Helge Hellevang

On the potential for CO 2 mineral storage in continental flood basalts - PHREEQC batch- and 1D diffusion-reaction simulations

Continental flood basalts (CFB) are considered as potential CO2 storage sites because of their high reactivity and abundant divalent metal ions that can potentially trap carbon for geological timescales. Moreover, laterally extensive CFB are found in many place in the world within reasonable distances from major CO2 point emission sources.Based on the mineral and glass composition of the Columbia River Basalt (CRB) we estimated the potential of CFB to store CO2 in secondary carbonates. We simulated the system using kinetic dependent dissolution of primary basalt-minerals (pyroxene, feldspar and glass) and the local equilibrium assumption for secondary phases (weathering products). The simulations were divided into closed-system batch simulations at a constant CO2 pressure of 100 bar with sensitivity studies of temperature and reactive surface area, an evaluation of the reactivity of H2O in scCO2, and finally 1D reactive diffusion simulations giving reactivity at CO2 pressures varying from 0 to 100 bar.Although the uncertainty in reactive surface area and corresponding reaction rates are large, we have estimated the potential for CO2 mineral storage and identified factors that control the maximum extent of carbonation. The simulations showed that formation of carbonates from basalt at 40 C may be limited to the formation of siderite and possibly FeMg carbonates. Calcium was largely consumed by zeolite and oxide instead of forming carbonates. At higher temperatures (60 – 100 C), magnesite is suggested to form together with siderite and ankerite. The maximum potential of CO2 stored as solid carbonates, if CO2 is supplied to the reactions unlimited, is shown to depend on the availability of pore space as the hydration and carbonation reactions increase the solid volume and clog the pore space. For systems such as in the scCO2 phase with limited amount of water, the total carbonation potential is limited by the amount of water present for hydration of basalt.

The Potential for Low-Temperature Abiotic Hydrogen Generation and a Hydrogen-Driven Deep Biosphere

The release and oxidation of ferrous iron during aqueous alteration of the mineral olivine is known to reduce aqueous solutions to such extent that molecular hydrogen, H2, forms. H2 is an efficient energy carrier and is considered basal to the deep subsurface biosphere. Knowledge of the potential for H2 generation is therefore vital to understanding the deep biosphere on Earth and on extraterrestrial bodies. Here, we provide a review of factors that may reduce the potential for H2 generation with a focus on systems in the core temperature region for thermophilic to hyperthermophilic microbial life. We show that aqueous sulfate may inhibit the formation of H2, whereas redox-sensitive compounds of carbon and nitrogen are unlikely to have significant effect at low temperatures. In addition, we suggest that the rate of H2 generation is proportional to the dissolution rate of olivine and, hence, limited by factors such as reactive surface areas and the access of water to fresh surfaces. We furthermore suggest that the availability of water and pore/fracture space are the most important factors that limit the generation of H2. Our study implies that, because of large heat flows, abundant olivine-bearing rocks, large thermodynamic gradients, and reduced atmospheres, young Earth and Mars probably offered abundant systems where microbial life could possibly have emerged.

On the forcing mechanism for the H 2 -driven deep biosphere

Heat produced in the mantle and core of the Earth by the decay of radioactive elements and mineral fusion results in large-scale mantle convection. The outer shell of the Earth that floats on the convective mantle is divided into rigid lithospheric plates. Subduction of dense cold plates into the mantle leads to plate tectonics. At divergent plate margins, heat is dissipated through hydrothermal convection cells. As ocean water is entrained into hydrothermal cells it interacts with fresh magmatic rocks and liberates ferrous iron. This iron reduces the ocean water to such an extent that it decomposes and forms hydrogen. Molecular hydrogen, as the most reduced component in the system, forms a basal component to a deep dark biosphere powered by metastable redox gradients. In this paper we review the driving force behind a hydrogen-driven deep biosphere. We present abundant observations of hydrogen produced at natural hydrothermal settings as well as in laboratory experiments. The key mineral reactions responsible for the bulk of this hydrogen production are then presented. A division of the reaction progression into an oxidized state and a reduced state is suggested. The amount of hydrogen produced is insignificant in the oxidized state whereas it becomes proportional to the amount of ferrous iron oxidized in the reduced state. The bulk of basalt-hosted aquifers are expected to reside in the oxidized state because of the low content of ferrous minerals, whereas abundant olivine in ultramafic-hosted systems is responsible for large-scale hydrogen production. Today the majority of the seafloor is basaltic. The Archean seafloor on the other hand consisted of fewer ultramafic exposures, but was dominated by ultramafic magnesium-rich lavas with a higher potential for hydrogen production than the present seafloor.

