Quantitative visualization of gas dynamics during electrical resistance heating

Paul R. Hegele, Kevin G. Mumford

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Contaminated Sites and Remediation I

ABSTRACT: Electrical resistance heating (ERH) is an in situ thermal treatment (ISTT) technology used to remediate contaminated soil and groundwater by increasing subsurface temperatures to achieve boiling of water and non-aqueous phase liquids (NAPLs), including chlorinated solvents and petroleum fuels. Once the boiling point is reached, gas bubbles nucleate, grow and connect throughout the heated zone. Mass removal is achieved through transport of the gas phase to recovery points such as soil vapour extraction or multiphase extraction wells. This transport can be driven by pressure gradients through a connected gas phase, or by the buoyant transport of gas bubbles. An understanding of gas bubble nucleation, growth and connection mechanisms before the onset of gas transport is fundamental to the design and operation of ISTT technologies, because the removal of NAPL mass from the subsurface cannot be achieved without gas capture. Despite its importance, there is limited experimental work concerning gas dynamics during ERH. The purpose of this work is to (i) develop an experimental apparatus that allows heating by ERH and visualization using a light transmission method, which is a laboratory technique that allows quantitative visualization of fluid saturations, and (ii) investigate the onset of gas production and gas connection during heating. Multiphase flow in groundwater, including the flow of gases, is characterized by the hysteretic relationships of capillary pressure, saturation and relative permeability (e.g. Corey, 1994). The data on which these constitutive relationships are based are typically generated experimentally by subjecting a sample to external drainage and imbibition cycles, where capillary pressure is controlled at the sample boundaries. In these experiments, the onset of nonwetting fluid (including gas) transport occurs at the emergent gas saturation (Sgm), when a network of connected nonwetting fluid channels first spans the sample. Sgm has been shown experimentally to coincide with the inflection point of a capillary pressure-saturation (Pc-S) curve in homogeneous material (White et al., 1972). Unlike external drainage, gases generated by heating or depressurization do not enter from a boundary and, instead, nucleate internally throughout the sample. The mechanisms of internal drainage have been studied in the solution gas drive (e.g. Sheng et al., 1999) and carbon dioxide sequestration literature (e.g. Zuo et al., 2012). In these applications, the onset of nonwetting fluid mobility occurs at the critical gas saturation (Sgc), when nucleated bubbles connect such that they can become mobile. Values of the critical gas saturation reported in the literature vary from 0.5% to 38%, depending on factors such as the rate of depressurization, degree of supersaturation, experimental apparatus and definition of nonwetting fluid mobility (i.e. buoyancy of a disconnected gas cluster, connection throughout the sample, or production at a boundary) (Sheng et al., 1999). In the bench-scale ERH experiments presented here, a heated sample underwent internal drainage and Sgc was identified when the gas first appeared mobile in the images, based on the subtraction of successive images that clearly showed local imbibition during mobilization. These Sgc values were compared to Sgm values derived from capillary-pressure saturation curves to investigate the validity of using traditional multiphase flow concepts to describe gas mobility and recovery during ISTT. The apparatus consisted of a thin experimental cell (40 cm tall x 20 cm wide x 1 cm thick) with tempered borosilicate glass walls in a polytetrafluoroethylene (PTFE) frame. This allowed for visualization as well as temperature, electrical and chemical compatibility. A light emitting diode (LED) panel with a diffuser was used as a light source, which runs cooler than incandescent light sources and avoids problems associated with flickering that is typical of fluorescent lights. A digital single-lens reflex (DLSR) camera mounted on a tripod was used to capture transmitted light intensities at 1 min intervals during heating. Deionized water with 2.0 g/L of dissolved sodium chloride (NaCl) was used as an electrolyte to conduct electricity between two graphite electrodes, and voltage was controlled externally with a variable autotransformer. The electrodes were placed 16 cm apart in a silica sand pack (0.7 mm median grain size; Schroth et al., 1996), with one electrode in the middle of the pack and the other at the top boundary. The cell was outfitted with six PTFE-coated thermocouple probes to measure temperature and a digital ammeter to measure alternating current. Because thin cells required for light transmission suffer from significant conductive heat loss through their walls, dissolved carbon dioxide (CO2) was added to the pore water to generate gas at temperatures below the boiling point of water. This solution was prepared by bubbling CO2 gas into a reservoir containing the electrolyte solution, and then siphoning the solution into the bottom of the experimental cell after packing. The pH, electrical conductivity and temperature of the CO2-NaCl-H2O solution in the reservoir were measured during the course of the siphon to ensure that the dissolved CO2 was close to saturation at atmospheric pressure. The light transmission images were processed using methods described in Niemet and Selker (2001) to obtain saturation fields. A total of five replicate heating experiments were performed.

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