State-of-the-Art of Monitoring Methods to evaluate Storage Site Performance

 
1. INTRODUCTION
 
2. MONITORING TECHNIQUES
 
3. MONITORING CONCEPTS – STATUS QUO
 
4. SETTING UP A SITE-SPECIFIC MONITORING PLAN
 
5. CONCLUSIONS AND RECOMMENDATIONS
 
6. REFERENCES
 
 
2.1 CO2 plume migration in the storage reservoir
 
2.2 Surface uplift
 
2.3 Induced seismicity and mechanical reaction of overburden
 
2.4 Faults
 
2.5 Abandoned wells
 
2.6 Overlying and adjacent aquifers
 
2.7 Freshwater aquifers
 
2.8 Near surface eco-compartments
 
 
2.3.1 Induced seismicity
 
2.3.2 Passive seismic monitoring
 
» 2.3.3 Mechanical reaction of overburden
 
 

2.3.3 Mechanical reaction of overburden

The caprock and overburden are an integral part of a CO2 storage project. The caprock must be able to bear the change in stress fields during and after injection (Shukla et al., 2010). When stress is applied to a porous material, part of stress is supported by the matrix material and other part is supported by the fluid in the pores. The part of the stress supported by the matrix material is termed 'the effective stress' and it determines the deformation of the rock frame. Injection of CO2 will increase the pore pressure in the target reservoir. This will decrease the effective stress at the injection well due to changes both in pore pressure and the external stress, leading to expansion of the reservoir rocks. This expansion will also lead to deformation of the rocks (a small amount of compaction) in the overburden (Verdon, 2010; Verdon et al., 2011). Small Reservoirs that are softer than the overburden are more prone to stress arching, where much of the load induced by injection is accommodated by the overburden. These smaller reservoirs are more likely to generate fracturing, both inside and above the reservoir. This may be an important criterion when selecting potential CO2 storage sites and for monitoring (Verdon, 2010).

Deformation of the overburden can cause a problem for storage complex integrity if fractures and faults are created or re-activated, providing a leakage pathway for CO2. Most important impacts of a mechanical deformation of the overburden include:

  • Hydraulic fracturing: Rutqvist and Tsang (2002) mention that the greatest risk of rock failure is at the lower part of the cap rock because of the strongly coupled hydromechanical changes which are generated as a result of reduction in the effective mean stress induced in the lower part of the cap rock. The lower layers of the caprock possess a very high propensity to hydraulic fracturing.
  • fault reactivation: Any slight change in the stress conditions or in permeability of the caprock, could lead to the reactivation of existing faults or slips. The propensity for shear reactivation of faults increases due to any increase in the aquifer pressure during the injection period and the development of poro-elastic stresses in the rocks towards the bottom of the reservoir.
  • Fluid-flow driven pressure: Upward pressure is exerted on the caprock layer when the CO2 changes its phase from supercritical to liquid or to gaseous form, after injection or when a density-driven flow takes place. This could trigger the initiation of microcracks which can eventually lead to macro-level fracturing of the caprock (Shukla et al., 2010).

Indications for these processes can be derived, e.g. from passive seismic monitoring. For applicable monitoring techniques to follow surface deformation see Section 2.2.

Geomechanical modelling of the subsurface is necessary in any storage site assessment and should focus on the maximum formation pressures that can be sustained in a storage site. As an example, at Weyburn, where the initial reservoir pressure is 14.2 MPa, the maximum injection pressure (90% of fracture pressure) is in the range of 25-27 MPa and fracture pressure is in the range of 29-31 MPa (Semere, 2007). For geomechanical modelling it may be important to consider reservoir heterogeneity. Differences in porosity through a reservoir imply differences in rock fabrics. Differences in grain-size can exert significant influence on elastic stiffness. Differing degrees of carbonate cementation will produce different elastic stiffness as well. Small heterogeneities will probably not lead to changes in the shape of stress loops around the reservoir. Larger scale heterogeneous zones may act to change the nature of the geomechanical response of a reservoir. To assess these impacts, a geostatistical model could be used, which varies the difference in mechanical properties between the heterogeneous zones and the 'background' reservoir material, the proportion of the reservoir made up of the 'heterogeneous' material, and, importantly, the characteristic length scale of the heterogeneous zones (Verdon, 2010).

To guarantee security, site operators must be able to demonstrate that geomechanical deformation will not be of sufficient magnitude to damage the cap rock. Operators must also ensure that CO2 injection will not induce earthquakes on any nearby faults (Verdon, 2010). This can best be achieved by combining appropriate monitoring tools and geomechanical modelling.