GAS INJECTION EXPERIMENTS AT DIFFERENT SCALES: TRANSITION BETWEEN BUBBLY AND CHANNELIZED FLOW

Different gas flow pattern can occur during gas injection of CO₂ or O₂ into water-saturated porous media, like saline aquifers or contaminated aquifers. In general the gas flow can be classified into coherent (continuous) and incoherent (discontinuous) gas flow. The transition between these two flow regimes depends on injection rate and pore size distribution. An understanding of the relevant processes that control the different flow regimes, will lead to a better design of field experiments. For instance the interplay between channelized and bubbly flow is the basis to design oxygen gas injection for stimulating in-situ bioremediation. CO₂-gas injection for CCS-technology works also in the region where both gas flow regimes can occur or transitions between them.

The paper gives an overview about interesting visualization experiments of different flow regimes (basic research). We investigate the corresponding flow pattern at pore scale, subscale and REV-scale. We consider the stability of a single gas channel at pore scale using a strict thermodynamical approach calculating the free energy by a variational method. The main advantage of this method is that undulating gas channels can be treated. The conceptual model of an undulating gas channel is superior to the conceptual model of a straight capillary which is used throughout the literature. For the first time we investigate the role of viscous forces on the transition between coherent and incoherent gas flow. Invasion percolation models, which are often used for studying this transition, neglect viscous forces. We describe the main flow characteristics by continuum modelling at REV-scale. Furthermore, we discuss the interesting subscale approach [Stauffer et al., 2009], in order to describe the channelized flow pattern.

At field scale, we discuss two different field applications of direct gas injection: 1. Oxygen gas injection for stimulation of BTEX-degradation and 2. CO₂-gas injection for CCS-technology. To measure and estimate the heterogeneous in-situ gas saturation we used dense sensor arrays of optodes, redox sensors and TDR-sensors. To measure dissolved gas transport we used SF6-tracer technique. We present TOUGH2-simulations in order to study gas flow under low- and high pressure gas injection within heterogeneous aquifers.