The event described here can be limited to specific settings and might not be encountered in every geothermal projects. More details about this topic are presented in a report that can be found on GEOENVI website.
With depth varying between a few hundreds and a few thousands of meters, geothermal resources are explored, developed and produced through well drilling. To reach the targeted resource, the well is expected to intersect one or several aquifers of different quality and property that are separated by impermeable levels. Without the adoption of proper mitigation measures, during development (in particular drilling), and operation of a geothermal industrial plant, there is a risk of accidental connection of aquifers via the wellbore or disturbance of non-targeted aquifers with fluid intrusion (geothermal fluid, testing fluid, drilling mud, etc.).
The phenomena are driven by differential hydraulic pressures between layered aquifers, and can have several causes: well barrier and integrity failures due to poor cementation practices; mechanical damage during well development; corrosion and scaling; geo-mechanical disturbances; underground blowout; thermal stress and material failure or degradation; aging over the life cycle of operations. It can be triggered both during the drilling process and through all life stages of a geothermal project. It can also result from improper reinjection applications.
The consequences of these phenomena are diverse and may include potential aquifer and freshwater resource contamination and depletion, loss in geothermal potential, affect other underground uses, ground elevation at surface from formation swelling, collapse or landslides from formation dissolutions.
The risk of affecting groundwater aquifers is well controlled in geothermal operations thanks to mitigation actions taken, from the conception phase to day-to-day operations (e.g. best practices or legislation requirements).
Preventive and remedial solutions can be implemented to mitigate the risk of connection between aquifers and their disturbance at all stages of geothermal operations. The well design and more specifically materials chosen for casing when developing a drilling program and monitoring the work during cementation and tubing placement are essential to prevent effects on groundwater aquifers. A good knowledge of the geological and hydrogeological setting at the well site is essential. The monitoring of the reservoir behaviour, the control of casing and tubing and maintenance operations also contribute to prevent and mitigate aquifer interconnection and contamination. Corrective solutions implemented to stop or confine potential leakages can be carried out through direct well operations and work-over using patch or new casing.
The table below provides an overview of this topic in term of risk and impact assessment i.e. its causes, consequences, the phases concerned, the influencing contexts or the principals monitoring and mitigation measures that can be adopted.
