4) Mud Removal

Mud Removal

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Objectives of Primary Cementation • Provide complete isolation of zones

(Hydraulic Bond) • To support the casing • Protect casing string

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(Shear Bond)

Mud Removal • Most important aspect of cement job • A 3-step process before cementing •

Hole cleaning

•

Conditioning the drilling fluid Displace the drilling fluid from the annulus

•

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Mud Removal • Hole Cleaning • • • •

Controlled & optimized mud properties Wiper trips > 95% Total hole volume in circulation Caliper log

• Conditioning Mud • • • •

Break gel strength Lower ty + pv Drill solids < 6% Determine MPG to find qmin for all-around flow

• Displace Mud from Annulus • • •

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Optimized slurry placement ---> CemCADE Casing centralization optimized (STO > 75%) Casing movement

Criteria for Effective Mud Removal Cementing Operation: • • • • • •

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Centralize casing Casing movement Scratchers Wiper plugs Washes and spacers Flow regime selection

The Ideal Wellbore Casing BHST at top of cement >BHCT at TD

Annular gap

Minimum: 3/4” Ideal: 1 1/2”

Properly conditioned hole and mud No sloughing

Gauge diameter

NO LOSSES

Uniform as possible ( no washouts or restrictions)

NO FLOW

Casing centered in borehole

Thin, impermeable mud filter cake (not gelled or unconsolidated)

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Accurate BHST and BHCT

Influence of the Casing Stand-Off

Di Do

Vnar

8

Vwide

Newtonian Fluid - Effect of STO

The Effect of the Casing Stand-Off on the Annular Flow is Qualitatively Equivalent to the Following Flow Pattern Q

D2

D1 L

P L V2

V1

Q

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Newtonian Fluid of Viscosity µ Density  In Laminar Flow : • 1. Velocity L

11

P = 32µ V1 = 32µ V2 D 12 D 22

Newtonian Fluid of Viscosity µ Density  In Laminar Flow : • 1. Velocity

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P = 32µ V1 = 32µ V2 L D 12 D 22 V2 = (D2)2 V1 (D1)2

Newtonian Fluid of Viscosity µ Density  In Laminar Flow : • 1. Velocity

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P = 32µ V1 = 32µ V2 L D 12 D 22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1

Newtonian Fluid of Viscosity µ Density  In Laminar Flow : • 1. Velocity

P = 32µ V1 = 32µ V2 L D 12 D 22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1 V2 = 4V1 (For 67%)

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Newtonian Fluid of Viscosity µ Density  In Laminar Flow : • 1. Velocity P = 32µ V1 = 32µ V2 L D 12 D22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1 V2 = 4V1 (For 67%) • 2. Reynolds Number

Re2 = V2 D2 = 4V12D1 = VD µ µ µ

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Newtonian Fluid of Viscosity µ Density  In Laminar Flow : • 1. Velocity P = 32µ V1 = 32µ V2 L D 12 D 22 V2 = (D2)2 V1 (D1)2 If D2 = 2D1 V2 = 4V1 (For 67%) • 2. Reynolds Number

Re2 = V2 D2 = 4V12D1 = VD µ µ µ Re2 = 8Re1 (For 67%)

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Laminar Flow in Eccentric Annulus Non-parallel plate model Ri/Ro = 0.8 1000 500

Vwide / Vnarrow

n = 1.0 n = 0.5 n = 0.2

100 50

10 5

1

17

0

10

20

30

40

50

Stand-off %

60

70

80

90

100

In Turbulent Flow 2 1.75

• Velocity p



L

= =

If D2 = 2D1

1 D ) 0.241 x  0.75 x µ0.25 x( V 4 D1 4.75 V  2 1.75 0.241 x  0.75 x µ0.25 x( 2 D2 ) 4 D24.75 1

V2 =( D2 )0.714 V1 D 1 V2 = 1.64V1 (For 67%)

• Reynolds Number

Re2= V2 D2 = 1.64V12D1 = 3.28V1D1 µ µ µ Re2 = 3.28Re1 (For 67%)

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Turbulent Flow in Eccentric Annulus 1000 500

Vwide / Vnarrow 100 50

n = 1.0 n = 0.5 n = 0.2

10 5

1

0

10

20

30

40

50

60

API Stand - Off (%)

19

70

80

90

100

Casing Centralization • Relative Variation of flow rate ratio as a function of eccentricity 18

