Mud Removal- NExT

Mud Removal

Copyright © 2001 – 2013, Schlumberger. All rights reserved

Objectives of Primary Cementation

Provide complete isolation of zones

– (Hydraulic Bond)

To support the casing

– (Shear Bond)

Protect casing string

2


Mud Removal & Cement Isolation

The Most and First important aspect of cement job

A 3-step process before cementing

1. Hole cleaning

+ Conditioning the drilling fluid

2. Displace the drilling fluid from the annulus

& Replace the mud by cement slurry

3. Cement is setting, properties and Isolation

should not be affected by contamination (mud..)

Avoid mud channelling

3


Efficient Cement Placement

Check for efficient mud removal to prevent mud channeling

and to ensure zonal isolation

Optimize

casing centralization

fluid properties :mud – spacer – slurry(ies)

pumping rate

Select Displacement Regime

Turbulent Flow

Efficient Laminar Flow

Select Preflushes & Spacers

Ensure

Flat interfaces between fluids

Avoid static mud

Wall cleaning 4

Bulk mud removal


The Flow of Fluids

Shear Stress τ

Shear Rate γ =

Apparent Viscosity

r

V1

F A A

A A

V 2

1 Poise = 100 centiPoise = 0.2089 lbf.sec/100ft2 - 1 Pa.s = 2.089 lbf.sec/100ft2

Oilfield units

dv

dr

v v

r

2 1

2100ft

secflb

rateshearstressshear

·

6


Flow Curves - Fluids Classification

Power Law

NEWTONIAN or NON-NEWTONIAN

Shear rate

LAMINAR FLOW

TURBULENT FLOW

T R A N S I T I O N

T R A N S I T I O N

Z O N E

Z O N E

Shear rate

Shear Stress Bingham

Plastic

Shear

Stress

T R A N S I T I O N

T R A N S I T I O N

Z O N E

Z O N E

Herschel

Bulkley

Power law & Herschel Bulkley fluids :

shear thinning fluids

Drilling and Cementing fluids : HB behaviour (API 13D:2006 – SPE98743) 7


Flow Models

For mathematical representation, following models are used:

1. Newtonian model t =

2. Bingham plastic model t = ty + p = p + ty /

ty BinghamYield p plastic viscosity

3. Power Law Model t =K = K

(Pseudo plastic model)

K consitency index (lbf.s^n/100ft²)

n flow behaviour index (dimensionless)

4. Herschel Bulkley model t = ty + K

(Pseudo plastic model with a Yield)

g ·

g · g ·

g n · g n-1 ·

g n · g n ·

g · ty + K

=

8


Fluid Flow Property Measurements

PROPERTIES MEASURED:

Shear stress

Shear rate

Gel strength

EQUIPMENT USED: “Fann” 35 (12 speed)

Ramp up then Ramp down

Readings @ 3, 6, 30, 60, 100, 200, 300 rpm.

3 and 6 rpm not used for Bingham model .

Rotational speed is proportional to shear rate

With R1B1 combination 100 rpm = 170 sec-1

Bob deflection is proportional to shear stress

With R1-B1 - Spring 1 , spring factor SCF =1.065

SCF*t

9


Flow of Fluids

1. Laminar Flow

Plug flow is sub-laminar flow

2. Turbulent Flow

In fluid mechanics two types of flow are defined:

V = 0

V = 0

V max

• Sliding motion

• Velocity at the wall = 0

• Velocity is maximum at the centre

Vmax = 2 V V = Average particle velocity

DIRECTION OF FLOW

• Swirling motion

• Average particle velocity is

uniform throughout the pipe

Laminar and Turbulent Flow regimes are found anywhere (pipe, concentric or

eccentric annuli)

10


Flow in Eccentric Annuli

Always

Vw > Vn

11


Wall Shear Stress

0

WSS

4 DL

(Dout - dint) DP

WSS = D DP

4 DL WSS =

In Pipe In Concentric Annulus

If WSS > ty of displaced fluid in narrow gap,

then flow occurs.

