Explosive Science: a Sexy New Answer to Climate Change

      
            What can rock deformation do for you?  Maybe turn your car exhaust into limestone!

            Climate change is one of the most talked about issues of the last two and a half decades.  Though its existence is now generally accepted, debates about its causes, extent and what’s to be done continue to rage; however, it has become clear that one major cause of climate change is increased levels of carbon dioxide gas in the atmosphere.  Diverse solutions have been proposed, from massive reforestation efforts to iron-seeding the oceans to stimulate algal blooms, all of which have encountered resistance from various environmental and/or industrial groups.

Industrial processes release carbon dioxide and other byproducts into the atmosphere.

One of the proposed technologies that has emerged at the front of the pack is geological carbon sequestration, or the practice of pumping carbon dioxide into the ground to remove it from the atmosphere.  This technology generally assumes two forms, reservoir sequestration and mineralization.

Reservoir sequestration involves storing carbon dioxide as a gas trapped in non-economic coal seams or dissolved in non-potable aquifers.  This method is popular because it is essentially already in use by the hydrocarbon drilling industry in the form of enhanced oil recovery.  Several notable drawbacks include contamination of potable aquifers, acidification of groundwater and concomitant changes in the mechanical behavior of reservoir rocks and catastrophic degassing and asphyxiation.

Mineralization takes advantage of natural reactions of minerals from deep within the earth with carbon dioxide in the atmosphere to form stable carbonate rocks.  The main benefit of this method is that it is safe.  The carbon dioxide gets stored as stable carbonate rocks, and is therefore relatively static.  The drawbacks of this method are mostly cost related.

Carbonate minerals precipitated in old mantle rock.

The reaction of deep minerals and carbon dioxide happens spontaneously, but slowly on human time scales.  To speed up the reaction, the materials must be ground to increase the amount of material exposed to carbon dioxide and heated to speed up the reaction.  Mining, transporting, and heat treating the rocks then manufacturing filters to put on smokestacks world-wide would be too energy intensive to make a real impact on carbon emissions.

However, there may be another way.

The reaction of carbon dioxide with mafic minerals actually releases energy in the form of heat, so once initiated the reaction can be self-sustaining.  The reaction also has a positive change in volume.  A runaway positive volume change reaction in a confined space is usually referred to by another name: a bomb.  If we can harness this power of this natural bomb to fracture the rocks, creating pathways for fluid flow and exposing fresh reactive material, we could maintain a self-heating, self-pulverizing in-situ carbon dump.  BAM!

           In our lab, we have been designing an apparatus to run carbon dioxide-rich fluids through reactive materials during active deformation and monitor the changes in mechanical and transport property behavior.  We plan to test different pressure and temperature regimes to see if we can constrain under which conditions this reaction driven cracking might take place.  This is still a new area of research, so there is much to learn.  The myriad possibilities make it truly exciting science!

First image from the Library of Congress.
Second image from http://cain.ice.ucdavis.edu/repository/SerpWebSoilPics.htm 

Shale Reservoirs - Natural Gas Generation & Extraction

Shale is a fine-grained, laminated sedimentary rock composed of clay- and silt-sized sediments typically deposited in low-energy environments, along with algal-, plant- and animal-derived organic matter.  Formation of natural gas within shale primarily occurs via thermogenic degradation (cracking) of organic material (kerogen) through time, although biogenic production can also occur.  As the organic-rich shale rocks are buried and subjected to increasing pressures and temperatures (at a typical geothermal gradient of 30^oC/km), breakdown of organic matter to petroleum hydrocarbons begins to occur.  The oil generation window occurs at approximately 60-120^oC (2-4km depth), while the gas generation window occurs at 100-200+^oC (3-6km depth).  The thermal maturity of the shale rock, generally measured by vitrinite reflectance (%R_o), is thus used to determine the type and quantity of hydrocarbons present.  Vintrinite, composed of cellulose and lignin, is a common organic constituent in coal and woody kerogen that has a vitreous luster and characteristic reflectance when heated.  Gas-generating shale rock has typical % R_o values ranging from 1.5-3.0%, where as oil generation predominantly occurs in sources rocks with a <1.5% R_o.

