gas migration
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- What is Gas Migration?
- Gas Migration is the invasion of formation fluids into the annulus, due to a pressure imbalance at the formation face ( loss of Hydrostatic pressure)
- What are the Consequences of gas migration?
- Poor zone isolation, Blow out, Repair required, Environmental damage
- 3 Paths for gas migration.
- Channel, Invasion during setting cement, Set cement failure
- Cause of a channel
- Inadequate mud removal, excessive free fluid in deviated well
- Cause of invasion
-
Fluid-loss during and after placement
Chemical shrinkage
Gel strength development
Free Fluid
Highly Permeable Slurry - 4 Phases of setting cement
- liquid, early gelation, hydration, set
- CHP
- Critical Hydration Period
- PDA
- Pressure Decline Limit
- Dynamic Fluid Loss
-
Increased rheology
Change in other slurry properties - Static Fluid Loss
-
Loss of overbalance
Promote early gelation
Bridging
Accelerated hydration kinetics
Recommended fluid loss: < 50 mL/30 min - Cause of set cement failure
-
Down hole stresses
Change in Temperature
Change in Pressure
Poor Interfacial Bonding (Microannulus)
High Shrinkage - Essential for Successful Cementing of Gas Wells
- mud removal, cement slurry design, set cement mechanical properties, cement hydration, fluid density control
-
Essential for good mud removal
and schlumberger's slurry solution. -
Centralization
Conditioning of mud to follow the Density and viscosity hierarchy
Pipe movement during placement
Displacement regime and rate
Spacers/Pre-flushes:
WELLCLEAN II - Ideal Slurry Properties that minimize the gas migration during cement setting and schlumberger's solution
-
During Placement:
Appropriate Rheology for good mud/spacer removal
Zero Free Fluid (Inclined Section)
Fluid Loss less than 50 ml/30min
After Placement:
Short transition time from 100 to 500 lb/100ft2
Low Fluid Loss to avoid early Gelation
Pore throat plugging materials
GASBLOK - Cement that can withstand cyclical stress variations avoiding set mechanical failure.
- Flexstone
- Physical means of preventing gas migration
-
Annular Pressure
Multistage Cementing
Reduced cement column length
ECP / CFP
Casing seal ring
Increase mud density - Cement solutions to gas flow
-
Compressible Cements
Thixotropic cements
Right angle set cements
Surfactant cements
Expansive cements
Microsilica Cements
Impermeable cements - Slurry property optimization to prevent gas migration
-
Free water
Fluid loss control
Gelation control
Thickening time
Hydration kinetics
Slurry density (overbalance)
Special properties - API testing for testing and handling
- 10
- Recomended lab tests for gas migration
-
Thickening Time
Compressive Strength
Fluid Loss
Rheology
Slurry Stability
Free Fluid & Sedimentation or Settling
Static Gel Strength Development
Gas Flow Test - What is the lab equipment used to evaluate gas migration? What does it test for?
- Vane Rheometer, Measures true static gel strength
- Gas flow analyzer
- ?
- Slurry properties of GASBLOK
-
Well dispersed, thin, non gelling slurry
Excellent Fluid loss control
Film is Impermeable to gas during the liquid to solid transition period - GasBlok density range
- 8-24 ppg
- Temperature range for gasblok
- <375F
- Gasblok D codes low med high temp
- D500, 600, 700
- GASBLOK* Slurries Design Considerations
-
<50mL/30 min fluid loss
Dispersed to minimize gel strength development (<35 lb/100 ft2 )
Rheology for displacement (ELF)
Slurry stability
Short transition times (Bc)
Ty <10-15 lb/100 ft2 - GASBLOK* Concentration depends upon two factors
- BHST & SVF (Soild Volume Fraction)
- Stabalizer for gasblok 700,600
- D701,D135
-
D600G > 2 gal/sk =
D600G < 2 gal/sk = - gasblok system , fluidloss system