Basalt Core Research Articles

3D Quantification of Pore Networks and Anthropogenic Carbon Mineralization in Stacked Basalt Reservoirs.

Basalt formations are promising candidates for the geologic storage of anthropogenic CO2 due to their storage capacity, porosity, permeability, and reactive geochemical trapping ability. The Wallula Basalt Carbon Storage Pilot Project demonstrated that supercritical CO2 injected into >800 m deep Columbia River Basalt Group stacked reservoir flow tops mineralizes to ankerite-siderite-aragonite on month-year time scales, with 60% of the 977 metric tons of CO2 converted within 2 years. The potential impacts of mineral precipitation and consequent changes in the rock porosity, pore structure, pore size, and pore size distributions have likely been underestimated hitherto. Herein, we address these knowledge gaps using X-ray microcomputed tomography (XMT) to evaluate the pore network architecture of sidewall cores recovered 2 years after CO2 injection. In this study, we performed a detailed quantitative analysis of the CO2-reacted basalt cores by XMT imaging. Reconstructed 3D images were analyzed to determine the distribution and volumetric details of porosity and anthropogenic carbonate nodules in the cores. Additional mineralogic quantification provided insight into the overall paragenesis and carbonate growth mechanisms, including mineralogic/chemical zonation. These findings are being used to parametrize multiphase reactive transport models to predict the fate and transport of subsurface CO2, enabling scale-up to commercial-scale geologic carbon storage in basalts and other reactive mafic-ultramafic formations.

Heavy magnesium isotopic compositions of basalts erupted during arc inception: Implications for the mantle source underlying the nascent Izu-Bonin-Mariana arc

Basalts formed during the early development of an arc are usually buried, but they record critical information relating to the mantle source of the overriding plate prior to the addition of subducting slab components. Basalts recovered at Site U1438 of International Ocean Discovery Program (IODP) Expedition 351, in the Amami Sankaku Basin (ASB), formed during the transition from forearc basalt (FAB) and boninite eruptions to stratovolcano developments in the Izu-Bonin-Mariana (IBM) arc. In this study, we present magnesium (Mg) isotopic data (δ26Mg) for the ASB basalt core samples, formed at arc inception in the absence of down-going slab components, and report the implications of these data for the characteristics of the mantle sources underlying the proto-IBM arc. The δ26Mg values of the ASB basalts range from −0.21‰ to +0.08‰, with an average value of −0.13 ± 0.07‰. These values are systematically higher than those of mid-ocean ridge basalts (MORBs) (−0.25 ± 0.06‰). No obvious effects of post-eruptive alteration, fractional crystallization, partial melting, or subduction component addition can be identified in the Mg isotopic compositions either of the ASB basalts or their mantle source, given the absence of correlations between the δ26Mg values and the proxies of these processes (e.g., K/Nb, Nb/Zr, Ba/La, Nb/Y, etc.). We conclude that the high δ26Mg values of the ASB basalts are inherited from their mantle source that was enriched in heavy Mg isotopes. Melting of an ultra-depleted, refractory spinel peridotite source containing talc-bearing serpentinite components can explain the origin of the heavy Mg isotopic compositions in the ASB basalts. The presence of water-rich serpentinite components in the mantle provides a new perspective on how a refractory mantle source can undergo partial melting at temperatures and pressures similar to the formation of MORBs without the influence of subducted materials, which has important implications for the initiation of plate subduction. Furthermore, the Mg isotopic contributions of subducted additions and mantle source materials should be carefully discussed in future studies due to the heterogeneity of Mg isotopes in the mantle.