          Figure 1. Summary of the oil and gas formation process.

In addition to the thermal maturity of the source rock, the volume of produced gas is also dependent on the source of the organic matter and overall TOC content.  Kerogen is the insoluble portion of organic matter that is not extractable using organic solvents (versus bitumen).  There are four main types of kerogen, differentiated by their composition.  Type III, composed of woody terrestrial source material, is typically known to generate gas.  Globally averaged organic contents of shale are estimated to range between 2-10% carbonaceous material, with higher percentages being indicative of deposition under anoxic, reducing environments.  In general, the higher the TOC content within shale, the higher the probability of finding economically viable quantities of gas.

Once generated, natural gases are stored in natural fractures and pores between individual shale grains, and are also adsorbed onto the surfaces of, and also pores within, kerogen and clay particles.  Due to shale’s extremely low porosity and matrix permeability (< 1 millidarcy, mD), the organic-rich “black shales” can serve as both source rock and reservoir, trapping large estimated quantities of natural gas. 

                                  


                             Figure 2. Conventional and unconventional petroleum reservoirs.

 It is also this characteristic of shale structure that makes extraction of gas difficult and uneconomical, and thus why shale has been historically viewed as an “unconventional” gas resource.  However, current technological advances in horizontal drilling and fracture stimulation technology (ie: hydraulic fracturing) have enhanced access to natural fractures within shale and successfully increased matrix permeability, allowing the extraction of shale gas to become more economically viable energy.

Numerous questions and concerns are arising regarding the environmental effects of such drilling and extraction practices, with particular concern of the fluids used in hydraulic fracturing.  What is the exact composition of the “hydrofrack” fluids? What are the geochemical reactions of the fluids interacting with the shale?  What will happen to the integrity of the shale reservoir and surrounding rock formations over time?  While residual fluids left in the shale reservoir become trapped within fractures and pore spaces, ultimately inhibiting further natural gas generation or extraction over time?

My work ultimately aims to determine the chemical and physical interactions of different types of fracking fluids (water-based vs. CO₂-based) on shale rock formations of varying TOC and carbonate contents, thermal maturity and brittleness index, and thus determine their impact on gas recovery efficiency.  A more complete and quantitative understanding of the impacts of hydraulic fracturing fluids in shale reservoirs is imperative if we are to continue to depend on natural gas as a major energy resource.

References:

Arthur, J.Daniel, Bohm, Brian, and Layne, Mark, 2008.  Hydraulic Fracturing Considerations for Natural Gas Wells of the Marcellus Shale.Ground Water Protection Council Annual Forum.

U.S. Department of Energy (DOE), Office of Fossil Energy and National Energy Technology Laboratory, 2009.  Modern Shale Gas Development in the United States: A Primer.  DE-FG26-04NT15455.3-5.

http://oilandgasgeology.com/

http://www.glossary.oilfield.slb.com/search.cfm

http://www.3legsresources.com/media/A%20guide%20to%20shale%20gas.pdf  (Figure 1)

http://www.ems.psu.edu/~pisupati/ACSOutreach/Petroleum_2.html (Figure 2)

Synchrotron X-ray Tomography: A New Technology for Studying the Earth's Mantle

When one thinks of the standard tools geologists use to study the Earth, a particle accelerator is unlikely to come into mind. Nevertheless, Dr. Wenlu Zhu of the University of Maryland is utilizing the X-ray radiation given off by a synchrotron, which is a type of particle accelerator, to image the melt structures of synthetic partially molten mantle rocks. Zhu proposes that by imaging the melt distribution in three-dimensions, one can more completely quantify the transport properties of the partially molten region beneath ocean ridges, where plates are created. Zhu is the lead researcher on the project, which is a collaborative effort, involving scientists from the University of Maryland, Woods Hole Oceanographic Institute, and Argonne National Labs.

