The problems associated with inefficient hole cleaning include: 1. Decreased bit life and slow penetration rate resulting from regrinding of drill cuttings. 2. Fill-ups near the bottom during trips when the mud pumps are off. 3. Increase in annular density and, in turn, annular hydrostatic pressure of mud. The increased hydrostatic pressure of mud can cause the fracture of an exposed weak formation resulting in lost circulation. 4. Bridging and Packing leading to pipe sticking.

Hole Cleaning and Stuck pipe  As many as 1/3 of Stuck Pipe events in non deviated wells are hole cleaning related  In high angle wells, as much as 80% of Stuck Pipe is hole cleaning related

UNDERSTANDING HOLE CLEANING IS THE KEY TO PREVENTING STUCK PIPE RELATEDNPT& $$$

Plastic Viscosity  As total solids increase, free liquid decreases  Less available liquid to move in  More chance of physical interference as particle move  PV increases as solids increase  Think of traffic on the highway  Road is the Liquid  Cars are the solids  What is happening to the cars?  What happens to hole cleaning efficiency?

Conclusion: High Plastic Viscosity DOES NOT help hole cleaning

Yield Point  It is the force required to start things moving  Due to Electro-Chemical interactions of Particles  Force to start particles shearing by each other  YP is the intercept of the Rheology Curve  YP is a STRESS (lb/100ft2) (like a Pressure) +

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+

+

+

+

+

+

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Platelets of hydrated Gel behave like little magnets  Repulse Each Other  Also attract Each Other

Annular velocity & Flow profiles The AVERAGE upward speed of the mud in the annular space

24.5xQ AV  Dh2  Dp2 Velocity depends on where you are in the flow stream

AV= ft/min, Q= gpm, D=in

Flow regimes

LOW

FLOW RATE

Plug Flow

If we pump faster

Laminar Flow

If we pump really fast

Turbulent Flow

HIGH

Almost all the mud flow with the same velocity Except wellbore wall , there is no fluid movement

Center of the wellbore has a higher flow velocity. There is no flow on the wellbore wall

Flow is chaotic Flow on the wall of the well bore is not zero

What flow profile cleans the hole “best”?  Turbulent! But!??

 Pressures and rates can be really high  High YP muds may not be easy to push into turbulence

Cuttings Behavior in Laminar flow  Cuttings move fastest in the center of the stream  Cuttings tend to “slide” towards the wall of the hole  Then slowly settle…

Laminar Flow and Cuttings Recycling  As cuttings move to the wall  Fluid is moving faster on one side of the cutting relative to the other  Bernoulli Lift sucks the cutting back into the flow stream It is obvious that laminar flow can only be a compromise rather than a first choice This compromise must be optimized

Approaching the best Flow Profile Since turbulent flow is difficult to achieve:  Laminar flow with flat flow profile  Adjust Rheological parameters for a flatter profile

Effect of a high PV on the flow profile

 As cuttings slide towards the wall  Profile becomes steeper

The “Best” laminar flow in straight hole

 YP flattens the curve  PV elongates the curve  Low n flattens the curve but has side effects  LSRYP is important to carry cuttings on the flat profile

Best Flow Profile: High YP & LSRYP - PV As Low As Possible

Free Settling - Forces acting on a cutting Let’s consider a cutting in a well Gravity is acting DOWN on the cutting Buoyancy is acting UP on the cutting: • It makes the cutting lighter The viscous drag around the cutting is acting UP on it: the cutting has to force its way through the mud • It makes the cutting slower The cutting is accelerating until the forces are balanced It then falls at constant velocity = Free settling at terminal velocity

A particle falling at terminal velocity Force due to gravity is counterbalanced by - Buoyant force - Viscous drag around the particle

Vslip

STOKESLAW 2  part  mud d part  138*







In a perfectly Newtonian Fluid, In a completely laminar environment, With perfectly spherical bodies, With Re < 0.1

Cuttings transport in straight holes

CUTTING MOVES UP

VTransport=Vann- Vslip

Circulating Mud Moves Cutting up Vann Cutting Falls Down

Vslip

HOLECLEANING EFFICIENCY=VT/Vann

Hindered Settling Here is our Static Cutting in a Mud Column Mud has Static Gel Strength (lb/100ft2) Solids will fall at their terminal velocity ( based on MW, Density, Viscosity) As the cutting falls, it displaces it’s own volume of fluid upward as it moves downward Each cutting that drops, pushes nearby cuttings up This dramatically slows the slip velocity in static mud

This is called “HINDERED SETTLING”

Transport Ratio A transport ratio is a measure of the efficiency of cuttings transport. It is defined as:

