Geological CO2 storage and other emerging subsurface activities - Best practice: monitoring strategy & methods for groundwater protection
Emerging subsurface activities (ESA) describe a set of methodologies and technologies using the earths subsurface for energy production or capture and storage of carbon dioxide. The earth’s heat is used as a clean source of energy (deep geothermal systems, DGS), process-related CO2 emissions can be stored in suitable geological formations (geological CO2 storage, GCS) and since the technique of horizontal drilling was developed, the exploitation of unconventional reserves of natural gas via hydraulic fracturing (shale gas extraction, SGE) expanded. At the same time, 97% of global freshwater resources are stored in the earth’s subsurface, too, so that exploitation interests may come into conflict with the issue of groundwater and environmental protection. Main objective of deliverable D 3.1 of the COSMA-1 project therefore was to identify best practices of monitoring for geological carbon storage, deep geothermal systems and shale gas extraction projects with special focus on groundwater protection. Chapter 2 summarizes current groundwater monitoring standards, including monitoring network designs for emission-based (operators) and immission-based (water suppliers) monitoring. It further presents an identification of hazards related to ESA and a brief overview about the state of regulation. Finally, knowledge gaps concerning groundwater protection are identified. Chapters 3 to 5 describe for each of the above-named types of ESA the project stages and according monitoring needs and methods. Main target was to identify the key parameters and monitoring network designs ensuring reliable groundwater monitoring. As the most relevant hazards were drilling fluids, fracking fluids and brine migration as well as the mobilisation of methane, and the most likely pathways are leakages due to insufficient well integrity, for all three ESA types, pressure, temperature and TDS were recommended as key monitoring parameters. For shale gas extraction, in addition methane emission should be monitored. Key to any monitoring is i) the baseline sampling prior to the start of subsurface activities and ii) the adequate delineation of the area of review. All further monitoring to be implemented base on site-specific considerations and the authorities’ priorities. In any case, monitoring network should include the up-gradient, down-gradient and depth component. Monitoring wells and equipment should cover the full extension of horizontal bores and additional wells should be placed above potential pathways for fluid (or brine) migration as e.g. fault systems. The use of abandoned wells for monitoring is also recommended. The conception of appropriate monitoring strategies has further to be coordinated with the competent authorities, which have to control the compliance with all requirements. Therefore, site operator and water producer should report their monitoring plans and data at regular intervals to the competent authorities. The findings were summarized by transferring them to a risk management matrix following the Water Safety Plan (WSP) approach (WHO 2009). For shale gas extraction, deliverable D 3.2 will add specific mitigation measures to reduce the previously identified risk of negative impacts on shallow groundwater. Geological carbon storage was further investigated by means of the development of a coupled model for a theoretical case study site in the North-Eastern German Basin in the scope of work package 2 of the COSMA-project (D 2.3).