Where Lower Calcite Abundance Creates More Alteration: Enhanced Rock Matrix Diffusivity Induced by Preferential Dissolution

Abstract

Fractured rocks are essential for flow, solute transport and energy production in geosystems. Existing studies on mineral reactions in fractured rocks mostly consider single mineral systems where reactions occur at the fracture wall without changing rock matrix properties. This work presents multicomponent reactive transport numerical experiments in a fractured rock from the Brady’s field, a geothermal reservoir at a depth of 1,396 m in the Hot Springs Mountains, Nevada. Initial porosity, permeability, mineral composition (quartz, clay, and calcite), and fracture geometry are based on microscopy characterization and X-ray tomography. The model was calibrated using a CO2-saturated water flooding experiment. Three numerical experiments were carried out with the same initial physical properties however different calcite content. Although total dissolved masses are similar among the three cases, abundant calcite (50% (v/v), calcite50) leads to a localized, thick zone of large porosity increase while low calcite content (10% (v/v), calcite10) creates an extended and narrow zone of small porosity increase resulting in surprisingly larger change in effective transport property. After 300 days of dissolution, effective matrix diffusion coefficients increase by 9.9 and 19.6 times in calcite50 and calcite10, respectively, inducing corresponding 2.1 and 3.2 times rise in the slopes of power law tailing, a measure of transport properties. This counterintuitive results suggest that lower abundance of reactive minerals leads to greater alteration in the fractured media. Detailed analysis show that the effective rates of the fast-dissolving calcite are limited by diffusive transport in the altered matrix and the shape of the altered zone. In contrast, the while effective dissolution of slow-dissolving quartz depends on effective diffusion within the entire rock matrix. Calcite dissolution only occurs at the thin altered–unaltered matrix interface of tens of micrometers thickness occupying less than 1% of the total calcite content. In contrast, all quartz are effectively dissolving. This work highlights the importance of mineralogical complexity in determining mineral dissolution and rock matrix property evolution.

Distinct Source Water Chemistry Shapes Contrasting Concentration‐Discharge Patterns

“…More fractured regolith enables vertical flow from the shallow zone into deeper reservoirs, thereby increasing the connectedness and alleviating the differences between shallow and deep zones. Generally, lower permeability contrasts between reactive and nonreactive zones enhance the similarity of water chemistry (Wen & Li, , ; Wen et al, ). Musolff et al () represented catchment structural heterogeneity as mobile and immobile zones and analyzed several solutes in over 61 catchments.…”

Understanding watershed hydrogeochemistry: 2. Synchronized hydrological and geochemical processes drive stream chemostatic behavior

“…The product of kinetic rate constant and the surface area for chlorite (m 2 /m 3 ) (6.5 × 10 −10 mol/m 3 /s) is close to rates estimated from field data (7.6 × 10 −10 mol/m 3 /s) [ Jin et al ., ]. The specific surface area used here reflects the effective surface area that are truly reacting [ Li et al ., ; Moore et al ., ; Salehikhoo and Li , ; Wen et al ., ]. The fact that we need to lower SSA to reproduce data reflects the commonly observed laboratory‐field rate discrepancy [ White and Brantley , ].…”

Modeling Reactive Transport Processes in Fractures

“…At the boundary between the two domains, diffusive fluxes are balanced. The study of (Wen et al 2016) represents a simplified implementation of the hybrid approach, in which fracture aperture is discretized and assigned velocity values according to a modified parabolic profile that corrects for roughness. Using this model, the authors investigated the evolution of the diffusivity of the rock matrix bordering a rough fracture as a result of preferential dissolution of calcite in the rock matrix, and its dependence on calcite abundance.…”