Andrew J. Luhmann

Classification of Thermal Patterns at Karst Springs and Cave Streams

Thermal patterns of karst springs and cave streams provide potentially useful information concerning aquifer geometry and recharge. Temperature monitoring at 25 springs and cave streams in southeastern Minnesota has shown four distinct thermal patterns. These patterns can be divided into two types: those produced by flow paths with ineffective heat exchange, such as conduits, and those produced by flow paths with effective heat exchange, such as small fractures and pore space. Thermally ineffective patterns result when water flows through the aquifer before it can equilibrate to the rock temperature. Thermally ineffective patterns can be either event-scale, as produced by rainfall or snowmelt events, or seasonal scale, as produced by input from a perennial surface stream. Thermally effective patterns result when water equilibrates to rock temperature, and the patterns displayed depend on whether the aquifer temperature is changing over time. Shallow aquifers with seasonally varying temperatures display a phase-shifted seasonal signal, whereas deeper aquifers with constant temperatures display a stable temperature pattern. An individual aquifer may display more than one of these patterns. Since karst aquifers typically contain both thermally effective and ineffective routes, we argue that the thermal response is strongly influenced by recharge mode.

Experimental observation of permeability changes in dolomite at CO2 sequestration conditions.

Injection of cool CO2 into geothermally warm carbonate reservoirs for storage or geothermal energy production may lower near-well temperature and lead to mass transfer along flow paths leading away from the well. To investigate this process, a dolomite core was subjected to a 650 h, high pressure, CO2 saturated, flow-through experiment. Permeability increased from 10(-15.9) to 10(-15.2) m(2) over the initial 216 h at 21 °C, decreased to 10(-16.2) m(2) over 289 h at 50 °C, largely due to thermally driven CO2 exsolution, and reached a final value of 10(-16.4) m(2) after 145 h at 100 °C due to continued exsolution and the onset of dolomite precipitation. Theoretical calculations show that CO2 exsolution results in a maximum pore space CO2 saturation of 0.5, and steady state relative permeabilities of CO2 and water on the order of 0.0065 and 0.1, respectively. Post-experiment imagery reveals matrix dissolution at low temperatures, and subsequent filling-in of flow passages at elevated temperature. Geochemical calculations indicate that reservoir fluids subjected to a thermal gradient may exsolve and precipitate up to 200 cm(3) CO2 and 1.5 cm(3) dolomite per kg of water, respectively, resulting in substantial porosity and permeability redistribution.

Permeability reduction produced by grain reorganization and accumulation of exsolved CO2 during geologic carbon sequestration: a new CO2 trapping mechanism.

Carbon sequestration experiments were conducted on uncemented sediment and lithified rock from the Eau Claire Formation, which consisted primarily of K-feldspar and quartz. Cores were heated to accentuate reactivity between fluid and mineral grains and to force CO(2) exsolution. Measured permeability of one sediment core ultimately reduced by 4 orders of magnitude as it was incrementally heated from 21 to 150 °C. Water-rock interaction produced some alteration, yielding sub-μm clay precipitation on K-feldspar grains in the cores upstream end. Experimental results also revealed abundant newly formed pore space in regions of the core, and in some cases pores that were several times larger than the average grain size of the sediment. These large pores likely formed from elevated localized pressure caused by rapid CO(2) exsolution within the core and/or an accumulating CO(2) phase capable of pushing out surrounding sediment. CO(2) filled the pores and blocked flow pathways. Comparison with a similar experiment using a solid arkose core indicates that CO(2) accumulation and grain reorganization mainly contributed to permeability reduction during the heated sediment core experiment. This suggests that CO(2) injection into sediments may store more CO(2) and cause additional permeability reduction than is possible in lithified rock due to grain reorganization.

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

Four 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.

Spring Characterization Methods & Springshed Mapping

A summary of springshed delineation techniques using the integration of historic dye tracing information, borehole geophysics and chemistry measurements. Fountain, Mahoney, Waterhole, Wykoff, Starless River and Cold Spring springshed delineations included. Interpretations are subject to change as new traces and information becomes available. A collaborative effort between the University of Minnesota and the Department of Natural Resources.

Dye tracing within the St. Lawrence confining unit in Southeastern Minnesota

A single trace near the Fillmore-Winona County Border near the City of Rushford, Fillmore County, Minnesota. The trace began in Winona County and did not cross the county boundary. Primary goal of the study was to delineate the springshed feeding a cluster of trout streams. A collaborative effort between the Minnesota Department of Natural Resources and the University of Minnesota.

PHYSICAL AND CHEMICAL CHANGES FROM BASALT-CO2-RICH FLUID INTERACTION DURING FLOW-THROUGH EXPERIMENTS AT 150°C AND 150 BAR

To further our understanding of the relationship between environmental change and hominin evolution, at an important archeological and paleontological locality, XRD bulk analysis was completed on 1,183 core samples from a drilling core collected in West Turkana, Kenya. The core itself covers about 1.354 -1.85 Ma. Most minerals present are detrital feldspars, mica, and quartz. The authigenic minerals present are mostly carbonates, zeolites, and sulfur bearing minerals such as gypsum and pyrite. The few metal oxides present may be derived from paleosols in the core. This particular study is focused almost entirely on the data gathered from the XRD analysis of clay samples. We analyzed 70 samples from the submicron fraction for clay mineralogy. Both oriented and randomly oriented analyses were completed in order to characterize the clay minerals present.

Goliath’s Cave, Minnesota: Epigenic Modification and Extension of Pre-Existing Hypogenic Conduits

Introduction The Devonian, Ordovician, and Cambrian sedimentary rocks of southeast Minnesota (Mossler, 2008) host a variety of caves, sinkholes, sinking streams, blind valleys, large springs, and other karst features. The formations are relatively flat-lying, dip regionally to the southwest at a few m/km, and have been above sea level and subject to erosion since mid-Cretaceous time. All of southeast Minnesota has been glaciated several times during the Pleistocene but has not been covered with ice during the last two major glacial cycles. The highest concentration of karst features is in Fillmore County along the southern border of Minnesota. Fillmore County contains more mapped karst features than all of the rest of Minnesota combined (Gao et al., 2005).