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Mud Removal Module CMT 018 26-Jun-00 1
Objectives of Primary Cementation • Provide complete isolation of zones
(Hydraulic Bond) • To support the casing
(Shear Bond) • Protect casing string
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Mud Removal • Most important aspect of cement job • A three step process before cementing • • •
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Hole cleaning Conditioning the drilling fluid Displace the drilling fluid from the annulus
Mud Removal •
•
•
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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 • 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
Fluid Calipers • To determine circulation efficiency or amount
of fluid which is moving in the wellbore. • Procedure :
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•
Run multi-arm open-hole caliper log and determine total hole volume.
•
Circulate at cementing rate and determine mud pump efficiency
•
Drop marker or tracer in staged intervals
•
Monitor returns for marker
•
Calculate volume circulated from rate and time (Should be ± mechanical caliper volume)
•
Increase rate and re-calculate efficiency
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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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
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
Bow Spring Centralisers
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Weatherford Spiragliders
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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
•
DANGER : 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
Weatherford Non-Rotating Plugs
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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
Family of Spacers • MUDPUSH XT •
Turbulent spacer for water based muds
• MUDPUSH XS •
Turbulent flow spacer for water based muds in saline (Salt) environments
• MUDPUSH XL •
Effective laminar flow spacer for water based muds
• MUDPUSH XTO, XSO, XLO also exist •
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Applications are same as above but in OIL BASED MUDS.
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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