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.
Blowouts are uncontrolled flows of formation fluid from drilled wells. They are very rare incidents, that may occur from natural conditions (e.g. the drilling through an over-pressure zone), or in relation to drilling operation (e.g. if pressure inside the well falls below the saturation pressure and steam is formed) (Holmgren, 2018). In a blowout, fluid is ejected to the surface. Water located above the inflow zone is ejected rapidly out of the well, followed shortly after by steam and water.
Geothermal blowouts differ from blowouts that happen during drilling for oil and gas. They are generally easier to handle, less dangerous and by using the normal procedure of drilling and construction this risk is very limited. The drill rig is equipped with tools to monitor and prevent blowouts to reach the surface. The most important part of the drilling equipment is the Blowout preventers (BOPs) which have the purpose to close the well during drilling operations, if control of the formation fluid is suddenly lost.
The prevention of blowouts effects occurring underground is treated in Interconnection of aquifers and disturbance of non-targeted aquifers. The prevention of geothermal gas emission beside blowout is described in Degassing.
The table here 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.
Holmgren, M. (2018). ). X-Steam. Properties of water and steam (Steamtables). Add- in for Excel. Retrieved from http://www.me.ua.edu/me215/f07.woodbury/ExcelStuff/XSteam-v2a.xlsm