Experiments have already been conducted on similar synthetic systems. However, these studies rely on 2D measurements to infer the transport properties and therefore, paint an incomplete picture of the melt structure. At the Advanced Photon Source, Zhu et al. rotate their cylindrical, partially molten samples 180° through an x-ray beam in .12° increments to build digital 3D representations of their partially molten samples. Using the digital melt data, one can perform a so-called numerical experiment to simulate magma flowing through their samples. By doing this, Zhu et al. are able to quantify flow properties such as permeability. Permeability is defined as the capacity for the material to allow fluid, in this case magma, to pass through it. It is a fundamental property of the rock.

Figure 1: Diagram of imaging technique.

Figure 2: 3D image of melt distribution of olivine-melt from Zhu et al. (2011).

Preliminary results look promising. However, extrapolating these to the Earth may be challenging. So far, the only experiments that have been conducted have used the simplest system, which is olivine-melt. However, in the mantle, there is an assortment of minerals. Adding different mineral types may significantly alter the flow properties of the rock. In addition to having a compositional variety, the Earth’s mantle is moving at a rate of about 10 cm yr^-1. Although slow, this motion causes deformation that may give way to drastically different melt structures. The experiments currently being conducted by Zhu and her team are performed in isostatic equilibrium, and therefore, do not take in account the motion of the mantle. However, Zhu et al. have made plans to investigate the role of composition as well as deformation on the flow properties of partially molten mantle rocks.

Synchrotron radiation is a useful tool for studying magmatic processes in the Earth. So far, research performed by Zhu et al. has led to the first direct measurements of permeability for synthetic, partially molten mantle rocks. If their results help better constrain the melt transport properties at ocean ridges, they may help geologists better understand the mantle melting elsewhere on the Earth, such as at subduction zones where volcanoes usually form.

Zhu, W., Gaetani, G.A., Fusseis, F., Montési, L.G.J., De Carlo, F. (2011), Microtomography of Partially Molten Rocks: Three-Dimensional Melt Distribution in Mantle Peridotite, Science: 332 (6025) 88-91, [DOI:10.1126/science.1202221].

Tracking Potential Water on Europa

Europa's surface in true color (left) and false color (right). Courtesy of NASA/JPL.

Europa is a moon with a surface predominantly covered by ice orbiting the gas giant Jupiter.   The Voyager mission imaged a majority of Europa’s surface in the late 1970s.  The Galileo mission was launched in 1989 to orbit the Jovian system.    It obtained higher resolution of sections of Europa’s surface.  These missions revealed an ice-covered surface that resembled a ball of twine, covered with linear and curvilinear ridges.   Data from these missions also revealed that Europa generates its own magnetic field, which provides evidence that Europa has a briny ocean beneath the surface.

Image of a ridge on Europa courtesy of NASA/JPL.

Europan ridges are of specific scientific interest because they not only reveal information about geologic processes but also the potential for the existence of shallow water.  The interaction of water on Europa with the surface could have implications for the astrobiological potential of the icy satellite.  The reactions of water and the radiation that bombards the surface of Europa could cause chemical reactions that may be beneficial for the existence of life in the water.

One hypothesis for Europan ridge formation is that shallow water intrusions within the ice shell could freeze and expand to create the surface features of the ridges.  As the water freezes and expands it exerts a significant amount of pressure on the surrounding ice and may create cracks within the ice.   These cracks may serve as a conduit for water to reach the surface of Europa.

As the water intrusion freezes it exerts pressure in all 

directions and can cause upward flexure (red) and sideways compression and flexure (blue) to form a ridge.

In order to explore the water intrusion hypothesis several questions must be answered.  How much pressure can the freezing water exert on the surrounding ice?   Is this pressure enough to crack the ice?  How deep can the water intrusion be and still be able to create these ridge features.  These questions are being evaluated currently through the use of numerical models and may provide the answer to where is the water on Europa.

