KEM-19a Long term term fluid migration, leakage and hydromechanical risks and monitoring strategies Groningen

The specific objective of this research is to define the regional hazard and risk assessment method and practices, taking into account post abandonment vertical and horizontal pressure (re-)distribution and migration of fluids (gases and formation water) through aquifers, faults and existing abandoned wells. The assessment method also needs to include ground motions (subsidence and small earthquakes) associated with the fluid migration up to the moment a new equilibrium in the subsurface is reached. This capability is required to quantitatively assess long term leakage hazards for unwanted emissions in drinking water aquifers and at the surface, as well as post abandonment ground motion risks.
The quantitative leakage hazard assessment, based on the norms for acceptable levels of various substances in groundwater resources or emissions or building codes, would define the long term leakage and ground motion risks. This would be the basis for defining long term monitoring strategies – if needed – using wells in and above gas, oil, or thermal depleted aquifers, as well as geophysical techniques.
The abandonment of the Groningen gas field, including neighboring gas fields in the province of Friesland and below the Waddenzee in the same geological formation, may serve as a first case.

FULL RESEARCH REQUEST

The project developed a regional hazard and risk assessment approach related to fluid migration and ground motion after the abandonment of multiple gas extraction and/or gas storage reservoirs in the Netherlands. The work consisted of four main tasks: i) a literature review on processes and impacts associated with long-term fluid migration and wellbore leakage in abandoned gas reservoirs, ii) quantitative analysis of post-abandonment pressure redistribution and its effect on ground motion, iii) quantitative analysis of fluid migration along wells and development of conceptual models for migration along geological pathways, and iv) recommendations for a quantitative risk assessment framework and monitoring strategies for effects of subsurface mining activities in the Netherlands after their abandonment.

The KEM-19 research project results are documented in two reports and a summary in Dutch:

Literature review on post-abandonment risks report

Evaluation of fluid migration and ground motion risks report

Research project summary

The extensive literature review provides a useful overview of i) evidence of connectivity between gas reservoirs and connectivity to adjacent/overlying aquifers and the potential for long-term fluid migration and pressure equilibration, ii) potential leakage of gas to the (near) surface and its consequences, and iii) possible consequences of fluid migration and pressure redistribution in terms of subsidence and seismicity. The second report includes: risk identification, quantitative hazard and risk assessment analyses, monitoring and risk management options en recommendations.

Post-abandonment risks can occur 10’s to 100’s of years after production at 100’s to 1000’s of meters away from production sites. Risks which are studied are related to subsidence and uplift, induced seismicity, fluid migration from the reservoir, and interference of depleted reservoirs with new subsurface activities. The project has investigated and quantitively analyzed into all of these factors based on regional data for the Groningen area since this area has multiple gas fields and a lot of data accessible.

For the Groningen gas field, the maximum additional post-abandonment subsidence expected after 100 years due to fluid pressure redistribution is largest to the northwest of the Groningen gas reservoir, between 9 and 13 cm, and to the west of the reservoir, between 4 to 7 cm. The maximum uplift has been calculated to about 1 cm above the southern part of the gas reservoir. There is some uncertainty to these results related to the pressure in the Groningen and surrounding gas fields reservoir and the degree to which the aquifer compacts, leading to the recommendation to continue monitoring the subsidence to validate the models used.

Related to the risk of induced seismicity, which can cause damage to the built environment at the surface, it is shown that re-activation of the type of faults which earlier has caused the largest earthquakes in Groningen (faults with no offset) is very unlikely to happen as it would require pressure reductions that are far from what is expected in the aquifers. In aquifer zones with zones of significant pressure reduction, re-activation of other fault types (faults that acts as boundaries to the aquifers and faults with offsets) can occur because of pressure depletion, as confirmed by previous seismic events related to the Groningen field. Additional local analysis of fault mechanical properties is needed in the southwest aquifers of the Groningen field to better assess the seismic risk related to specific sites.

The project has, further, revealed that leakage along wells or geological pathways, which potentially can cause risk to drinking water, is unlikely for the Groningen field. In a worst-case scenario of leakage along a damaged well, the leakage would still be small (a few tons per year into aquifers and the atmosphere). Shallow gas accumulations could be a potential risk of leakage but are to a small degree present in the Groningen area. Further monitoring and investigation of local sites is necessary to draw a more definite conclusion related to this leakage risk.

The project was evaluated by the KEM scientific expert panel.

KEM PROJECT EVALUATION

The KEM panel considers all research questions to have been adequately addressed and all deliverables to have been provided. Both quality and quantity of the work performed in this project was high. Results of this project and its recommendations are useful to the government and the regulators in formulating policies, informing the public, and companies that are involved in subsurface production or storage activities in the Netherlands. The results of the project form a useful basis for defining local risk assessment studies and long-term monitoring strategies in specific regions, also related to possible interference with new subsurface activities.