Understanding diffusion-controlled bubble growth in porous media using experiments and simulations

Nucleation and subsequent expansion of gas bubbles in porous media is relevant to many applications, including oil recovery, carbon storage, and boiling. We have built an experimental setup using microfluidic chips to study the dynamics of bubble growth in porous media. Visualization experiments of the growth of carbon dioxide bubbles in a supersaturated dodecane solution were conducted. We show that bubbles grow as dissolved gas molecules inside the oversaturated liquid diffuse to the gas-liquid interface. Bubbles expanding inside a porous medium displace the liquid phase until the cluster of the gas-filled pores becomes connected to the outlet at the critical gas saturation, which is used as a measure for the total liquid displacement. Our experiments uniquely focus on the growth of a single bubble and show that larger pressure drops lead to faster bubble growth while resulting in lower critical gas saturations. A nonlinear pore-network model is implemented to simulate bubble growth. We compare model predictions for bubble growth dynamics to our experimental results and present the need for further theoretical development to capture deviations from invasion-percolation when a large pressure drop is applied.

Anaerobic biodegradation of dissolved ethanol in a pilot-scale sand aquifer: Gas phase dynamics

Groundwater contamination from ethanol (e.g., alternative fuels) can support vigorous biodegradation, with many possible reactions producing dissolved gases. The objective of this study was to improve the understanding of the development and evolution of trapped gas phase changes occurring within an ethanol plume undergoing biodegradation. The experiment performed involved highly detailed spatial and temporal monitoring of gas phase saturations using Time Domain Reflectometry probes embedded in a 2-dimensional (175 cm high × 525 cm long) synthetic aquifer (homogeneous sand tank with horizontal groundwater flow). Ethanol injection immediately promoted gas-producing reactions, including: fermentation, denitrification, sulphate-reduction and iron(III)-reduction, with methanogenesis developing between 69 and 109 days. Substantial in situ increases in trapped gas were observed over ~330 days, with maximum gas saturations reaching 27% of the pore volume. Despite sustained gas production, this maximum was never exceeded, likely due to the onset of gas phase mobilization (i.e., ebullition) upon reaching a buoyancy-capillarity threshold. Reductions in the quasi-saturated hydraulic conductivity, resulting from the gas phase accumulation, were restricted by ebullition to a factor of ≤2; but still appeared to alter the groundwater flow field. Overall, trapped gas saturations exhibited high spatial and temporal variability, including declines within the plume and increases outside of the plume. Influential factors included vertically-shifting ethanol inputs and resultant secondary redox reactions, microbial controls on redox zonation, ebullition, and altered groundwater flows. These observations have implications for the transport of gases and volatile compounds within plumes and above the water table at sites with groundwater contamination from ethanol or other highly degradable organics.

Generalized network modeling: Network extraction as a coarse-scale discretization of the void space of porous media

A generalized network extraction workflow is developed for parameterizing three-dimensional (3D) images of porous media. The aim of this workflow is to reduce the uncertainties in conventional network modeling predictions introduced due to the oversimplification of complex pore geometries encountered in natural porous media. The generalized network serves as a coarse discretization of the surface generated from a medial-axis transformation of the 3D image. This discretization divides the void space into individual pores and then subdivides each pore into sub-elements called half-throat connections. Each half-throat connection is further segmented into corners by analyzing the medial axis curves of its axial plane. The parameters approximating each corner-corner angle, volume, and conductivity-are extracted at different discretization levels, corresponding to different wetting layer thickness and local capillary pressures during multiphase flow simulations. Conductivities are calculated using direct single-phase flow simulation so that the network can reproduce the single-phase flow permeability of the underlying image exactly. We first validate the algorithm by using it to discretize synthetic angular pore geometries and show that the network model reproduces the corner angles accurately. We then extract network models from micro-CT images of porous rocks and show that the network extraction preserves macroscopic properties, the permeability and formation factor, and the statistics of the micro-CT images.

Gas production and transport during bench-scale electrical resistance heating of water and trichloroethene

The effective remediation of chlorinated solvent source zones using in situ thermal treatment requires successful capture of gas that is produced. Replicate electrical resistance heating experiments were performed in a thin bench-scale apparatus, where water was boiled and pooled dense non-aqueous phase liquid (DNAPL) trichloroethene (TCE) and water were co-boiled in unconsolidated silica sand. Quantitative light transmission visualization was used to assess gas production and transport mechanisms. In the water boiling experiments, nucleation, growth and coalescence of the gas phase into connected channels were observed at critical gas saturations of Sgc=0.233±0.017, which allowed for continuous gas transport out of the sand. In experiments containing a colder region above a target heated zone, condensation prevented the formation of steam channels and discrete gas clusters that mobilized into colder regions were trapped soon after discontinuous transport began. In the TCE-water experiments, co-boiling at immiscible fluid interfaces resulted in discontinuous gas transport above the DNAPL pool. Redistribution of DNAPL was also observed above the pool and at the edge of the vapor front that propagated upwards through colder regions. These results suggest that the subsurface should be heated to water boiling temperatures to facilitate gas transport from specific locations of DNAPL to extraction points and reduce the potential for DNAPL redistribution. Decreases in electric current were observed at the onset of gas phase production, which suggests that coupled electrical current and temperature measurements may provide a reliable metric to assess gas phase development.

The critical gas saturation in a porous medium in the presence of gravity

The critical gas saturation, S(gc), denotes the volume fraction of the gas phase at the onset of bulk gas flow during the depressurization of a supersaturated liquid in a porous medium. In the absence of gradients due to viscous or gravity forces, S(gc) is controlled by nucleation, capillary forces, and the rate of decline of the supersaturation. In this paper we address one important additional effect, that of buoyancy. We use 2-D pore-network simulations, based on invasion percolation in a gradient (IPG), and corresponding scaling relations to obtain the dependence of S(gc) on the gravity Bond number, B, under conditions of slow growth, namely when mass transfer is sufficiently fast. The critical gas saturation approaches two plateau values at low and high Bond numbers. In the in-between region it scales as a power law of B, which for a 2-D lattice is S(gc) approximately B(-0.91).