Observing the evolution of geometry and flow in dissolving rocks

Dissolution of porous media is a complex process involving nonlinear couplings between flow, transport, the evolving geometry of the media, and the process of dissolution itself. In some cases these couplings lead to the formation of intricate patterns, the characteristics of which depend strongly on flow, mineral dissolution rate, and initial pore space geometry. In particular, finger-like channels, termed "wormholes", are spontaneously formed where the majority of flow is focused. Capturing the dynamics of wormhole growth has so far been largely limited to numerical models, with few studies observing their time evolution in real rocks. In this study we capture the dynamics of both wormhole growth and their alteration of flow in the rock by placing the experiment within neutron and X-Ray tomographs and scanning while actively dissolving limestone cores. 

To observe the evolution of wormhole geometry limestone samples are dissolved in a cell translucent to X-Rays and neutrons. For each experiment a high (30-35 micrometer) resolution scan was taken of the initial sample geometry, as well as the geometry after dissolution. During acidization tomography was performed at 60-70 micrometer resolution with acquisition times ranging from three to six minutes. For several experiments dissolution was paused and and a contrasting agent injected to visualize the flow field within the sample. Flow field experiments were performed with neutron tomography by first injecting heavy water, followed by light water as the contrast agent, and with X-Ray tomography by injection a solution of potassium iodide into light water. Results of dissolution experiments show that wormhole growth can be tracked at sufficiently high spatial and temporal resolution to measure changes to the pore space.

These experiments highlight the importance of the near-tip region on the dynamics of wormhole propagation. In particular, focusing of the flow is shown to take place not only within the wormhole but also significantly (>5mm) into the porous region past the wormhole tip. These "virtual channels" link the tip with the neighboring regions of high porosity. Several such virtual channels can exist, indicating potential paths of further growth, and demonstrate the strong coupling of flow and geometry evolution. Additionally, we observe a dramatic dependence of the dissolution patterns on the initial pore structure, in particular the total initial porosity, distribution of pore sizes and connectivity of the pore space. In pore spaces with poor connectivity and low porosity the wormholes tend to be very tortuous and thin. Such wormholes advance through rapid, almost discontinuous jumps, guided by the above-described pre-focusing mechanism. On the other hand, the advancement of a wormhole in a well-connected rock is much more diffuse, controlled by merging between neighboring pore spaces.