The rock physics and geochemistry of carbonates exposed to reactive brines

Abstract

When carbonate‐rich rocks are brought into contact with an acidic brine their mechanical and acoustic responses depend on many factors including pH, porosity, permeability, effective stress, and time. These complexities hinder the understanding of processes such as hydrothermal fluid circulation, seismicity, and deep burial diagenesis. The present study addresses how different lithofacies exposed to the same reactive brine undergo varying degrees of transformation and whether it is possible to remotely detect these phenomena in the Earth. Results are presented from fluid injections carried out on a large and varied set of calcareous rocks under hydrostatic stress. The output brine was analyzed for dissolved mineral concentrations and the rock porosity, permeability, axial strain, ultrasonic velocity, and images from electron microscopy were contrasted before and after injection. Microbialites were found to be the lithofacies most vulnerable to changes in their transport properties. However, all samples irreversibly compacted with the greatest strain in the most porous and permeable cores. The most extreme structural changes discovered through imaging were the welding of microporous zones, grain sliding, and the fracturing of various phases. Observations are consistent with a chemically enhanced weakening of the rock frame that generated compliant pores. The associated decrease in velocity of the dry rock can be approximated with linear relations that depend on both porosity and effective stress.

Reactive transport under stress: Permeability evolution in deformable porous media

“…21, 23, 38, 28, 45, 27, 44, 34].In this paper, we present a novel pore-scale model, which, to the best of our knowledge, is the first to describe the permeability evolution caused by coupling between dissolution and compaction in a porous medium. Our simulations show that increasing stress inhibits the permeability evolution, requiring longer time or larger volumes injected to reach a certain permeability, in agreement with experiments [41,17,9]. We also expose the impact of the dissolution regimes (Da), that is how stress effect changes between wormholing and a more uniform dissolution.…”

“…Relatively fewer works addressed porous media such as soils [8] and rocks [e.g. 46, 11, 47, 16, 6, 29, 32, 23], mostly focusing on the evolution of the mechanical properties.The evolution of transport properties in a porous rock sample has been addressed by recent experiments, in which rock cores subjected to external stress were flushed by a reactive fluid [43,41,17,42,40,9]. These experiments show considerable reduction in mechanical stiffness, and inhibition of permeability enhancement, in spite of net removal of solid mass.…”

Permeability Evolution of a Cemented Volcanic Ash During Carbonation and CO2 Depressurization

“…Benchtop, ultrasonic P wave (1.0 MHz) and S wave (2.25 MHz) velocities were also acquired for these samples in the dry condition from pulse time‐of‐flight measurements using pairs of contact transducers (12 mm in diameter) powered by a 5055PR, all manufactured by Panametrics. Acoustic coupling between the transducers and samples was aided by a high viscosity bonding medium (Clark & Vanorio, ). A relatively low water saturation ( S w ) for CVA5c was used in the initial test runs of our carbonation and depressurization setup and was obtained through re‐equilibration with ambient relative humidity (~55%) in the laboratory.…”

7. Seismic, rock physics, spatial models, and their integration in reservoir geophysics

“…The col lection of papers in Palaz and Marfurt (1997) provide an overview of rock properties and seis mic signatures in carbonate reservoirs. Because of their susceptibility to chemical processes, rock physics modeling in carbonates must take into account not only the physical effects of pore mi crogeometry but also geochemical effects of flu idsolid interaction (Vanorio et al, 2011;Clark and Vanorio, 2016).…”