Borehole magnetic fabric anomalies in KBH 07 Deccan basalt core from Deep Continental Drilling, western Maharashtra, India

Borehole magnetic fabric anomalies in KBH 07 Deccan basalt core from Deep Continental Drilling, western Maharashtra, India

Frictional Behavior of the Stressed Basalt Fracture

The frictional properties and sliding stability of rocks are critically important in several fields, e.g., earthquake dynamics, extraction of underground resources, and sequestration of carbon dioxide. Here a self-developed triaxial apparatus was employed to conduct sliding tests under the coupled hydro-mechanical (HM) conditions. We determined the evolution of frictional strength in the stressed basalt fracture. The relations were established through velocity-stepping (VS) experiments on split basalt core subject to various stress, infill, and hydraulic conditions. The basalt exhibited velocity-strengthening behavior under cyclic velocities of 1–30 μm/s. Moreover, the frictional property was observed to be stress- and infill-dependent, showing lower frictions at both larger effective normal stresses and larger infill thickness. Besides, the sliding stability of the basalt fracture is found to be velocity- and stress-dependent. In particular, an unstable slip of a stick-slip was observed at a small velocity of 1 μm/s, but stabilizes with increasing shearing velocities at about 6 μm/s. Meanwhile, there is a transition from stable slip to unstable slip with the increasing effective normal stress, which is 5 MPa. Whereas, the thickness of the gouge as large as 2 mm does not seem to influence the sliding stability, even though the gouge thickness will significantly weaken the frictional strength of basalt fracture. The results suggested that the evolution of basalt fracture friction is controlled considerably by hydro-mechanical conditions, the sliding velocity, and gouge thickness. The frictional strength tends to vary with specified hydraulic, mechanical, gouge-bearing conditions. Furthermore, the frictional behavior transits between stable- and unstable-slip based on the velocities, stress pore pressure, and gouge thickness. These conclusions are drawn from constrained conditions.

Comparison of neurotransmitter levels, physiological conditions, and emotional behavior between isolation-housed rats with group-housed rats.

Brain monoaminergic neurotransmitters, such as dopamine (DA), serotonin (5-HT), and noradrenaline (NA), play crucial roles in neuronal and physiological functions, including social behaviors. Isolation housing may induce behavioral and neurochemical abnormalities in rats, although its influence on neurotransmitter levels remains obscure. This study investigated the influence of isolation- or group-housing on core body temperature (Tcore ), locomotor activity (ACT), emotional behavior, and neurotransmitter levels in male Wistar rats. Behavioral changes were monitored using the open field test (OFT) and social interaction test (SIT). After 4weeks, brain tissues were collected to quantify 5-HT, DA, and NA concentrations. Body weight and basal Tcore during both the light and dark phase were higher in isolation-housed than in group-housed rats, although no significant difference was seen in ACT. No significant differences were observed during the OFT. Isolation-housed rats showed increased line crossing and decreased social behavior during the SIT. Isolation-housed rats exhibited decreased levels of 5-HT in the caudate putamen and amygdala, and elevated and decreased NA levels in the paraventricular hypothalamic nucleus and hippocampus, respectively. However, DA levels were unaffected. Thus, housing environments may affect brain areas that regulate various neuronal and physiological functions, such as memory, stress responses, and emotional behavior.

Structural characteristics of columnar jointed basalt in drainage tunnel of Baihetan hydropower station and its influence on the behavior of P-wave anisotropy

Researches about the structural characteristics of rock mass are the basis for understanding their failure modes, mechanism properties as well as evaluation of structural stability during the process of excavation. Natural columnar jointed basalt has complex structures and exhibits remarkable anisotropy and size effect. In this study, the observation window method was firstly applied to describe the natural columnar joints and pillar shapes at engineering scale and then the structural characteristics of columnar joints were analyzed. The roughness of columnar joints scanned from large-size rock core obtained at drainage tunnel #1-1 was secondly investigated based on the 3D scanning method. Some techniques, such as field statistical analysis and CT scanning were used to analyse the characteristics of internal implicit joints in columnar jointed basalt. Thirdly, the large scale columnar jointed basalt cores were tested by P-wave, which assisted to investigating the anisotropy of columnar jointed basalt. The experimental and analytical results indicate that, at the engineering scale, natural columnar jointed basalt is heterogeneous, exhibiting the features of mosaic structure and mainly quadrilateral in shape. At the large-size rock core scale, the roughness of columnar joints is low with the value of 5.558. At the basalt column scale, the internal implicit joints are not randomly distributed, instead, showing obvious tendency. The internal implicit joints and columnar joints are important factors contributing to the special failure mode of columnar jointed basalt. At the rock core scale, with increasing angle between the P-wave propagation direction and column, the anisotropy of columnar jointed basalt core gradually increases and the degree of anisotropy is proportional to the number of columnar joints. The internal implicit joints could also affect the P-wave anisotropy of basalt column and the results in this study can provide some references for projects related to columnar jointed rock mass.

