Transient Behavior of the Solidified Magma Region in the Direct Utilization of Magma Energy

Abstract

In order to study the process of solidification, remelting and melting down of the subsurface heat exchange system for the direct heat extraction from molten magma, the transient behavoir of the solidified region, which is formed around a wellbore and is accompanied by a mushy region outside, is analyzed for both the closed and open heat extraction systems. The analysis utilizes the so-called enthalpy method that is a finite difference technique developed for analyzing heat conduction in a system with phase change between solid and liquid. It was revealed that the growth of solidified region due to heat extraction almost stopped in a fairly short time period in the case of the closed system. However, in the case of the open system, the solidified region is still growing even 1200 hours after heat extraction starts. The melting down analysis revealed that the heat extraction system disappeared due to melting down within a very short time period, e.g. nearly a day for a certain kind of rock, in the case of the closed system. In the case of the open system, on the other hand, the solidified region survives nearly a month at least after a complete stoppage of heat extraction.