Examination of CO2-SO2 solubility in water by SAFT1. Implications for CO2 transport and storage.

Removal of toxic gases like SO2 by cosequestration with CO2 in deep saline aquifers is a very attractive suggestion from environmental, human health and economic point of view. Examination of feasibility of this technique and forecasting the underlying fluid-rock interactions requires precise knowledge about the phase equilibria of the ternary mixture of SO2-CO2-H2O at conditions relevant to carbon capture and storage (CCS). In this study, a molecular-based statistical association fluid theory (SAFT1) model is applied to estimate the phase equilibria and aqueous phase density of mixtures. The molecules are modeled as associating segments while self-association is not allowed. The model is tested for different SO2 concentrations and for temperatures and pressures varying between 30-100 °C and ∼6-30 MPa, respectively. Comparison of the results of this model against the available experimental data of binary systems demonstrates the capability of this equation of state, although, in contrast to the previous works, no temperature dependent binary interaction coefficient is applied. The results show that the total solubility of SO2 + CO2 in water varies exponentially with respect to SO2 concentrations, i.e., at low concentrations of SO2, total changes in solubility of the CO2 in water is negligible.

Evolution of lavas in the Late Ordovician/Early Silurian Solund‐Stavfjord Ophiolite Complex, West Norway

The geochemistry of the volcanic sequence of the Late Ordovician/Early Silurian Solund-Stavfjord Ophiolite Complex (SSOC) of the Western Norwegian Caledonides has been investigated along 12 stratigraphic sections spaced over a lateral distance of ∼60 km. The metabasalts commonly show a cyclic organization, comprising sheet flows followed by pillow lavas and with volcanic breccias at the top of some volcanic units. The proportions of the volcanic rock types vary considerably along-axis, even within short distances. The robust nature of the sheeted dike complex, the proportion and regional distribution of the various volcanic rocks, the aphyric nature of the metabasalts, as well as the predominance of Fe-Ti basalts suggest that the SSOC formed at an intermediate to fast spreading ridge, probably within a back-arc basin. The variations in the concentrations of the incompatible (e.g., Zr) and compatible (e.g., Cr) elements in the lavas are substantial, i.e., from 49–384 ppm and 66–443 ppm, respectively. Nd isotopic ratios show only minor variations, suggesting that the lavas were generated from an isotopically uniform source. The variations in the incompatible elements (represented by Zr) define stratigraphic units ∼25 to 150 m thick, with either gradual decreases or increases in Zr concentrations, while Cr may show different trends. The different Zr-Cr covariations are interpreted in the light of a numerical model in which fractional crystallization and mixing of various magmas are the principal processes. Through upwardly Zr-increasing units, Cr generally decreases and estimated magma densities increase, compatible with progressive fractional crystallization. However, through upwardly Zr-decreasing units, Cr and estimated magma densities either increase or decrease, trends that are attributed to hybridization in frequently replenished magma chambers. Within short lateral distances (<1 km), the geochemical stratigraphy changes dramatically, excluding eruption from a homogeneous, axially continuous magma chamber. Instead, we propose that the volcanic sequence was fed from small, separate magma lenses that evolved independently of each other.

Constraints on natural global atmospheric CO2 fluxes from 1860 to 2010 using a simplified explicit forward model.