References and Further Reading: 

Cassidy, T., Coll, P., Raulin, F., Carlson, R. W., Johnson, R. E., Loeffler, M. J., Hand, K. P., et al. (2010). Radiolysis and Photolysis of Icy Satellite Surfaces: Experiments and Theory. Space Science Reviews, 153(1-4), 299-315. doi:10.1007/s11214-009-9625-3

Christopher F. Chyba and Cynthia B. Phillips. (2002). Europa as an Abode of Life. Origins of Life and Evolution of Biospheres (Vol. 32, pp. 46-47). doi:10.1111/j.1744-618X.2010.01158.x

Han, L., & Showman, A. P. (2008). Implications of shear heating and fracture zones for ridge formation on Europa. Geophysical Research Letters, 35(3), 6-10. doi:10.1029/2007GL031957

The Melt Redemption

The spreading center axis of a mid-ocean ridge is always believed to be the only exit for the upwelling melts beneath it to erupt. However, recent observations suggest that those seemingly abiding melts have probably already found a secret passage to escape. Throughout the global mid-ocean ridge system, mid-ocean ridges are offset by transform faults. These transform faults, with efficient conductive cooling, may produce a thickened lithosphere that directs melts away, but recent research showed that, the thickened crust along transform faults might indicate efficient melt extraction toward transforms.

Laura and Laurent (2011) apply a 3D model of melt migration and extraction to investigate the unusual thickened crust along an oceanic transform at fast spreading ridges, and suggest that a melt extraction zone (MEZ) should be responsible for the melt redemption. Melt migration at mid-ocean ridge system can be modeled by a 2-D process with two steps: (1) vertical upwelling under buoyancy within the asthenosphere; (2) lateral migration along a low-permeability boundary (permeability barrier) inclined towards the ridge axis within the thermal lithosphere. However, in 3-D, the thermomechanical  structure of the transform will affect the melt pathway to the ridge axis and result in thickened crust. Laura and Laurent solve 3-D model based on a thermal structure that incorporates rheological feedbacks associated with brittle deformation and hydrothermal cooling, and emphasize the potential structural controls on melt extraction. Their results show that an MEZ, interpreted as structural damage such as faults and dikes, intersects the permeability barrier, redistributes excess ridge crust to the transform domain, and accounts well for the unusual crustal thickness at intermediate and fast slipping transform faults observed.

Conceptual geometry of melt extraction zone (MEZ) (A) ssociated with a fast-spreading mid-ocean ridge, (B) associated with a transform fault.

(A) Isoviscous model results for crustal thickness for the simplified ridge-transform cases and varying the distance of lateral extraction; (B) schematic representation of crustal accretion at ridge segments with no shunting; (C) Schematic representation of crustal redistribution into the transform domain when shunting is allowed.

However, at ridges with slow and ultraslow spreading rate, no thickened crust has been observed along transform faults. Thus, more questions emerge. If there is any MEZ around slow to ultraslow slipping transform? What prevents these MEZs from acting efficiently? What’s the relationship between the depths of MEZ & permeability barrier and the spreading rate? My research will be focusing on these questions and I will be modeling the mid-ocean ridge systems with different spreading rate, and finally find out the reasons for the different crustal thicknesses among transform faults with different slipping rates.

Reference

Hebert, L. B., and L. G. J. Montési (2011), Melt extraction pathways at seg- mented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform, Geophys. Res. Lett., 38, L11306, doi:10.1029/2011GL047206.

The Strength of Mineral Growth

Salt crystal growth has the power to destroy cinderblocks, roadways and building walls from the inside out.  Surprised well take a look at this:

Figure 1: Sample of Cordoba Cream limestone exposed to Sodium 

Sulfate solution for 47 days. [Image source: G. Scherer]

This cinder block was infiltrated by a sodium sulfate solution through capillary action.    Tiny grains of salt crystals grew both on the inside and outside of this block.  The ones that grew from the internal pore spaces caused all of the damage seen.  The engineers performing this experiment were determining the conditions for which crystal growth will occur within the blocks causing instability and ultimately failure.  As you can see they definitely achieved their goal.  It was determined that the presence of a supersaturated fluid alone would not allow the crystals to continue their growth beyond simply filling the pore spaces of the material.  There must be a disjoining force to prevent the crystal from coming in direct contact with the pore space walls, allowing continued replenishment of the source fluid and thus continued crystal growth.

Figure 2: Views of typical crystallization patterns [Image source: C. Noiriel et al.]

  

So how does this knowledge help a geologist?