Transport Ratio  FT 

Transport Velocity Annular Velocity

Vslip VT 1  FT  Va Va The slower the cuttings are removed, the lower the transport ratio, and the higher the concentration of cuttings in the annulus: it is an excellent measure of the carrying capacity of a particular drilling fluid  If Slip Velocity =0 Transport Ratio=1 (perfect cleaning)  If VSlip = Va then FT will be zero (no cleaning, no settling)  If VSlip > Va then FT will be negative (no cleaning, settling)

Cuttings concentration in the annulus  As a bit drills it generates cuttings Q s  A b * ROP Q

s

Qs

 0 .7854 d 2 * ROP * 7 .4805 ( gpm ) h 144 * 60 d h2 * ROP  ( gpm ) 1471

 In the annulus, there is a fraction f of cuttings and (1-f ) of mud, both of which have an upward velocity

Qs VT  and fAannulus

Qmud VAnnulus  1 f Aannulus

Cuttings concentration in the annulus FT 

VT VAnnulus

Qs fAannulus  Qmud 1 f Aannulus

Q h f  s  Qs  FQ t mud

d 2 * ROP d h2 * ROP  1471FQ t mud

Annular Mud Weight  sf  m (1 f ) Thus we can define the fraction of solids in terms of the rate of generation of cuttings and of the flow velocity of mud

Cuttings Concentration To prevent hole problems, it is generally accepted that the volume fraction of cuttings (or concentration) in the annulus should not exceed 7%. Therefore, the design program for mud carrying capacity should also include a figure for the drill cuttings concentration in the annulus.

Example Assume we are going to drill a 17-1/2” hole at 150 ft/hr with 9.5 ppg mud. We can pump at 750 gpm Assume that we have 20” 94# casing and 5” DP. Assume a cuttings rise velocity of 30 ft/min and a Section TD of 3,000 ft  What is the Transport Ratio?  What is the Return Mud Weight?  How fast can I drill so that I never have a drill solids concentration higher than 7%?  My boss does not want to drill that slow. He wants to drill at 200 ft/hr, then circulate for a while to reduce the hydrostatic. How Long do you have to circulate to have an average 7% DS content in the hole. What would you recommend and why?

Transport Efficiency and YP (120 ft/min Annular Velocity) For ranges normally used YP Changes have marginal effect on Hole Cleaning

Transport Ratios Based on Hopkin's Slip Velocities

95.0

Transport Efficieny (%)

85.0

75.0

65.0

To be effective must be very aggressive

55.0

45.0

35.0 0

10

20

30

40

Yield YeildPoint Point

50

60

70

80

Mud weight and cuttings transport Effect of MW on Cuttings Transport

95.0%

Transport Ratio

85.0% Water

75.0%

10ppg 16 ppg 20 ppg

65.0%

55.0%

45.0%

35.0% 0

10

20

30 40 Yield Point

50

60

70

Drill Pipe Rotation  As the rpm increases, the pipe rattles around the hole  Mud drags on pipe due to its gels  Viscous torque  Produces a velocity vector at a tangent to the pipe  Increases Velocity at the wall  The velocity at the pipe wall is no longer zero  Keeps cuttings off the walls

VTang

pipeOD  RPM *  * * eff 12

Drill Pipe Rotation CENTRIFUGAL EFFECT

ROTATING

TORQUE EFFECT

NO ROTATION

(VELOCITY GRADIENT)

  

Increases Velocity at the wall Keeps cuttings off the walls (Velocity and mechanically) Contributes more as angle increases

Hole cleaning in low angle wells (<30 deg)  Transport Velocity is the key  To increase Cuttings Removal Efficiency (Transport Ratio)  Increase Annular Velocity  Increase Mud Weight  Increase Mud Properties (YP, Gel Strength)

Straight Hole Cleaning Parameters ‘High’

Cuttings density

Influence on cuttings

Mud Weight

Cuttings size

transport

Flow Rate

LSYP YP RPM

‘Low’

PV ‘Low’

Ability to control

‘High’

Hole cleaning in low angle     

Cuttings will settle in mud YP and Gels are important for hole cleaning PV tends to steepen the flow profile and reduce hole cleaning Keep the PV as low as possible The flattest flow profile possible cleans the hole best Pipe Rotation increases the annular velocity (marginal in vertical wells)

HOLE CLEANING IN DIRECTIONAL WELLS

Cleaning inclined wellbores 1

WHAT WORKS HERE 2

MIGHT WORK HERE WILL NOT WORK HERE

3

4

0

30

60 Hole Inclination

90

Hole Cleaning: Difficulty vs. Angle Difficult

I

II

III

IV

Easy

0

30 Inclination

60

90

As angle increases things change… THE CONCENTRATION OF CUTTINGS AT THE LOW SIDE INCREASES 

Locally overloads the mud system (>5%)

CUTTINGS ARE NO LONGER BEING SUPPORTED BY THE MUD  Some cuttings are supported by the wellbore itself A CUTTINGS BED BEGINS TO FORM 

The Stabillity is

highly dependant on the angle

WHY?? GRAVITY WORKS AGAINST US  Cuttings distribution changes FLOW PROFILE CHANGES 

Carrying capability changes

PIPE IS ECCENTRIC 

Does it “stir” the mud?

