Laboratory Observations of an Advancing Boiling Front in a Porous Medium and Correlation to Self Potential Measurements

We present new laboratory observations of an advancing boiling front radiating from an injection point in a simulated geothermal reservoir. The Berea sandstone model is a 26 cm cube that is either heated to a dry 125°C, or saturated with 145°C steam in two separate testing configurations. Cool water is applied at the sample center, and the advancing boiling front is identified by a radial array of 4 temperature sensors spaced at 1 cm intervals from the injection point. Using this data, the location of the vaporization front is observed, and temporal variations noted. Temperature information is correlated to self potential measurements made at the injection point. We show that these measurements can identify the point at which local fluid flow changes from 2-phase water / steam to single phase water, or that point when 2-phase flow ceases at the injection point. This is made possible by a large difference in resistivity between water and steam, and the resulting increase in self potential coupling for 2-phase flow. Furthermore, pressure fluctuations resulting from 2phase flow in a porous medium can be observed in the self potential record and used to indicate the state of local pore fluid. Extrapolation of this work to field sites will lead to a better understanding of vaporization at injection points and more economical injection strategies.