The role of fractures on coupled dissolution and precipitation patterns in carbonate rocks

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Abstract

A series of laboratory experiments is presented which investigates the influence of fractures on the evolution of hydraulic conductivity and porosity caused by flow, dissolution and precipitation—and the interplay among them—in carbonate rocks. We inject input solutions of HCl and H2SO4, at different flow rates, into carbonate rock samples containing different configurations of fractures. As a consequence, host rock dissolves and gypsum subsequently precipitates. These experiments allowed us to determine effects of fracture orientation, fracture wall roughness, fluid flow rate and chemistry, and coupled dissolution/precipitation reaction mechanisms on overall patterns of hydraulic conductivity and porosity evolution. To separate the relative effects of these parameters, flow experiments used quasi-two-dimensional (2D) rock fractures, three-dimensional (3D) intact rock cores, and 3D rock cores containing different fracture configurations. Changes in pressure gradient along the sample, recorded at specific time intervals during the experiments, were used to calculate the overall evolution of hydraulic conductivity. The effluent acid was analyzed for Ca2+ and SO42- concentrations to estimate corresponding porosity changes. After each experiment, the rock sample was retrieved and sectioned in order to study the pore space geometry, micromorphology, and distribution of precipitated and dissolved minerals. We find that fracture sample geometry and chemical composition of the reacting fluid are the two main factors most strongly influencing precipitation and dissolution patterns within a fracture. The interplay of these factors is controlled largely by the flow rate of the injected fluid. In 3D systems, we find that fracture orientation controls whether precipitation or dissolution is the dominant process: a through-flow fracture led to a dissolution-dominated system, in contrast to an isolated fracture which led to a precipitation-dominated system under the same experimental conditions. Moreover, comparison of the hydraulic conductivity and porosity evolution among the intact core, the isolated fracture and (multiple) fracture system experiments demonstrates that, under the same flow conditions, cores containing isolated fractures clog more rapidly than intact cores, while cores with multiple fractures clog even more rapidly than the isolated fracture systems. Finally, in spite of the complex coupling of flow and reaction processes between intact rock and fractures, good agreement was obtained between time-varying estimates and experimentally obtained values of system hydraulic conductivity for a core sample containing a through-flow fracture.

Introduction

The importance of fractures in the transport of contaminants in groundwater systems is well-known. While the hydraulic conductivity of fractures is usually orders of magnitude larger than that of the host rock matrix, dissolution and precipitation processes can modify significantly the physical and chemical properties of fractured porous media. Qualitative and quantitative description of these coupled processes, and their interaction with both anthropogenic and natural pollutants, is fundamental to management and protection of water resources, as well as to the analysis of geological formations and to processes of interest in petroleum and chemical engineering.

A specific instance in which coupled precipitation and dissolution phenomena occur arises in oil reservoir exploitation. Acid is often injected in an attempt to increase hydraulic conductivity in enhanced oil recovery operations, as well as to reduce the resistance to fluid flow during oil well maintenance procedures. However, some dissolution products can precipitate and ultimate clog the pore space. For example, injection of HF acid into sandstone led to partial pore clogging by precipitated silica [3]. Similarly, Rege and Fogler [13] showed that injection of HCl into carbonate rock results in significant precipitation of ferric hydroxide. Another situation for which coupled precipitation and dissolution phenomena can occur is associated with waste rock deposits from mining operations.