Evaluation of the geothermal potential of the western Snake River Plain based on a deep corehole on the Mountain Home AFB near Mountain Home, Idaho

A geothermal exploration corehole was drilled to a total depth of 1821.5 m on the Mountain Home Air Force Base near Mountain Home, Idaho. The corehole was used to collect an unusually large amount of data, including uniaxial compressive stress (UCS) experiments on core samples, to evaluate the geothermal potential of the western Snake River Plain. In addition, unlike many exploration holes in this region, a fluid entry was encountered at 1745.3 m and flowed artesian to the surface. A maximum temperature of 149.4 °C was calculated for the entry. A temperature log run on the corehole from 3 to 1675 m is nearly linear with little variation. The average geothermal gradient is 73 °C/km, and the average heat flow between 200 and 1500 m is 102 ± 15 mW/m2. Chemical analyses of a sample from the fluid entry suggest that a significant proportion of the water is not meteoric. Five geothermometers show equilibrium temperature in the range of 133–157 °C. Furthermore, based on the unconfined UCS experiments on basalt core samples, a brittle unit was found to comprise the fractured reservoir that the geothermal water flows from, while an overlying ductile unit acts as a hydrothermal caprock. This implies that the reservoir/caprock pair may be a target for future exploration wells drilled to delineate the extent of the potential resource and the boundaries of the connected fracture network.

Multimodal imaging and stochastic percolation simulation for improved quantification of effective porosity and surface area in vesicular basalt

Abstract Improved methods for predicting fluid transport and vesicle connectivity in heterogeneous basalts are critical for determining the long-term reaction and trapping behavior of sequestered carbon dioxide and maximizing the efficiency of geothermal energy production. In this study we measured vesicle geometry, pore connectivity, and vesicle surface area of three basalt cores from the CarbFix carbon storage project in Iceland using a combination of micro-computed tomography, clinical computed tomography, and micro-positron emission tomography. A vesicle percolation simulator was then constructed to quantify vesicle connectivity across a complete range of porosities, pore size distributions, and microporosity conditions. Percolation simulations that incorporate important geologic features such as microporosity are able to describe the trend of experimental measurements made in this study and in previous work, without relying on statistical or empirical techniques. Simulation results highlight and quantify the trade-off between storage capacity and reactive surface area in high porosity basalts. Experiment and simulation results also indicate that there is very limited connected pore space below total porosity values of 15%, guiding improved site selection for large scale CO2 storage projects. Use of this stochastic percolation simulation method for basalt storage reservoir evaluation will enable more accurate storage capacity and mineral trapping estimates.

Ultrasonic P- and S-Wave Attenuation and Petrophysical Properties of Deccan Flood Basalts, India, as Revealed by Borehole Studies

