• Problems in CO₂ injection are analogous to the other injection problems in petroleum
• The same design/analysis and field technology can be applied
• EPSL group/ TAURUS experience:
  - PWRI (produced water reinjection)
  - Tight gas water fracturing
  - Fractures in oil sands
  - Sour gas injection at frac pressure
  - N₂ injection in coal seams
  
  
  Seal Integrity
   
• Issues:
      - Slip at the aquifer/ shale boundary ( risk if low friction angle layers exist )
      - Opening dormant fractures in the overburden
      - Fracturing through the seal (rare?)
      - Leaks behind pipe
• Driven by shear stress development at the interface and expansion of the target zone
  
  
  Possible modes of communication with shallower zones
  
  

Fault Reactivation

• Initial stress state is important (some faults may be close to critical initially)
• Fault strength against slip depends on effective normal stress and shear stress
• Require large scale modeling
• Fault behavior modeled by joint mechanics

  Competition between increased                               Decreased eff. normal stress and
   eff. normal stress and shear stress                           shear stress both waken the fault

Fracturing

• Injectivity may decrease in time due to:
       -Dry out
       -Chemical reactions and formation damage
       -Overall pressurization of the storage area
• It is desirable to maximize the inj rates
• Fracturing pressures are function of poroelastic and thermal stresses created by injection

Requires coupled geomech modeling


    Storage Capacity
 

• Compressibility of high
  perm sands is nonlinear
   
• Storage capacity increases
  as pressure increases and
  eff stress decreases