Electrical Resistance Heating of Clay Layers in Water-Saturated Sand

摘要

Electrical resistance heating (ERH) experiments were performed in a two-dimensional water-saturated porous medium comprising an electrically conductive, low-permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. These were compared to experiments performed in homogeneous silica sand. All experiments were performed in the absence of a non-aqueous phase liquid (NAPL) or dissolved volatile organic compound (VOC). Temperature monitoring showed preferential heating in the clay lens and higher overall heating rates throughout the test cell compared to the homogeneous case. Gas production was localized around the sand-clay interface due to high temperature and low capillary displacement pressure. Above the clay lens, unexpected temperature plateaus were observed, similar to those observed in previous experiments during NAPL-water co-boiling. A conceptual model based on the consumption of thermal energy as latent heat of vaporization in the highly localized heating and gas production region adjacent to the clay lens is proposed to explain the temperature plateaus. Supporting data is drawn from images of the gas phase and electric current measurements. These results show that the use of co-boiling plateaus as an indicator of NAPL-water co-boiling could be misleading during applications of ERH at sites containing electrically conductive, low-permeability clay lenses embedded within less electrically conductive, higher-permeability sands.