Vertical Wells Homogeneous Suspension  Gravity aligned with flow  Cuttings scattered evenly in the mud  More at the wall due to flow profile

As Angle Increases Heterogeneous Suspension High side

Low Side

 Gravity no longer aligned  Shorter distance to travel  Cuttings are forced to the wall  By flow profile  By gravity  A lot more cuttings on the Low Side.

AS ANGLE INCREASES  Segregation is obvious  Mud profile changes  Cuttings in the top move with the mud  Others move slower  Cuttings on the bottom Slide down  Cuttings recycling  Build up and get back in the flow stream  VERY COMPLICATED MECHANISMS

Gravity forces segregate cuttings The fluid tracks the wellbore but gravity still points to the center of the earth. Cuttings tend to accumulate on the “Low Side” of the hole. As long as the cuttings are in suspension they behave more or less as we expect. If they are not in suspension they are forced to “ Low Side” of the hole where they may slide down. Low Side

The flow profile is non-symmetrical

AN EQUILIBRIUM IS FORMED SOLIDS

 Tops has fast moving thin mud

10 ft/min

 Bottom has high solids heavy mud

150 ft/min 100 ft/min

 Solids “ fall out” of the mud

100 ft/min

50 ft/min

 Boundary layer at the mud cuttings interface

0-3 ft/min

50 ft/min

DP

0-3 ft/min

FLOW PROPERTIES ARE CRITICAL We would like to have a FLAT profile High YP Low PV High LSYP No “re-drilled” solids

Boycott settling

Rapid settling of individual particles onto the existing bed once a critical mass has collected Particles can slide down the wall of the annulus very rapidly

Saltation transport

Low Side Lift and Drag moves the cuttings back into the flow stream Cuttings can move in the flow but now gravity pushes it back down Cuttings fall back to bottom almost immediately

Cuttings bed  IF A BED IS FLUIDIZED  Transmits hydrostatic  Responds to hydraulic forces  Can be cleaned and moved easily

 IF A BED IS NOT FLUIDIZED  No hydrostatic contribution  Responds to mechanical forces  Very difficult to clean and move (mechanical agitation)

Cutting beds  Stable non moving Cuttings Bed

80 Deg

 Dynamic cuttings bed

35 Deg

60+ DEG: region of stable cuttings Beds >30&<60 degrees >60 degrees

Cuttings Transportation

Dune Transport

Moving Bed

Rapid settling of individual particles onto the existing bed

Stationary Bed

Hole Cleaning in various angles

Increasing annular velocity

Zone 2

Zone 1 Zone 3

 Cuttings beds start forming at angles above 30°  Hole angles between 30°- 60° are hardest to clean

Zone 4

Zone 1-Efficient hole cleaning Zone 2 - Good hole cleaning with moving cuttings beds Zone 3 - Slow removal of cuttings Zone 4 -Some hole cleaning, cuttings bed formed Zone 5 - No hole cleaning

Zone 5 0

30 60 Well inclination (degrees)

90

OPTION ONE: TURBULENCE RULES Static S Sttaattiicc

50 5500ffpm ppmm

fpm 1150 15500fp pmm 100 fpm 110000fp pmm

If you pump hard enough you may clean the hole

What flow cleans the hole the best ? TURBULENT BUT

 Pressures and rates can be really high  High YP muds may not be easy to push into turbulence  Most wells are in laminar flow  Turbulence flow may lead to hole wash out  Pump capacity may not be sufficient to provide high enough flow rates  High flow rate= High ECD= Lost circulation

OPTION TWO: ROTATING PIPE

As RPM Increases pipe rattles around the wellbore

Eventually it acts like a gyroscope. Pipe rotates around the wall in a counter rotational manner.

Pipe rotation is critical  Viscous mud less effective with eccentricity and no rotation

 Viscous mud much more effective with eccentricity and rotation

Hole Cleaning: flow parameters  Turbulent Flow  High-velocities and eddies can erode beds and transport cuttings

 Laminar Flow  Lower flow rates, rheology and rotation are critical

AAvv== 220000 fp fpm m AAnngglele ==9900 cccc==44% % ++00.5 .5XXCCDD 115500 rp rpm m

AAnnggle le == 5555 RRoota tatio tionn == 115500 rp rpm m

The conveyor belt

Rotation creates fluid movement in the bed Pipe rotation around the wall creates velocity at the cuttings bed Velocity lifts the cuttings and causes frictional drag Cuttings are lifted to where mud is moving

Pipe rotation limitation

THE TRICK TO THE PROBLEM IS TO GET THE CUTTINGS ONTO THE CONVEYOR BELT!!

