| Title | THEORETICAL MODELS OF MAGMA ASCENT THROUGH A GEOTHERMAL BOREHOLE |
|---|---|
| Authors | D. Dempsey, C. McConnochie |
| Year | 2021 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Magma, geothermal, drilling, eruption, borehole, fluid dynamics, crystallization |
| Abstract | As drilling technology improves and shallower resources become tapped, interest is growing in deeper, hotter geothermal resources. In volcanic settings, the possibility of encountering a magmatic intrusion increases with each metre of borehole. Such intrusions may not be resolved by surface geophysical surveys, particularly if they are small, deep or sit beneath other bodies that shadow their presence. Further, if the intrusion contains a mobile fraction of partial melt, this could enter the borehole and begin to ascend. Magma has been encountered at least three times during geothermal drilling: in Iceland, Kenya and Hawai’i. In the latter instance, dacitic magma at a depth of 2.5 km entered the borehole and ascended 5.5 m over several minutes before stopping. Several international projects have proposed to drill shallow magma bodies or their thermal aureoles for scientific inquiry, energy generation or hazard mitigation. However, the possibility of uncontrolled magma ascent within a wellbore has not been quantitatively addressed. Here, we present an analysis of the competing time scales controlling the thermal survival of a magma column ascending the borehole. Rise time is most rapid for low viscosity magmas (like basalts) under large overpressure that are encountered at shallow depths in large-diameter boreholes. This magma must reach the surface before it is stalled by crystallization – either through radial cooling or growth of a crystal plug that exerts a drag on the borehole wall. Magma rise is also rapidly accelerated in the presence of a lowviscosity outer annulus of degassed volatiles, although it is not clear if this configuration could persist stably. Investigating a typical parameter range, we show that rhyolite magmas are too viscous to feasibly ascend wellbores, which is consistent with the experience at Hawai’i and Iceland. In exceptional circumstances, low-viscosity basaltic magmas could reach the surface before freezing, such as occurred in Námafjall geothermal field in 1977 during the Krafla Fires. |