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
 
 
4.1 Mapping of relevant areas
 
4.2 Definition of monitoring objectives and intensity
 
» 4.3 Selection of methods and specification of measurements
 
4.4 Examples
 
 

4.3 Selection of methods and specification of measurements

The EU CCS Directive does not specify the method or monitoring technology that should be used, but requires that the choice is based on best practice available at the time of design. As stated in Section 3.2.1, Annex II of the CCS Directive does give some guidelines for the selection of monitoring technology. Technologies that can detect migration pathways of CO2 in the subsurface and at the surface, areal/vertical distribution of CO2 plume and technologies that can provide wide areal spread of the complete storage complex and beyond are recommended.

The resolution of a specific monitoring method depends on the instrument specifications, but also on site-specific conditions. The monitoring instrument's ability to measure the distribution, phase and mass of CO2 in a subsurface reservoir varies with geology of the site and surrounding area, target depth, ambient conditions of temperature, pressure and water saturation underground as well as by the theoretical sensitivity of the techniques or measurement instruments themselves. For example when acquiring seismic data onshore it makes a large difference if the geophone is placed in soil with good coupling and little background noise (e.g. no noise from surrounding traffic or industry).

At the general level for any site the main questions that need to be considered are according to the Guidance Document 2 (2011):

  • Which methods are relevant for the specific site?
  • What is the resolution of monitoring in detecting leakage?
  • How accurately can leakage be quantified?
  • What quantity of CO2 can be resolved in the plume or deep subsurface?
  • If continuous monitoring is considered in order to increase time sampling, what shall be the lifespan of the system?

Detecting and quantifying leakage

For offshore sites, the North Sea basin Task Force (NSBTF, 2009) suggests to use a model driven approach where simulations are combined with data collection. For the North Sea, a good strategy would be to use "geophysical methods like seismic data (detection of gas chimneys) or sea bottom echo-sounding (detection of pockmarks) and then sample these leakage areas for direct CO2 detection repeatedly. Based on the sampling profiles an estimate can be made of leakage rates in time for the area. In case of wellbore leakages an additional monitoring programme in and around the well is suggested" (NSBTFf, 2009). Similar to sea bottom echo-sounding, other techniques that are able of detecting gas bubble streams in the water column, such as hydroacoustic techniques, may be employed for large area surveys, as outlined in Section 2.8.1.

For onshore sites there are several technologies to choose from as described in Section 2.3.4. Both direct methods for leakage detection and indirect methods where, e.g. ecosystems, groundwater or isotopic signatures are monitored can give reliable indications of irregularities. The main challenge for measuring absence of leakage with both direct and indirect detection methods consists of temporal and spatial coverage. At present there is no technology that can detect CO2 releases at the surface ‑ diffuse or localised, strong or weak ‑ in an area corresponding to the size of the underground pressure plume. Therefore, a range of technologies are likely to be required to increase the probability of leakage detection. Given a storage complex size of more than a few hundreds of km2 in comparison to potential surface leakage diameter of less than 1 m2, the chances of missing a leak are high. To ensure leakage detection a comprehensive monitoring strategy should be implemented comprising techniques with different spatial and temporal coverage and resolution.

Definition of an adequate spatial and temporal coverage based on identified risks is the best strategy to employ. A plan for intensified monitoring in the event of irregularities is an important part of a good monitoring strategy. For this, the sensitivity and reliability of different techniques to quantify a potential leakage needs to be considered. An overview of the capabilities of currently employed monitoring techniques for quantifying leaking CO2 is given in IEAGHG, 2012.

Fig. 4-2 gives an overview of surface, near-surface and subsurface monitoring methods used in the large-scale CO2 injection demo projects In Salah (onshore), Sleipner (offshore) and Snøhvit (offshore) (after Wildenborg et al., 2009).

FIG 4.2

Fig. 4-2: monitoring techniques deployed at large injection sites (adapted from Wildenborg et al., 2009). Selected techniques are indicated by yellow boxes.

Quantifying CO2 in the plume

Strategies for monitoring and quantifying CO2 in the subsurface have been successfully applied in several projects. Repeated 3D seismic surveys with an interval of several years in the onshore Weyburn oil field and the offshore Sleipner CO2 storage site have shown that deep seismic methods can be used to quantify CO2 with sufficient accuracy. However, the success rate depends on the target depth, reservoir quality, caprock and overburden. The optimal target depth with current technologies is 500 - 3000 m according to the Guidance Document 2 (2011).