Petrophysical properties and ultrasonic P- and S-wave attenuation measurements on 35 Deccan basalt core specimens, recovered from Killari borehole site in western India, provide unique reference data-sets for a lesser studied Deccan Volcanic Province. These samples represent 338-m-thick basaltic column, consisting four lava flows each of Ambenali and Poladpur Formations, belonging to Wai Subgroup of the Deccan volcanic sequence. These basalt samples are found to be iron-rich (average FeOT: 13.4 wt%), but relatively poor in silica content (average SiO2: 47.8 wt%). The saturated massive basalt cores are characterized by a mean density of 2.91 g/cm3 (range 2.80–3.01 g/cm3) and mean P- and S-wave velocities of 5.89 km/s (range 5.01–6.50 km/s) and 3.43 km/s (range 2.84–3.69 km/s), respectively. In comparison, saturated vesicular basalt cores show a wide range in density (2.40–2.79 g/cm3) as well as P-wave (3.28–4.78 km/s) and S-wave (1.70–2.95 km/s) velocities. Based on the present study, the Deccan volcanic sequence can be assigned a weighted mean density of 2.74 g/cm3 and a low Vp and Vs of 5.00 and 3.00 km/s, respectively. Such low velocities in Deccan basalts can be attributed mainly to the presence of fine-grained glassy material, high iron contents, and hydrothermally altered secondary mineral products, besides higher porosity in vesicular samples. The measured Q values in saturated massive basalt cores vary enormously (Qp: 33–1960 and Qs: 35–506), while saturated vesicular basalt samples exhibit somewhat lesser variation in Qp (6–46) as well as Qs (5–49). In general, high-porosity rocks exhibit high attenuation, but we observed the high value of attenuation in some of the massive basalt core samples also. In such cases, energy loss is mainly due to the presence of fine-grained glassy material as well as secondary alteration products like chlorophaeite, that could contribute to intrinsic attenuation. Dominance of weekly bound secondary minerals might also be responsible for the generation of microcracks, which may generate squirt flow in saturated samples. Hence, we argue that the Deccan basalts attenuate seismic energy significantly, where its composition plays a major role.

CO2 Mineral Sequestration in Naturally Porous Basalt

Continental flood basalts are extensive geologic features currently being evaluated as reservoirs that are suitable for long-term storage of carbon emissions. Favorable attributes of these formations for containment of injected carbon dioxide (CO2) include high mineral trapping capacity, unique structural features, and enormous volumes. We experimentally investigated mineral carbonation in whole core samples retrieved from the Grand Ronde basalt, the same formation into which ∼1000 t of CO2 was recently injected in an eastern Washington pilot-scale demonstration. The rate and extent of carbonate mineral formation at 100 °C and 100 bar were tracked via time-resolved sampling of bench-scale experiments. Basalt cores were recovered from the reactor after 6, 20, and 40 weeks, and three-dimensional X-ray tomographic imaging of these cores detected carbonate mineral formation in the fracture network within 20 weeks. Under these conditions, a carbon mineral trapping rate of 1.24 ± 0.52 kg of CO2/m3 of basalt per...

Evolution of permeability and fluid pathways in the uppermost oceanic crust inferred from experimental measurements on basalt cores

Evolution of permeability and fluid pathways in the uppermost oceanic crust inferred from experimental measurements on basalt cores

A 1251m-thick Deccan Flood Basalt Pile Recovered by Scientific Drilling in the Koyna Region, Western India

ABSTRACT A 1251 m thick succession of Deccan lava flows has been recovered by scientific drilling in the Koyna region, western part of the Deccan Volcanic Province (DVP). Forty-six lava flows, starting with the first flow overlying the granitic basement in the region, have been identified through mesoscopic studies of cores obtained from a 1500 m deep borehole KBH7 in Panchgani village of Patan sub-division. Mesoscopic observations supported by petrographic studies bring out the characteristics of the basaltic pile. The salient findings are as follows. The thickness of Deccan trap in the Panchgani area is 1251.20 m, comprising a succession of 40 simple lava flows and 6 compound lava flows. The contact with the underlying granitic basement occurs over a short span of 90 cm. Infra-trappean sediments are absent. Overall, ∼74% of the basaltic pile is constituted of massive basalt and ∼24% vesicular/amygdaloidal basalt. Red bole horizons are observed in 7 flows. Nine giant plagioclase basalt (GPB) flows are identified in the single vertical section. The long basalt core may serve as a model Deccan trap section for correlation with the flows exposed at the surface in the Koyna and surrounding region and support detailed investigations to constrain the duration of Deccan volcanism.