Mud, cement slurry : fluids with a yield (Herchel Bulckley, Bingham)

12


Effect of Casing Stand Off

The Effect of the Casing Stand-Off on the Annular Flow is

Qualitatively Equivalent to the Following Flow Pattern

Q

D1 D2

V2 V1

Q

L DP

DL

13


Newtonian Fluid of Viscosity µ Density ρ

In LAMINAR FLOW :

Velocity

Reynolds Number

DD

VV

2

1

2

2

1

2

DDIf 2 12

DVDVDV 111122

2

824Re

%67Re8Re 12for

Pipe stand off : 67%

14


Laminar Flow in Eccentric Annulus

Non-parallel plate model Ri/Ro = 0.8

0 10 20 30 40 50 60 70 80 90 100

1000

500

100

50

10

5

1

Vwide /Vnarrow

Stand-off %

n = 1.0

n = 0.5

n = 0.2

W

% Stand-off = w

RH - RC X 100

RC

RH

15


Newtonian Fluid of Viscosity µ Density ρ

In TURBULENT FLOW Velocity

Reynolds Number

714,0

1

2

1

2

DD

VV

DDIf 2 12

V2 =1 .64 V1 for 67% stand off

DVDVDV 111122

2

28,3264,1Re

Re 2 = 3.28 Re 1 for 67% Stand off

16


Turbulent Flow in Eccentric Annulus

0 10 20 30 40 50 60 70 80 90 100

1000

500

100

50

10

5

1

Vwide / Vnarrow

API Stand - Off (%)

n = 1.0

n = 0.5

n = 0.2

W

% Stand-off = w

RH - RC X 100

RC

RH

17


Eccentric annulus : Flow rates & regimes

Laminar Flow

Velocity Profile

(Sliding motion)

Turbulent Flow

Velocity Profile

(Swirling motion)

NO

NO

YES

YES

HB

Fluid

18


Flow Regime Comparison

Turbulent

Centered Annulus

Laminar

Eccentered Annulus

Turbulent

19


Turbulent Flow Reynold’s number correction

RH

RC

W

% STO = X 100 RH - RC

W

Correction factor to apply to centered annulus NRe

to provide turbulence in the narrow side

20


Turbulent Flow Guidelines

The best flow regime for displacement :

Cementing fluids in turbulent flow all around eccentered pipe

Function of standoff, annular flow rate, hole size

Contact time 10 min across zones of interest – Minimum contact time of 6 minutes – Must take u-tubing into account – Maximize by increased volume or decreased rate

For chemical wash Allows for contamination – consider a viscosity of 5 cp

BUT ! Due to differential density, interfaces are not stable in annulus :

Preflushes (Spacer/wash) density should be close and higher than mud density

turbulent flow ρ(2) displacing ρ(1) mud

21


Turbulent Flow Applications

Turbulent flow is generally achievable with difficulties :

To get a flow turbulent in the narrow size need to take in account – Annular size – Casing eccentration (stand off) – Rheology of cementing fluids : preflushes (spacer, washes), slurries

Turbulence in the narrow side could results in :

High rate, not compatible with pumping equipment

Well control : High friction pressure and risk of losses

Volume of preflushes/spacer : equipment, cost,…

Only for small casing diameters (< 7’’) in gauge hole with good centralization (> 80%) in a small annular gap.

If Turbulent flow not possible,to achieve a flat interface between fluids :

Use Effective Laminar Flow Conditions 22


Efficient Laminar Flow

Alternative flow to provide a flat displacement front

Four (+ 1) rules must be satisfied:

– Density differential

– Friction pressure hierarchy

– Minimum pressure gradient

• Mud in motion

• No Mud on the Wall

– Differential velocity criterion

– Turbulence to be avoided

23


Density Hierarchy

Density of the displacing fluid is greater than the density of the fluid

being displaced.

D ρ + 10%

spacer > 1.1 (mud)

cement > 1.1 (spacer)

24


Friction Pressure Hierarchy

Promotes a flat stable interface with less possibility of viscous fingering

Friction pressure of displacing fluid must be greater than friction pressure of fluid being displaced.

displacing displaced

> 1.2 DP

DL

DP

DL

D + 20% DL

DP

25


Minimum Pressure Gradient

To get a flow of the fluid (mud) all around the eccentric annulus

Wall Shear Stress must exceed the yield stress of the fluid on the narrow side:

Function of standoff

Applies only to fluids with a yield point

Translates into a lower limit for flow rate

Approximated from:

