Combined Neutron and X-ray Time-Resolved Tomography of Wormhole Growth in Dissolving Limestones

Abstract:

Dissolution of porous media introduces a positive feedback between fluid transport and chemical reactions at mineral surfaces leading to the formation of pronounced wormhole-like channels. While the impact of flow rate and reaction rate on the shapes of the wormholes is now well understood, much less is known about the dynamics of their propagation. In this study we capture the evolution of wormholes and their effects on flow patterns by in-situ imaging of dissolving limestone cores. We use a combination of X-Ray and neutron tomography: the former to image the evolving pore space, the latter for imaging of the flow paths. Combined neutron X-Ray tomography was conducted at Institut Laue-Langevin, with further X-Ray only experiments at Institute of Oil and Gas, Krakow, Poland. These experiments highlight the importance of the near-tip region on the dynamics of wormhole propagation. In particular, flow focusing is shown to take place not only within the wormhole but also in a relatively wide region past the wormhole tip. Thus, the flow channel network is more extended than the porosity network.
These "virtual flow channels" are guided by the heterogeneities within the limestone matrix. Several such virtual channels can exist, indicating potential paths of further wormhole growth, demonstrating the strong coupling of flow and geometry evolution. As the wormhole progresses the flow map changes rapidly, with new pathways opening and old ones abandoned. As a result, the final wormhole path through the sample is tortuous, with relatively sharp changes of the propagation direction. We also show the differences between the growth of dominant wormholes (formed at high flow rates) and conical ones (formed at lower flow rates), and characterize the evolution of their geometrical properties in time. The conical wormholes are seen to evolve in two stages: propagation stage (where they progress relatively fast through the sample, not unlike their dominant counterparts) and the maturation stage, where they mainly growth in size, acquiring the characteristic shape of an inverted cone.
Finally, we study dissolution patterns in porous calcarenites, characterized by large porosity and high connectivity of the pore space. In these rocks, the dissolution patterns are much more diffuse, controlled by merging between neighboring pore spaces.