Effect of an 860-m thick, cold, freshwater aquifer on geothermal potential along the axis of the eastern Snake River Plain, Idaho

A 1912-m exploration corehole was drilled along the axis of the eastern Snake River Plain, Idaho. Two temperature logs run on the corehole display an obvious inflection point at about 960 m. Such behavior is indicative of downward fluid flow in the wellbore. The geothermal gradient above 935 m is 4.5 °C/km, while the gradient is 72–75 °C/km from 980 to 1440 m. Projecting the higher gradients upward to where they intersect the lower gradient on the temperature logs places the bottom of the cold, freshwater Snake River Plain aquifer, which suppresses the geothermal gradient at this location, at least 860 m below the surface. The average heat flow for the corehole between 983 and 1550 m is 132 mW/m2. Although the maximum bottom-hole temperature extrapolated from a measured time–temperature curve was only 59.3 °C, geothermometers suggest an equilibrium temperature on the order of 125–140 °C based on a single fluid sample from 1070 m. Furthermore, below 960 m the basalt core shows obvious signs of alteration, including a distinct color change, the formation of smectite clay, and the presence of secondary minerals filling vesicles and fracture zones. This alteration boundary could act as an effective cap or seal for a hot-water geothermal system.

Permeability, porosity, and mineral surface area changes in basalt cores induced by reactive transport of CO2‐rich brine

AbstractFour reactive flow‐through laboratory experiments (two each at 0.1 mL/min and 0.01 mL/min flow rates) at 150°C and 150 bar (15 MPa) are conducted on intact basalt cores to assess changes in porosity, permeability, and surface area caused by CO2‐rich fluid‐rock interaction. Permeability decreases slightly during the lower flow rate experiments and increases during the higher flow rate experiments. At the higher flow rate, core permeability increases by more than one order of magnitude in one experiment and less than a factor of two in the other due to differences in preexisting flow path structure. X‐ray computed tomography (XRCT) scans of pre‐ and post‐experiment cores identify both mineral dissolution and secondary mineralization, with a net decrease in XRCT porosity of ∼0.7%–0.8% for the larger pores in all four cores. (Ultra) small‐angle neutron scattering ((U)SANS) data sets indicate an increase in both (U)SANS porosity and specific surface area (SSA) over the ∼1 nm to 10 µm scale range in post‐experiment basalt samples, with differences due to flow rate and reaction time. Net porosity increases from summing porosity changes from XRCT and (U)SANS analyses are consistent with core mass decreases. (U)SANS data suggest an overall preservation of the pore structure with no change in mineral surface roughness from reaction, and the pore structure is unique in comparison to previously published basalt analyses. Together, these data sets illustrate changes in physical parameters that arise due to fluid‐basalt interaction in relatively low pH environments with elevated CO2 concentration, with significant implications for flow, transport, and reaction through geologic formations.

Whole rock basalt alteration from CO2-rich brine during flow-through experiments at 150 °C and 150 bar

Four flow-through experiments at 150°C were conducted on intact cores of basalt to assess alteration and mass transfer during reaction with CO2-rich fluid. Two experiments used a flow rate of 0.1ml/min, and two used a flow rate of 0.01ml/min. Permeability increased for both experiments at the higher flow rate, but decreased for the lower flow rate experiments. The experimental fluid (initial pH of 3.3) became enriched in Si, Mg, and Fe upon passing through the cores, primarily from olivine and titanomagnetite dissolution and possibly pyroxene dissolution. Secondary minerals enriched in Al and Si were present on post-experimental cores, and an Fe2O3-rich phase was identified on the downstream ends of the cores from the experiments at the lower flow rate. While we could not specifically identify if siderite (FeCO3) was present in the post-experimental basalt cores, siderite was generally saturated or supersaturated in outlet fluid samples, suggesting a thermodynamic drive for Fe carbonation from basalt-H2O-CO2 reaction. Reaction path models that employ dissolution kinetics of olivine, labradorite, and enstatite also suggest siderite formation at low pH. Furthermore, fluid-rock interaction caused a relatively high mobility of the alkali metals; up to 29% and 99% of the K and Cs present in the core, respectively, were preferentially dissolved from the cores, likely due to fractional crystallization effects that made alkali metals highly accessible. Together, these datasets illustrate changes in chemical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO2.