Q mini = Mud circulation rate above MPG

STO (DOH-Dc)

4ty

>

No Flow

Laminar Flow

WSS > ty

DP

DL

26


MPG and Mud On the Wall

To displace Mud by Spacer and Spacer by Slurry:

Avoid Stable layer of mud (or Spacer) left on casing and formation

Wall shear stress (WSS spacer)

if WSS Spacer>ty,mud => no mud film

if WSS spacer <ty,mud =>Mud On the Wall

- Looks” like a channel

– But thicker on the formation

– Dehydration at the formation face

L

x

0

v t

tMax

(WSS)

Minimum Pressure Gradient for the mud Mud in motion (eccentered annulus size E) if

WSS mud = (1/4) (E) > Gel mud

DP

DL E

27


Friction Pressure vs. Rate

0.0 2.0 4.0 6.0Flow Rate (bbl/min)

15.0

10.0

5.0

0.0

Frictio

n P

ressure

(psi/1

000ft)

Mud

Tail Slurry

Spacer

Annulus ID : 9.625 in - OD : 15.000 in

29


Calculating Annular Shear Rate

g (2471) Q

(Do - Di)2 (Do + Di)

where:

g Annular Shear Rate

Q = Rate (BPM)

Do = Outer Diameter (in.)

Di = Inner Diameter (in.)

Fann 35 Speed

(RPM)

Shear Rate

(sec-1

)

300 511

200 340

100 170

60 102

30 51

6 10

3 5

.

.

In concentric annulus

Annular shear rate should be compared to Fann measurements

30


Differential Velocity Criterion

In mud channel situation to prevent channel growth :

Displacing fluid does not flow faster on the wide side than the narrow side of the annulus

Function of standoff and density differential

Imposes a maximum annular flow rate

V2 narrow side > V1wide side Q

D1 D2

V2 V1

Q

L

DP

DL

Velocity

dP/dz

V2 > V1

V2 < V1

DP (displacing narrow side) –DP (displaced wide side)+ D .g cos > 0

V1

V2

31


Fluids will naturally climb faster on the wide (upper) side

Density drives displacing fluid to the narrow side

– Density hierarchy and differential velocity

– For non-newtonian fluids additional viscosity effects

Horizontal wells

– Axial vs. azimuthal

– Dynamic vs. static channeling

Differential velocity vs. MPG

– Static channel depends on ty

Channeling

c > s s

MPG

Diff Vel.

Mud

On the Wall 32


Effective Laminar Flow

Minimum Annular Rate

MPG Exceeded

Beginning of 20% Pfriction

Arbitrary limit of 1 BPM

Beginning of stable front

Maximum Annular Rate

Turbulence of displacing

End of 20% Pfriction

Arbitrary limit of 40 BPM

End of stable front

Assume deviation even in “vertical” wells

0.02 to 0.04°/100 ft vertical deviation

0.05°/100 ft azimuthal bearing change

33


Casing Stand Off

For Mud removal efficiency Use casing Centralisers

Objective SO > 80%

( minimum 75%)

Vnar Vwide

Always, Vnarrow < Vwide

Di

Do

w RH - RC

% Stand-off = X 100

W

RC

RH

S.O % = 100 W/ (RH - RC)

34


Flow improvement in eccentric annulus

Casing movement : Reciprocation & Rotation ?

35


Reciprocation

Movement of casing up and down during the job

Must be done from the start of circulation to end displacement

20 to 40 feet stroke

Needs scratchers

(cable type) to be effective

Possible excessive

swab and surge pressures

Excessive pull and buckling

Casing may become stuck

during movement

Cannot be the only method

of mud removal

Sta

tic

Mu

d

Slu

rry

Poor

Stand Off

Slu

rry

Slu

rry

Improved

Stand Off

1 to 5 minutes per cycle

36


Rotation

Circular movement of pipe

Must be done from the start of circulation to end displacement

10 to 30 rpm

Scratchers help efficiency

Needs special rotary

cement heads and power swivels

Torque must be very closely

monitored

Cannot be the only method

of mud removal

More effective than reciprocation

Mud

Almost

Removed

Rotation

Started

Casing

Stationary

Flowing

Cement

Gelled Mud

37

Spacers & Washes Cementing Plugs

38


Contamination : Fluids Incompatibility

Results In:

