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
 
 

2.7 Freshwater aquifers

Since freshwater is a valuable commodity and protected good, the European Parliament and the Council adopted the Directive 2000/60/EC establishing the framework for Community action in the field of water policy (EU Water Framework Directive; WFD). The purpose of this Directive is to establish a framework for the protection of inland surface waters, transitional waters, coastal waters and groundwater. In Article 11 it is stated that "member states may authorise, specifying condition for (inter alia) injection of natural gas or liquefied petroleum gas for storage purposes into other geological formations where there is an overriding need for security of gas supply, and where the injection is such as to prevent any present or future danger of deterioration in the quality of any receiving groundwater". Accordingly, one of the monitoring purposes explicitly mentioned in the EU CCS Directive is detecting significant adverse effects on the surrounding environment, in particular on drinking water. Additionally, compliance with the EU Groundwater Directive (2006/118/EC) and the EU Water Framework Directive is required. Thus, freshwater aquifers that are used for drinking water production must be monitored, if there is a risk of pollution. Annex II part B and Annex III of the EU Groundwater Directive provide practical information for groundwater monitoring.

Groundwater protection requires monitoring at three principal levels, at which appropriate methods can be applied in a sequential manner with increasing effort and intensity (May, 2012):

  1. Observation of the storage reservoir and overlying deep aquifers should provide early indications of irregularities and the possible migration of fluids out of the storage formation. In case such indications occur, monitoring can be focussed on potential connections to shallow aquifers.
  2. If there are indications for leakage of fluids out of the storage complex, along pathways towards shallow freshwater aquifers, monitoring plans can be intensified in order to detect signs of leakage into shallow aquifers.
  3. If groundwater contamination is detected, monitoring is needed to quantify the extent of the contamination (mass, fluxes, area, concentration etc.) in order to take appropriate actions to minimise the effects of the spill and eventually remediate the affected aquifer.

Impacts on groundwater may result from migration of CO2, mobilised fluids/substances or saline formation water into freshwater resources (IEA GHG, 2011; Lemieux et al., 2011). Freshwater monitoring can be used to assess changes through time and across an area using indicators such as pH, specific conductance, alkalinity, major and trace chemical constituents, dissolved gases including noble gases, stable isotopes, radio-isotopes (14C), and redox potential. Changes in the chemical composition of freshwater could be used to detect leakage or indications for the risk of water quality changes by saline formation water, mobilised fluids/substances or CO2 migration.

A monitoring programme should include both pre- and post-injection sampling and assessment of baseline water chemistry and mineralogy. Evaluating the extent of spreading CO2 may not be trivial as the lateral extension of aquifers can reach up to tens of km. Spacing of monitoring wells needs to consider site-specific controls on sensitivity and attenuation, risk factors, groundwater flow direction and rate, and account for non-geological storage changes (cf. Fahrner et al., 2012). For large area surveys, airborne electromagnetic methods can provide valuable information on a potential salt water intrusion into freshwater aquifers at shallow depth (e.g. Siemon et al., 2009).

Apart from possible effects of CO2 on water composition and quality, the availability of groundwater resources may also be affected by displacement of deep saline or shallower groundwater. This may be the case in shallow parts of formations, if saline water is used for down-dip CO2 storage. Any modification of the groundwater pressure regime may lead to changes in water table levels and, thereby, may affect flow rates and geometry of water bodies (IEA GHG, 2011).