Geothermal drilling fluids are a critical component to the overall success of a geothermal drilling program. The main purpose of drilling fluids during geothermal drilling is to:
The main type of drilling fluids used for drilling the overburden well sections, where the formation collapse is expected and temperature gradients are high, is based on bentonite. Such fluid types, however, after exposure to temperatures ranging between 150 and 200°C (Otte et al. 1990) experience a sharp increase in the viscosity which might result in dangerous stuck pipe incidents and long non-productive time. Other drilling fluid option, that will provide support to the well and allow for good cutting transport include polymer-based drilling fluids. Polymer additives available on the market today are limited to circulation temperatures of approximately 90°C. It is a common procedure to use mud coolers or cooling towers for lowering the temperature of the drilling fluid being recycled from the well during high-temperature drilling. One of the cooling towers installed while drilling one of the wells in the Reykjanes peninsula in south-west Iceland is presented below.
Fig. 1. Cooling tower used commonly in Iceland for high-temperature geothermal drilling
One of the main concerns of geothermal drilling, especially in over-pressurized zones is exposing the drilling fluid to high-temperatures, especially during long periods of drilling stoppage, and simultaneously degrading its properties. In projects such as the IDDP-2, constant total losses throughout drilling operations and highly fractured rock formations (Friðleifsson et al. 2017a and 2017b), resolved the problems connected to the degradation of drilling fluid properties and hole cleaning, as only clean water with polymer pills were used as drilling fluid and all created cuttings were lost in the heavily fractured geothermal reservoir. This phenomenon enabled reaching depths of nearly 4700 m. Different conditions were encountered during the drilling of the Venelle-2 well, which resulted in the usage of a water-based drilling fluid system with ilmenite and sepiolite as drilling suspending agents in order to resist high circulating temperatures and eliminate the risk of sagging. Selection of the drilling fluid in the Venelle-2 well proved to be successful, however, caused challenges during implementing standard well control procedures (Bretani et al. 2018). Improvements are needed for drilling fluids being able to maintain their properties at much higher temperatures as well as aid cleaning operations, provide good cooling capabilities and above that being environmentally friendly and biodegradable.
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Article based on a study by Kruszewski and Wittig titled “Review of failure modes in supercritical geothermal drilling projects” published in Geothermal Energy (2018) 6:28 (doi.org/10.1186/s40517-018-0113-4).