Detrimental Interface Reactions

High Rheological Properties

– Very high viscosities and high gel strengths

Change in Cement Slurry Properties

– Thickening time altered

– Increase in fluid loss

– Reduction in compressive strength

Reduction in Hydraulic Bond

Prevented By

Wiper Plugs in Casing

Compatible Preflushes in Annulus – Spacers and Chemical Washes

39


Cement placement: Down the casing

Down : Inside the casing :

Fluid interfaces are unstable (mud < spacer < cement)

Mechanical plugs should be used to separate the fluids

Top plug also designed to give indication of end of job (plugs bump on

landing collar)

Lack of bottom plug(s) will lead possibly to

– Fluid contamination (intermixing) or even fluid swapping

– Improper displacement in the annulus

– Poor cement at shoe (top plug scrapping mud film at casing wall)

40


Cement Wiper Plugs

Keep Fluids Separated in Casing and Reduce Contamination

Bottom Plug(s)

– Remove mud ahead of cement

– Prevent cement falling through lighter fluid ahead

– Wipe inner casing walls clean

– Use at least 1 ..or more if possible

• Long cemented interval

• Critical operation

Top Plug

– Separate cement from displacing fluid

– Positive indication of end of displacement

41


Why Run a Bottom Plug ?

Bottom plug wipes accumulated mud cake, scale, etc. from inner casing walls out through float equipment into annulus.

Volume of debris can be significant and fill-up shoe track if not removed ahead of the top plug.

Example : 9 5/8” 47 lb/ft (ID 8.681” 0.41cuft/ft) at 10000 ft (collar at 9820ft)

Volume of 1/32” film?

Height corresponding to this volume ?

Conclusions ?

SPACER

MUD MUD MUD MUD

SPACER

42


Why Run a Bottom Plug ?

Volume of residual mud scrapped by the top plug :

Π x ID x L x e

3.14 x (8.681 /12) x 9820 x (1/32 x 1/12) = 58.1 cuft

Length of 9 5/8 casing filled by scrapped mud

58.1 / 0.41 = 141.7 ft

Shoe track length : 180 ft

If an overdisplacement is occuring potential displacement of mud around

the 9 5/8 shoe

43


Cement placement: Up the annulus

Up : inside annulus

Fluid interfaces can be stable (mud < spacer < cement)

but casing has to be properly centralized.

Fluids density, rheology and pumping rate to be designed properly

depending on the flow regime (laminar vs. turbulent).

Improper displacement (design or execution) in the annulus will lead to :

– Mud/spacer channels in the annulus

– Mud/spacer films at the casing/formation walls

– Fluid contamination (intermixing)

44


Up the annulus to separate mud from cement :

USE SPACERS in order to prevent contact and incompatibilities

between drilling mud and cement slurries

Some mud additives are retarders for cement

– e.g. lignosulfonate (dispersants)

Others act as accelerators

– CaCl2

Drilling mud/cement mixtures can be very viscous (NABM):

– Absolutely avoided

– Higher friction pressures than expected

• possibly overcoming frac pressure

– Mixtures possibly very difficult to displace from the annulus (gelation)

Improve cement bonding by water wetting casing and borehole (NABM)

45


Spacers & Washes - Definitions

Compatible:

Capable of forming a mixture which does not undergo any undesirable chemical or physical reactions

Wettability:

The preferential adhesion of polar fluids, such as water, versus non-polar fluids, such as oils, to solid surfaces

46


Washes & Spacers

Spacers

Densified polymer fluids with insoluble weighting agent (generally barite)

Designed rheology for efficient laminar or turbulent flow displacement Fluid loss control should be required Contains always a surfactant when used with NABM

– compatibility,water wet surface.

Chemical Washes or Preflushes

Generally not densified (Brine) : water, diesel, or thin fresh mud CW contains additives to thin the mud, to control leak off as water wetting

surfactant (NABM) CW pumped in turbulent flow but are not really effective in annulus

– casing eccentration, Taylor instabilities

47


NABM - Spacer Surfactants Efficiency

48


Required Properties of Spacers

Compatible with all other well fluids

Stability (good suspending capacity)

Controllable density and rheology

Good fluid loss control

Environmentally safe and easy to handle in the field

Water Wet surface with NABM

50


Composition & Field Mixing Order

Water: Fresh or Brackish

Antifoam

Spacer Blend (viscosifier, leak off control)

Shearing and hydration

Salt (NaCl or KCl) : If required

Weighting Agent : CaCO3 < 1.35 sg (11.5 ppg)

Barite 1.35 –1.92 sg (11.5-16 ppg)

Hematite > 1.92 sg (16ppg)

Surfactant(s) for NABM : type and concentration depends on

base oil / spacer / mud used.