RH

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FLOW RATE RATIO

14

RC

12 10

W

8 6

% Stand-off =

4

w RH - R C

X 100

2 0

0

20

20

40 60 API % STAND-OFF

80

100

Types of Centralizers • Bow Spring (Spiral or Straight): •

Flexible bow springs

•

Centralizer OD slightly larger than OH size

• Rigid Bow (or Positive) type: •

Non-flexible O.D. (Slightly less than previous casing ID)

•

Use inside cased-hole sections

•

Effective in in-gauge OH intervals only

• Rigid Solid slip-on type:

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•

Solid body - no bows

•

Use: as per rigid type

Reciprocation

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•

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

•

1 to 5 minutes per cycle

•

Needs scratchers to be effective

•

Casing may become stuck during movement

•

Excessive swab and surge pressures may be created

•

Excessive pull and buckling

•

Cannot be the only method of mud removal

Rotation • Circular movement of pipe • Must be done from the start of circulation to end

displacement • 10 to 40 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

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Fluids Incompatibility • Results In: • •

Detrimental Interface Reactions High Rheological Properties • •

•

Change in Cement Slurry Properties • • •

•

Very high viscosities Very high gel strengths Thickening time altered Increase in fluid loss Reduction in compressive strength

Reduction in Hydraulic Bond

• Prevented By: • • • •

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Wiper Plugs Chemical Washes Spacers Compatibility Testing

Cement Wiper Plugs • Keep Fluids Separate in Casing and Reduce

Contamination • Bottom Plugs •

Remove mud ahead of cement

•

Prevent cement falling through lighter fluid ahead

•

Wipe inner casing walls clean

•

Use 2 or more if possible

• Top Plugs

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•

Separate cement from displacing fluid

•

Positive indication of end of displacement

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

shoetrack if not removed ahead of the top plug.

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•

EXAMPLE: 9 5/8” 47 lb/ft 10000 feet, collar at 9820 feet

•

Volume of 1/16” film?

•

Height corresponding to this volume?

•

Turbulent Flow Displacement Preferred and best flow regime

• Critical rate depends on: • • • •

Fluid rheologies Casing stand-off Annular gap, casing OD and bit size Formation fracture gradient

• Use Chemical Wash and/or Mudpush XT/S spacers: • •

10 Min. Contact time or 750 ft (use greater volume)) Spacer density to be close to that of mud

• Optimize cement slurry properties: • •

Minimum PV and TY without settling Fluid loss and free water controlled

• Water wet the casing and formation

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Effective Laminar Flow • Alternative flow regime when Turbulent flow is not

possible • Four criteria must be satisfied: • • • •

DENSITY DIFFERENTIAL (10%) MINIMUM PRESSURE GRADIENT (MPG) FRICTION PRESSURE HIERARCHY (20%) DIFFERENTIAL VELOCITY CRITERION

• Viscous spacer: Mudpush XL/XLO • • • • •

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Viscosity adjustable (Change D149 concentration) Volume to use: 500 ft or 60 bbls Use 20 - 40 bbls chemical wash Condition and clean mud Viscosify cement slurry when necessary

Chemical Washes • Water based fluids, low viscosity, density of water • Easy to pump in turbulent flow • CW7 for intermediate casings, water based muds •

41.5 gals water, 0.5 gals D122A

• CW100 for production casings, water based muds •

41.25 gals water, 0.5 gals D122A, 0.25 gals J237

• CW8 for intermediate casings, oil based muds •

41.25 gals water, 0.5 gals D122A, 0.25 gals F40 last

• CW101 for production casings, oil based muds •

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41 gals water, 0.5 gals D122A, 0.25 gals J237, 0.25 gals F40

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

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Events to be Recorded • Was the mud conditioned - rate and time? • How many centralizers were run and where? • Was the casing rotated and/or reciprocated? • Where the plugs correctly dropped? • What was the density and rheology of the spacers? • Was the correct volume of preflushes used? • The following data must be recorded on the PRISM: • • •

All densities, if possible of displacement fluid as well All flow rates, if possible of displacement as well All pressures

• Note any changes in flow rate, density, stoppages,

pressure peaks, etc.

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Conclusions • Condition mud prior to cementing • Centralize to give optimum casing stand-off • Rotate and/or Reciprocate casing •

Use cable-type scratchers when reciprocating

• Always use the bottom plugs: 2 preferred • Optimize slurry placement using CemCADE: •

Turbulent flow preferred, or

•

Effective laminar flow technique

• Use chemical wash pre-flushes • Control Mudpush spacer/cement slurry properties:

batch mix • Compatibility mud/cement/spacer : lab/field test

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