Land-use changes until the beginning of the 20th century made the terrestrial biosphere a net source of atmospheric carbon. Later, burning of fossil fuel surpassed land use changes as the major anthropogenic source of carbon. The terrestrial biosphere is at present suggested to be a carbon sink, but the distribution of excess anthropogenic carbon to the ocean and biosphere sinks is highly uncertain. Our modeling suggest that land-use changes can be tracked quite well by the carbon isotopes until mid-20th century, whereas burning of fossil fuel dominates the present-day observed changes in the isotope signature. The modeling indicates that the global carbon isotope fractionation has not changed significantly during the last 150 years. Furthermore, increased uptake of carbon by the ocean and increasing temperatures does not yet appear to have resulted in increasing the global gross ocean-to-atmosphere carbon fluxes. This may however change in the future when the excess carbon will emerge in the ocean upwelling zones, possibly reducing the net-uptake of carbon compared to the present-day ocean.

On Layer Specific CO2 Plume Distributions and Variability in Mineralization Potential

Candidate sandstone reservoirs for CO2 storage in the North Sea typically display layered geometries and varying degree of geological heterogeneity. The spatial distribution of depositional environments and the diagenetic imprint controls the reservoir properties. From correlation of well data (geophysical logs and rock samples) property grids may be constructed. Deterministic scenario modeling, taking the geological interpretation of facies into account, shows layer specific estimated dissolved volumes and lateral reaches of the plume. In comparison, standard averaging techniques such as harmonic mean for estimating the vertical permeability will yield a smaller plume front area and a mean vertical distribution. Using Eclipse 300 to illustrate the effects of averaging and PHREEQ-C to model the geochemical system, we demonstrate the range of mineralization potentials within the Johansen Formation (Northern North Sea) according to observed variations in mineralogy.

Carbon Capture and Storage (CCS)

Carbon Capture and Storage (CCS) involves capturing CO2, which is the process of separating CO2 from natural gas or flue (exhaust) gas, and then storing it so that it is not emitted to the atmosphere. The background for CCS is a concern that increasing atmospheric CO2 concentrations will cause global climate changes, ocean acidification and a sea level rise, with dramatic negative consequences for large populations (IPCC 2007).

Fracture Patterns in Organic Rich Mudstones - Implications for Primary Petroleum Migration

Scanning Electron Microscopy (SEM) of thin sections taken from organic rich Jurassic shale core samples from the Norwegian Continental Shelf reveal the existence of what appear to be fine scale fracture patterns filled with migrated petroleum oriented normal to depositional bedding. The suggested fracture patterns forms in shales rich in fine grained clays (smectite, kaolinite) at temperatures around 90 sC (2500m burial depth). Coarser grained shales appear not to be fractured on a thin section scale. Fewer and less extensive micro fractures are found in samples intermediate between the fine and coarse grained samples investigated. The mechanism responsible for the fractures is suggested to be pressure build up during initial maturation around isolated patches of organic material before a functioning migration network has been formed. This indicate that a certain amount of petroleum must be produced before an effective long range organic network functioning as a migration pathway is established in source rocks. The absence of fracture networks in coarser grained source rocks indicate that functioning migration networks are establish in such rocks before local fracture pressure is reached. This is probably due to higher permeability resulting in a more effective displacement of the continuous porewater phase.

Effect of Pore-Scale Mineral Spatial Heterogeneity on Chemically Induced Alterations of Fractured Rock: A Lattice Boltzmann Study

Fractures are the main flow path in rocks with very low permeability, and their hydrodynamic properties might change due to interaction with the pore fluid or injected fluid. Existence of minerals with different reactivities and along with their spatial distribution can affect the fracture geometry evolution and correspondingly its physical and hydrodynamic properties such as porosity and permeability. In this work, evolution of a fracture with two different initial spatial mineral heterogeneities is studied using a pore-scale reactive transport lattice Boltzmann method- (LBM-) based model. The previously developed LBM transport solver coupled with IPHREEQC in open-source Yantra has been extended for simulating advective-diffusive reactive transport. Results show that in case of initially mixed structures for mineral assemblage, a degraded zone will form after dissolution of fast-dissolving minerals which creates a resistance to flow in this region. This causes the permeability-porosity relationship to deviate from a power-law behavior. Results show that permeability will reach a steady-state condition which also depends on transport and reaction conditions. In case of initially banded structures, a comb-tooth zone will form and the same behavior as above is observed; however, in this case, permeability is usually less than that of mixed structures.