Rapid migration of CO2-rich micro-fluids in calcite matrices

Abstract

The transport of supercritical fluids is a determining factor for several geological processes and fundamental in predicting natural resource accumulation and distribution. Calcite, ubiquitous in most geological environments, may contain supercritical CO trapped under the form of fluid inclusions that may move through grain boundaries affecting the rock physical properties. However, despite macroscopic evidence for this process, until recent it was not possible to characterize this process at the nano-scale due to the difficulty of such observations. In this study, we report nanometer-scale observations on calcite crystal surfaces and demonstrate that stress with absence of visible deformation produces fluid leakage from fluid inclusions. Atomic Force Microscopy scanning experiments on freshly cleaved calcite crystals containing visible fluid inclusions revealed the spontaneous formation of nanometer-scale hillocks on flat crystal terraces in only a few minutes, without evidence of surface dissolution. The fact the hillocks formed on flat surface in a short time was unexpected and suggests deposition of material from the inner crystal to the surface through small-scale fluid migration. We estimated the rate of this fluid mobility is by several orders of magnitude higher than the diffusion rate through vacancies estimated in calcite crystals showing that CO-rich fluids through micro-pore and nano-pore spaces is in reality much higher than previously assumed using current predictive models.

Escape of Supercritical-CO2 Fluids Trapped in Calcite Nano-metric Pores

“…These inclusions form a thin, continuous film that may short-circuit transport. Recently, Atomic Force Microscopy observations [7] provided evidence for stressinduced fluid leakage from micrometric and nanometric fluid inclusions, and that this phenomenon took place in a very short time even at standard conditions. These experiments could show that even a stress lower that the Shear Modulus (modulus of rigidity measuring the stiffness of calcite macroscopic crystals) can produce disruptions responsible for fast fluid movement through a calcite crystal.…”

“…When observed at the nano-meter scale (C) this crystal is in reality constituted by crystallites having 10-20 nano-metre size (grains) forming grain boundaries. The interface between two grains is, 2D defect, generates a polycrystalline material where nano-fluids may move very fast(Zuddas et al 2018)…”