Simple??

Recommended maximum DS RPM for PDMs Curve section

Recommended maximum Drill string RPM for PDMs in Tangent or Straight Section

Absolute Maximum Drill string RPM for PDMS in Tangent or Straight Section

Correct RPM to load conveyor?  To assist hole cleaning the pipe must rotate to obtain velocity at the wall

 Dependant on     

Hole Size DP Size Hole Angle Eccentricity Sufficient RPM

 To benefit hole cleaning cutting must be in the flow stream

 Dependant on    

Cuttings Geometry Low Shear mud rheology Rheology in faster flow stream Flow Rate

Pills for hole cleaning • • • •

The use of the right sweeps may improve hole cleaning Combined rotation when pumping sweeps Monitor carefully sweeps Pill volumes depends on hole size (check hydrostatic pressure effect)

17 ½” or 16”

12 ¼”

8 ½”

50+bbl

50-30 bbl

20 bbl

Pills for hole cleaning High Viscosity Pills: A highly viscous pill will be more effective in vertical hole. At high angles, viscous pills deforms over the bed without disturbing the bed. Do not use as first option when annular space is restricted.

Low Viscosity Pills: The base fluid usually has a low viscosity and may become turbulent at lower flow rates. Use of a low viscosity pill alone may not be successful. It will not be able to carry the cuttings up a vertical section of the hole or suspend the cuttings when the pumps are stopped. (CAUTION)

Pills for hole cleaning Weighted Pill A weighted pill comprises base fluid with additional weighting material to create a pill weight 2 to 3 ppg heavier than the mud. This type of pill will aid hole cleaning by increasing the buoyancy of cuttings slightly. This type of pill is usually used as part of a tandem pill.

Tandem Pill (also called Combination pill ) This consists of two pills, a low viscosity pill followed by a weighted pill. Tandem pills can be very effective at stirring up cuttings. If the hole is full of cuttings and a tandem pill is pumped, there is a chance the amount of cuttings stirred up can cause a pack-off. (CAUTION)

Recommended Bottoms up Hole Size

Inclination

Circulation

17 ½” to 12 ¼”

> 30 deg

At least 3-4 btm-up circulations at optimum parameters.

17 ½” to 12 ¼”

< 30 deg

At least 2 btm-up circulations at optimum parameters.

8 ½” to 6”

> 30deg

At least 2 btm-up circulations at optimum parameters.

8 ½” to 6”

< 30 deg

At least 1.5 btm-up circulations at optimum parameters

Circulate until shakers are clean this may take several circulations, do not stop circulating if the well is not clean

REMEMBER – All cases          

Use highest annular velocity regardless of flow regime MW has a direct relationship to hole cleaning at all angles. YP has an impact by slowing the rate at which particles settle. Cutting need to be in suspension for YP to have an effect. LSYP is critical where velocity profiles are poor Dispersed muds can help cleaning by dissolving the cuttings, but may create washouts and create solids control problems. The most desirable mud is a clean mud with low PV Sweeps band aid poor rigs, poor muds, poor solids control and or poor practices. Monitor the sweeps and what they bring to surface Learn to listen to the well and respond as it talks to you

REMEMBER – 0-30 DEG WELLS  The best mud system considered for deviated wells should be modified     

versions of those proven effective in vertical and near-vertical offsets in the area. Maintain LSYP 0.4 - 0.8 times hole size Use the lowest PV you can Don’t let PV and or gels build with native solids. Do not expect pipe rotation to help as much as in directional wells Hole cleaning will be a problem first at doglegs, washouts and casing seats.

REMEMBER – Between 30 to 60 deg  Good Hole-Cleaning parameters considered in one interval may be inadequate in another interval.

 At low annular velocity & high hole angle expect avalanching (especially 30-60 degree).

 Hole-cleaning and well bore instability sometimes respond best to an increase in the mud weight.  An increase in annular velocity improves hole cleaning  Rotate pipe at high RPMs to prevent/remove beds.  Maintain LSYP at least equal to the hole diameter (in inches)  The higher the angle, the longer it takes a cutting to get out Fast pipe rotation coupled with mud weight and proper LSYP values is the only viable way to clean hole sections at angles between 30 and 60 degrees

Remember  The mud and pumps are a conveyor belt that can hold about 5-7 % drill solids at any time and place in a vertical well.  As the angle increases the capacity of the conveyor decreases. Do not out drill the conveyor belt  Remember that you have to clean all the well, not just the part you are drilling,  you can only drill as fast as you can clean the worst section of the hole.

Guidelines on mud properties Yield Point

Mud Weight (ppg)

Fann R6 & R3

> Hole Size (in)

LSYP {(2*R3) - R6}

> Hole Size (in)