51


Cement placement : Turbulent or Laminar Flow

In all cases Prevent Cement contamination by using/pumping

Along casing down to shoe :

– Separation plugs : Bottom & Top Plugs • 2 Bottom plugs if possible

– Chemical wash ahead plug : • Mud dilution and/or turbulence

Up along the annulus : Spacer ( laminar or turbulent)

Chemical wash (brine?) only with

• Compatible mud ( WBM) with slurry

• Low density mud (< 1.20 sg)

A must do for compatibility with Non Aqueous Base Muds : Chemical wash & spacer + surfactant

Weatherford plugs

52

Conclusions & Resume

Mud displacement and removal


Criteria for Effective Mud Removal

Cementing Operation :

Centralize casing

Casing movement

Wiper plugs

Spacer and Washes

Flow regime selection

With

Conditioned mud in hole

55


Mud Removal

Hole Cleaning

– Controlled & optimized mud properties

– Low gravity drill solids < 6%

– Break gel strength

• Wiper trip and intermediate circulation RIH casing

– > 95% Total hole volume in circulation (calliper fluid)

• Calliper log

Conditioning Mud

– Lower TY and PV, flat gel

– Clean hole, LGS < 6%

– Maximum flow rate compatible with minimum frac pressure

– Rate above minimum rate to flow all-around pipe (MPG)

56


Conclusions: Mud displacement

Centralize to give optimum casing stand-off (80% minimum 75%)

Rotate and/or Reciprocate casing

– Rotation is preferred

– Use cable-type scratchers when reciprocating

Always use a bottom plug: 2 preferred….when possible!

Optimise slurry placement using a simulator:

– Turbulent flow preferred, or in combination with

– Effective laminar flow technique

Use Chemical wash pre-flushes ahead bottom plug

Use Spacer to avoid contact mud/cement slurry

Control spacer/cement slurry properties: batch mix when possible

Compatibility test mud/spacer/cement slurry : lab/field test

58


Effective Laminar Flow displacement (1)

General Flow regime when Turbulent flow is not possible

Four criteria must be satisfied (for spacer and slurries)

Density differential (10%)

Minimum pressure gradient (MPG)

Friction pressure hierarchy (20%)

Differential velocity criterion

Wash : To clean inside casing ( turbulent flow)

Use 3 – 7 m3 (20 - 40 bbls) chemical wash

Turbulent flow inside casing

Ahead of bottom plug

59


Effective Laminar Flow displacement (2)

Viscous spacer

Conditioned and clean mud

Viscosity adjustable

– Higher than mud

– WSS > ty,mud

Volume to use: > 150 m - 10 m3 ( 500 ft or 60 bbls)

Surfactant with NABM for water wetability and compatibility

Slurry (ies)

Viscosity adjusted and higher than the spacer

Casing Centralization : Stand off > 75%

60


Turbulent Flow Displacement (1)

Preferred and best flow regime …..When possible

Applicable at least to Spacer when possible ( laminar slurry)

Critical rate depends on:

Fluid rheologies

Casing stand-off : 85% recommended, minimum 80%

Annular gap, casing OD and Open hole size (bit size)

Formation fracture gradient

61


Turbulent Flow Displacement (2)

Use Turbulent Spacer and/or Chemical Wash

10 min. Contact time (> 6 min) or 300m (use greater volume))

Spacer density to be close to that of mud

Wash applicable only with low density mud (< 1.20 sg -10 ppg)

Turbulent spacer + Wash to clean inside casing ( preserve the spacer for annulus)

Water wet casing and formation with NABM (surfactant)

Optimise cement slurry properties:

Turbulence at the lowest rate : Minimum PV and TY without settling

Fluid loss and Free Fluid controlled

62

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Mud Removal- NExT

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