            Well directly it doesn’t.  Beyond the immediate crystal growth occurs within pore spaces of material.  But with a deeper look into their study you can find applications for it that may have a great deal of interest for geologists; particularly in the fields of carbon sequestration and possibly even in the oil industry regarding fracking.  In both of these fields instead of allowing a fluid (oil) to rise to the surface we are artificially forcing fluids into a confined environment.  The pore spaces in this environment are not filled with air as one might think instead they are usually filled with a brine solution with a very complex chemistry.  So what happens when this brine solution gets exposed to mixtures that are foreign to the confining pressures of such an environment?  Well that is not fully understood but one scenario that may occur is the dehydration of the brine resulting in crystal growth within pore spaces….  That sounds familiar.

Figure 3: X-ray tomography of sample ADA-1 depicting

internal crystal growth[Image source: C. Noiriel et al.]

            However simply translating the engineers work to a completely different set of conditions is not practical as there are numerous variables that do not match with their study.  One such variable is the confining pressure under which these crystals may grow.  Fortunately for us we have the ability to simulate these pressures here at Maryland.  If crystal growth does occur to the point of cracking the pore space walls could this lead to instability within the reservoir?  Well with any luck we will be able to begin to better understand at least this part of the question.

Image Sources and Further Reading

Scherer, G.W., 2004, Stress from crystallization of salt, Cement and Concrete Research, 34(9): 1613-1624.

Noiriel, C., Renard, F., Doan, M-L., Gratier, J-P., 2010, Intense fracturing and fracture sealing induced 

by mineral growth in porous rocks, Chemical Geology, 269(3-4): 197-209.

Measuring both major and trace element concentrations using LA-ICP-MS

    All mass spectrometry can only measure isotopic ratios. To obtain element abundances from isotopic ratios, we need two kinds of standards–external standards and internal standards. External standards, with known composition, provide the coefficients that convert signal strength (counts per second) to chemical concentration. These coefficients are then applied to samples to determine the concentrations of unknowns. However, these coefficients may vary from standards to samples. For LA-ICP-MS, such variation is caused by the inconstant laser ablation rate on different matrix and instrumental drift. This is why we need internal standards, which correct the variation (Fig. 1). An internal standard is usually an element with known concentration in both standards and samples. To learn the concentration of an internal standard, EMPA is applied prior to LA-ICP-MS analysis.

Fig. 1 A schematic flow of LA-ICP-MS analysis.

    Following Liu et al. (2008), I am now developing a new LA-ICP-MS analytical method that replaces the conventional internal standard, which is one element, with the sum of major element oxides. This enables us to measure both major and trace elements by LA-ICP-MS alone in that the sum of major element oxides is approximately 100% (assuming H₂O, CO₂, halogens, etc. contribute little). Since the major difference lies in the internal standard, my task is to examine whether this new internal standard can effectively correct the variation in signal-strength/concentration coefficients, and which external standard would be the best with regard to both accuracy and precision.

Fig. 2 Plot of deviation using BHVO as external standard for major and rare earth elements. The pink shaded area denotes 10% deviation area from referred values.

    Five reference glass materials, including BHVO, BCR-2G, BIR-1G, KL-2G and ML-3B, were analyzed in our work. In each session, one of the glasses was assigned as the external standard while the rest as samples. As for precision, 1 σ uncertainty is better than 4% for most elements. Accuracy, however, is not as good. In Fig. 2, deviation relative to referred values frequently falls outside the ballpark (±10%), especially for rare earth elements. In order to monitor potential isobaric interferences, many elements were measured by more than one isotope. Isotopic ratios were plotted in Fig. 3, with true values plotted at the end of each series. Obviously, isotopic ratios obtained in my experiments agree well with the true values, which indicates that isobaric interference may not be a significant issue in my experiments. So far, it remains unknown what caused the accuracy problem in this work.

Fig. 3 Plot of isotopic ratios as monitors of isobaric interference.

Reference

Liu, Y-S., Hu, Z-C., Gao, S., Gϋnther, D., Xu, J., Gao, C-G., Chen, H-H., (2008) In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard. Chem. Geol. 269, 237-251.