Several theoretical and experimental studies have provided partial understanding of the evolution of hydraulic conductivity caused by precipitation or dissolution processes in single fractures [5], [7], [8], [10], [12] and fractured media [17], [18]. However, only a few studies have analyzed effects of coupled precipitation and dissolution on the temporal evolution of fluid flow and solute transport; moreover, these studies have focused on porous rocks without fractures. Rege and Fogler [13] examined both theoretically and experimentally a system of concurrent dissolution of calcium carbonate and precipitation of ferric hydroxide. They demonstrated that periodic fluctuations in the overall permeability, and the frequency of these fluctuations, are related to the interplay between geochemical reactions and flow rates. Singurindy and Berkowitz [15] investigated the evolution of hydraulic conductivity in uniform carbonate rock due to competition among flow, dissolution of calcium carbonate and precipitation of gypsum (CaSO4 · 2H2O; hereafter, we use the term gypsum). Their experimental study presented a range of injected H+/SO42- ratios and flow rates in which the effective hydraulic conductivity displays oscillatory patterns. This study provided a conceptual picture of carbonate rock evolution, describing a variety of possible system behaviors (e.g., system clogging by precipitation dominance or wormhole formation by dissolution dominance), and a detailed description of the factors controlling the oscillations. A parallel study [16] investigated the process of dedolomitization, which couples dolomite dissolution with calcite precipitation and magnesium liberation into solution. Here too, the interplay among flow, precipitation and dissolution processes led to oscillations in the temporal and spatial evolution of effective hydraulic conductivity and porosity.

In spite of the interest of these problems, few analyses exist and the available literature provides only partial understanding of hydraulic conductivity evolution by coupled flow, precipitation and dissolution in fractured rocks. A model simulation describing dissolution and precipitation of minerals during water-rock interaction in fractured granite is given by Sausse et al. [14]. The permeability calculation of this model is based on description only of the fracture network, with the microcrack (fracture) and matrix permeabilities being neglected. Another study of coupled precipitation and dissolution processes in fractured rocks is based on a multicomponent reactive transport model in fractured dolostone [1]. This 1D model accounts for dedolomitization during diffusion from a fracture towards the rock matrix and during advective flow along a fracture. Together, these processes were seen to cause changes in fracture/matrix volume and porosity.

The main aim of this current experimental study is to investigate the influence of fractures on the evolution of hydraulic conductivity structure, caused by the interplay of flow, dissolution of calcium carbonate, and precipitation of gypsum, in carbonate rocks. Specifically, we use laboratory flow cells to determine effects of fracture orientation, fracture wall roughness, fluid flow rate and chemistry, and coupled dissolution/precipitation reaction mechanisms on overall patterns of hydraulic conductivity and porosity evolution. To separate the relative effects of these parameters, we work with flow experiments in quasi-two-dimensional (2D) rock fractures, in three-dimensional (3D) intact rock cores, and in 3D rock cores containing different fracture configurations. While the flow and geochemical conditions considered here do not necessarily correspond to specific natural situations, the dynamic behaviors we investigate are relevant to, e.g., acid injection for enhanced oil recovery [13] and neutralization of sulfuric acid wastes [4].

Section snippets

Rock samples and fluid input solutions

Calcareous sandstone consisting of 97.5% CaCO3 and 2.5% SiO2, collected from the Rosh-Hanikra coastal area of northern Israel, was used as the host rock. Input solutions of two acids, hydrochloric and sulfuric, were injected into the rock samples as reacting fluid. Coupled calcium carbonate dissolution and gypsum precipitation occurred according to the following reactions [9]:CaCO3+2H+=Ca2++CO2+H2OCaCO3+2H++SO42-+H2O=CaSO4·2H2O+CO2

Calcium carbonate dissolution is a mass-transfer limited process

Experimental results

As outlined above, we first examine flow dynamics in the quasi-2D fracture systems. We then consider intact and fractured 3D core samples.

Conclusions

A range of evolutionary patterns of overall hydraulic conductivity occurs in systems for which fluid flow is accompanied by coupled precipitation and dissolution processes. Fractures present in such systems play a particularly important role. We have carried out a series of laboratory experiments which allow us to delineate, qualitatively and in part quantitatively, the effect of fractures on flow and dissolution/precipitation patterns. In the systems we considered, oscillations in the

Acknowledgement

We thank Dr. Ishai Dror, Simon Emmanuel, Dr. Eugenia Klein and Dr. Eytan Sass for assistance and useful discussions. The US-Israel Binational Science Foundation (Contract No. 1999142) and British Petroleum International are thanked for financial support.

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