Determining the influence of pore-scale geometry on wormhole formation

Chemical erosion of porous media is a complex process involving the interplay between flow, transport, reaction, and geometry evolution. Nonlinear coupling between these processes may lead to the formation of intricate patterns, the characteristics of which depend strongly on flow and mineral dissolution rates. In particular, in a broad range of physical conditions, long, finger-like channels or "wormholes" are spontaneously formed where the majority of the flow is focused. Wormhole formation is a multi-scale process, in which the dynamics of flow and dissolution at the pore-scale couple to the geometry of the emerging macro-scale features.

To determine the role of initial pore-scale characteristics (such as connectivity, topology and distribution of pore sizes) on wormhole formation we scan the initial geometry of limestone cores with X-Ray tomography, acidize the core to form wormholes, and rescan, comparing the resulting geometry to the initial pore configuration. For some experiments tomography is also performed during dissolution. To visualize preferential flow paths created both by dissolution and the initial pore geometry we perform contrast experiments with neutron tomography wherein a partially dissolved core is flooded with heavy water and then injected with regular water, or X-Ray tomography wherein it is flooded with regular water and then injected with an iodide solution. Additionally, we use initial scans as input to a numerical model developed to simulate flow, transport, and dissolution in porous media. The output geometry of the model is compared to that of the scanned final geometry to tune the fidelity of input data required to model features at larger scales. Initial results of in-tomograph acidization experiments show that wormhole growth can be tracked at sufficiently high spatial (30-70 micrometers) and temporal (3-15 minutes) resolution to measure core- and pore-scale changes. Contrast experiments highlight the combined importance of the wormhole structure and pre-existing pore network in further evolution of the wormhole with flow focusing in large pores near branching wormhole tips, similar to model predictions. Further, models are capable of predicting final wormhole geometries given initial conditions from tomographic core scans at 30-70 micrometer resolution.