Flow and Reaction in Carbonate Rocks: Calcite Dissolution Experiments Revisited
Simon Emmanuel and Yael Levenson
Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel
The dissolution of calcite in rocks influences a range of geological and industrial processes from the evolution of karst landscapes to the response of carbonate-hosted oil reservoirs to enhanced recovery techniques. Numerous studies have focused on determining reaction rate laws from individual calcite crystals, and it is often assumed that the empirical rate laws derived from such studies can be applied in a straightforward manner to describe calcite dissolution rates in porous and fractured rocks. However, as numerical models used to simulate the evolution of porosity and permeability in carbonate rocks often adopt such empirical rate laws, this assumption has a crucial impact on model behavior and, ultimately, predictive capability.
There are a number of reasons to think that mineral reactions within rocks cannot adequately be described by simply incorporating standard rate laws into continuum reactive transport models. Firstly, the coupled processes of fluid flow, solute transport, and mineral reaction may create different solute concentrations in adjacent pores, and potentially even within individual pores, thereby creating heterogeneous reaction rates that cannot be easily described in a continuum framework. Furthermore, from a thermodynamic point of view, the behavior of systems containing multiple crystals is inherently different to that of systems comprising single crystals; due to processes such as Ostwald ripening, in which large crystals grow at the expense of smaller crystals, reaction rates can vary from crystal to crystal, particularly when the system is close to equilibrium. Moreover, experiments are often carried out on large crystals, which are not significantly affected by interfacial energy effects; rocks on the other hand often comprise micron and nanometer size crystals which might be expected to grow at much slower rates. However, only limited attempts have been made to incorporate such pore scale effects into continuum models, and the processes that affect mineral reactions in porous media are still far from being fully understood.
One tool often used to infer mechanisms and reaction rates during crystal growth and dissolution is atomic force microscopy (AFM). Using the primary imaging mode, the surface of minerals and rocks can be scanned, yielding a precise topographic map with nanometer spatial resolution. Crucially for the study of mineral reactions, surfaces can be imaged in fluids using a flow-through chamber so that real time in situ changes can be measured during an experiment.
Most AFM studies examining calcite dissolution have tested the behavior of isolated crystals, typically in contact with an immobile fluid phase. In our work, however, we performed dissolution experiments using porous carbonate rock samples exposed to a constantly flowing fluid. We demonstrate how AFM can be used to estimate in situ reaction rates on these complex substrates, and we identify the primary dissolution mechanisms operating at the interface between a mobile fluid phase and a porous substrate. Critically, we observe a high degree of spatial heterogeneity that is directly related to the porous structure. The precise mechanism behind this behavior is discussed and we present a 3D numerical model that accurately predicts the dissolution patterns on a porous carbonate surface. We also discuss how continuum descriptions of reactive transport can be modified to incorporate such pore scale heterogeneity.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012