Effect of carbon dioxide sequestration on the mechanical properties of Deccan basalt

Carbon dioxide sequestration is considered to be an efficient method of curbing the release of carbon dioxide emissions into the atmosphere and to mitigating corresponding effects on climate change. In this regard, basaltic rocks are among the potential repositories due to their ability to trap carbon dioxide in form of carbonate minerals. A series of laboratory-scale saturation experiments was conducted on three types of basalts that were collected from different horizons of the Deccan Volcanic Province (DVP), India. The basalt cores were treated in a saturation chamber for different exposure periods under low-pressure and room-temperature conditions. A set of key mechanical, physical, and chemical properties of the pre- and post-treated samples were analyzed to identify saturation-related changes. The results show that exposure of the basalts to carbon dioxide has a strong effect on their strength. However, the extent of the effect is different for different types of basalts and is strongly controlled by the exposure time, the mineralogical composition, and rock texture. X-ray diffraction and scanning electron microscopy (SEM) show that saturation leads to dissolution of the mafic minerals, and precipitation of new carbonate minerals in the cracks and vesicles of the host rock. These changes in the mineralogical content and the development of micro-cracks in the samples are interpreted as being the primary factors that affect the loss of integrity in the rock.

Continental Drift and Plate Tectonics vis-a-vis Earth’s Expansion:Probing the Missing Links for Understanding the Total Earth System

Hypotheses like continental drift, plate tectonics or Earth’s expansion should not be considered as viable in view of solid and rigid state of mantle which, in contrast, would resist such phenomena. Based on Hilgenberg’s model of Earth’s expansion (1933), the author elucidates that before expansion when the Earth was small and devoid of oceans, its mantle must have been sufficiently fluid owing to association of ocean-forming water. Further, matching thickness of fluid outer core and extent of radial expansion of the Earth strongly support that the outer core was opened up as a void geosphere owing to planetary expansion. At the deep interior of the planet, owing to occurrence of a void or pseudo-fluid geosphere separating basaltic mantle and solid iron core, an additional force of reverse gravity would develop acting in opposite direction of normal inwardly directed force of gravity. This postulation leads us to consider that in the deep interior of the planet an upwardly directed force of gravitational attraction would act in a predominant manner, thereby sustaining sufficiently low temperature and pressure condition and magnetic nature of the inner core which completely agrees with observed features of terrestrial magnetism. Over the Earth’s surface the crustal layer was fragmented due to expansion while through the expansion cracks widespread incidences of magma emission occurred forming rudimentary ocean basins. With further expansion, these basins were expanded and filled up with water that degassed from the mantle associated with the process of magma emission while owing to desiccation, the mantle itself eventually turned into a rigid body. Before expansion of the planet when the iron core and the mantle were juxtaposed to each other, due to external magnetic influence the magnetic iron core was deflected causing major change in polar and equatorial disposition of the planet. Subsequently in younger geological period when due to expansion a major void geosphere was opened up between the iron core and mantle, external magnetic influences caused the magnetic core to execute smooth revolutions giving rise to new magnetic phenomena like pole reversal and polar wandering, documented over the planet’s younger strata. It may be noted that while due to expansion, the continental fragments would tend to move away from one another, owing to rotation of the planet along its axis of rotation some continental fragments came closer to each other or even collided to form mountain ranges.

