Nigel Moore, principal consultant at TNEI Services, explores the difference between assessment of risks and assessment of impacts.
Environmental risk assessments (ERA) are used when the consequences of development are uncertain. All on-shore deep-drilling exploration activities have this element of uncertainty to them which is why the energy and climate change department (Decc) requires shale gas operators to submit ERAs in advance of drilling activity.
An ERA must be informed by a probability measure. The risk of an impact is equal to its consequences multiplied by its probability. ERAs must address potentially significant environmental and human health risks and well operators must demonstrate that such issues have been identified and mechanisms put in place to manage them when drilling.
In contrast, environmental impact assessments (EIA) address outcomes where the consequences of development are predictable and can be assessed with a degree of certainty. EIAs consider the impacts derived from both planned (impact) and unplanned (risk) events associated with development. Accordingly, an environmental statement should be informed by the risks and mitigation measures identified within the ERA.
When producing ERAs, well operators must use Decc guidance which brings together the good practice of the environment department (Defra) , the Environment Agency and the Royal Society and Royal Academy of Engineering . The approach calls for the assessment of key risk categories that share common pathways and sensitive receptors.
Key risk categories associated with shale gas exploration
-
Seismicity and ground stability
-
Surface and sub-surface ground works
-
Water acquisition
-
Chemical mixing
-
Borehole installation and integrity
-
High volume hydraulic fracturing
-
Management of flow-back fluids
-
Gas management
-
Offsite disposal
-
Well closure and abandonment
For each risk category, the magnitude of the impact, the sensitivity of receptors and the effect of mitigation is considered first. These findings then inform an assessment of residual risk which combines the probability of an event occurring and the consequences for people and the environment if it does occur, making use of an ERA risk magnitude matrix as shown below.
ERA risk magnitude matrix
Consequence Probability Very low Low Medium High Very low Low Low Low Low Low Low Low Medium Medium Medium Low Medium Medium High High Medium Medium High HighIn practice, there are tried and tested design solutions and mitigation measures available to avoid or minimise unplanned or uncertain risks and environmental impacts to an acceptable level.
Generally, they are considered acceptable when they are as low as reasonably practicable. This encourages operators to move beyond minimum standards in a continuous effort for improvement. However, the threshold of acceptability can vary with the sensitivity of a location, for example, risks or impacts that are acceptable in urban locations may not be acceptable in rural locations.
Risks and impacts: striking the right balance for assessment
When considering shale gas exploration, the consequences of deep drilling at depths of one to two miles have so far been assessed as high; for example:
- Pollution: a well failure at depth contaminating subterranean water reserves; or
- Seismic activity: drilling causing ground instability.
However, the probability of such events occurring is considered to be low; given:
- Pollution: the high degree of well integrity on operational sites and the lack of pollution pathways between the deep sources and surface receptors,
- Seismic activity: the lack of seismic pathways between sources and surface receptor.
Accordingly, the risks engaged by deep drilling associated with shale gas exploration are likely to be low. Conversely, the consequences of above ground spills and contamination associated with day-to-day operations are likely to be of a smaller magnitude, but the probability would appear to be much higher given the existence of clear pollution pathways at ground level or just below, in other words, within the catchment of the water table or within 300 metres of the surface . It is important that EIA takes forward the findings of an ERA and is able to focus on the main likely environmental impacts.
Conclusion
The assessment of risks and impacts are indelibly linked; ERAs should inform EIAs. To date, the reporting of risks associated with shale gas exploration within the media has focused on the consequences of deep drilling with little regard for their probability or the risks from surface spills or contamination. The concern for assessors in the future is that this lack of balance in reporting must not lead to under-assessing probable risks while over-assessing improbable ones.