Generation of Undistorted Photomosaics of Cylindrical Vesicular Basalt Specimens

Photographic documentation of prepared rock core specimens may be required for scientific studies. For specimens that have surface features which vary circumferentially, it is advantageous to have a single photomosaic of the specimen surface rather than a series of surface photographs. A technique to develop a photomosaic from a series of overlapping images of prepared vesicular basalt core specimens is presented. The overlapping images of the specimen surface are subjected to an initial cropping, a geometric transformation, an intensity interpolation, a final cropping, and an image stitching algorithm. The final result is an undistorted photomosaic of the entire specimen surface. All steps except the initial cropping are implemented within MATLAB®.

Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific

AbstractDetermining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (∼100 Ma) and U1368 (∼13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 and 1.1 m/Myr−1, respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10–15 Myr of ocean crust formation. Both sites exhibit low‐temperature (<150°C) alteration (e.g., iron‐hydroxides, carbonate, and quartz) within a predominantly oxidative regime, albeit with markedly different alteration styles and intensity. Alteration at Site U1365, which is predominantly composed of sheet flows, occurs mainly near sheet flow boundaries and fractures. In contrast, Site U1368 comprises interlayered pillows and thin sheet flows that have been subjected to relatively even levels of alteration. Variation of alteration style and intensity between Sites U1365 and U1368 appear closely tied to lithology and crustal structure. Although alteration‐induced elemental changes at both sites are similar in, e.g., increasing K, Rb, U, Ba, and Fe3+ and decreasing Fe2+, Ca, and Ni, they show distinct differences in Th, which is significantly decreased at Site U1365 but relatively constant at Site U1368. At both sites enrichment of LREEs relative to HREEs is ascribed to alteration. The greater vein abundance and notably higher Fe3+/TiO2, K2O/TiO2, LOI/TiO2, and Rb/TiO2 ratios of representative samples at Site U1365 compared to Site U1368 are attributed to increased alteration intensity. This is mirrored by greater overall chemical change (Fe2O3, FeO, CaO, K2O, Li, Rb, Pb, and U) observed at Site U1365 than those of Site U1368 and other DSDP/ODP sites between 6 and 46 Ma. Since both Sites U1365 and U1368 endured only minimal sedimentation, we attribute the differences in overall chemical change across the two sites to duration of exposure to seawater.

Wellbore cement fracture evolution at the cement–basalt caprock interface during geologic carbon sequestration

Composite Portland cement–basalt caprock cores with fractures, as well as neat Portland cement columns, were prepared to understand the geochemical and geomechanical effects on the integrity of wellbores with defects during geologic carbon sequestration. The samples were reacted with CO2–saturated groundwater at 50°C and 10MPa for 3months under static conditions, while one cement–basalt core was subjected to mechanical stress at 2.7MPa before the CO2 reaction. Micro-XRD and SEM–EDS data collected along the cement–basalt interface after 3-month reaction with CO2–saturated groundwater indicate that carbonation of cement matrix was extensive with the precipitation of calcite, aragonite, and vaterite, whereas the alteration of basalt caprock was minor. X-ray microtomography (XMT) provided three-dimensional (3-D) visualization of the opening and interconnection of cement fractures due to mechanical stress. Computational fluid dynamics (CFD) modeling further revealed that this stress led to the increase in fluid flow and hence permeability. After the CO2-reaction, XMT images displayed that calcium carbonate precipitation occurred extensively within the fractures in the cement matrix, but only partially along the fracture located at the cement–basalt interface. The 3-D visualization and CFD modeling also showed that the precipitation of calcium carbonate within the cement fractures after the CO2-reaction resulted in the disconnection of cement fractures and permeability decrease. The permeability calculated based on CFD modeling was in agreement with the experimentally determined permeability. This study demonstrates that XMT imaging coupled with CFD modeling represent a powerful tool to visualize and quantify fracture evolution and permeability change in geologic materials and to predict their behavior during geologic carbon sequestration or hydraulic fracturing for shale gas production and enhanced geothermal systems.