Munetake Sasaki

Dissolution kinetics of anorthite in a supercritical CO2–water system

Abstract The Gibbs free energy change, ΔGr, dependence of the anorthite dissolution rate in a supercritical CO2-water system was measured as part of a geochemical assessment of CO2 geological sequestration. Being bounded at ΔGr crit, the critical ΔGr required for an opening of the etch pit at a screw dislocation, it has been accepted that mineral dissolution follows etch pit formation assisted by dislocations below ΔGr crit, whereas the horizontal step retreats without the etch pit above ΔGrcrit. The experiment described herein, however, revealed that another mode of dissolution occurs more distant from equilibrium by spontaneous formation of the etch pit over the entire surface, as observed on calcite. The dissolution rate is higher by more than one order of magnitude than that in the dislocation-assisted mode. Therefore, including the rate gap at ΔGrcrit, a nonlinear curve with three steps instead of a sigmoidal curve is proposed for the ΔGr dependence of the anorthite dissolution rate. Extremely slow rates were observed depending on observed points for the same ΔGr condition. Although the reason for such a rate difference remains unknown, it is likely related to the defect density on the crystal surface. It is possible that initial spreading of the dissolved surface attributable to the etch pit formation assisted by defects provides some trigger for subsequent explosive etch pit formation. These findings suggest that the initial transient process can strongly influence the kinetics of geochemical reactions that occur during CO2 geological sequestration.

Petrographic features of a high-temperature granite just newly solidified magma at the Kakkonda geothermal field, Japan

Abstract A 3729-m-deep geothermal research well, WD-1a, provides us with a unique opportunity to study initial petrographic features of a high-temperature granite just after solidification of magma. The well succeeded in collecting three spot-cores of the Kakkonda Granite that is a pluton emplaced at a shallow depth and regarded as a heat source of the active Kakkonda geothermal system. The core samples were collected at the present formation temperatures of 370, 410 and over 500°C. These samples are granodiorite to tonalite consisting mainly of plagioclase, quartz, hornblende, biotite and K-feldspar. A sample collected at a formation temperature of over 500°C possesses the following remarkable petrographic features compared to the other two samples. Interstitial spaces are not completely sealed. K-feldspar exhibits no perthite by the exsolution of albite lamella. Quartz includes glassy melt inclusions without devitrification. Hornblende is less intensively altered to actinolite, and biotite is not altered. This study directly confirmed that perthite in K-feldspar is a recrystallization texture formed at 410–500°C based on a comparison of the in situ temperatures of the samples. Chemical compositions of minerals were analyzed to compare temperatures determined from geothermometers in several publications to the in situ temperatures of the samples.

Fluid inclusion study of the Kirishima geothermal system, Japan

Abstract Gases from fluid inclusions in quartz and anhydrite were analyzed with a quadrupole mass spectrometer and a capacitance manometer. The quartz and anhydrite occur in hydrothermal veins in volcanic and pelitic rocks collected from geothermal wells in the Kirishima area, southwest Japan. The geothermal wells are located in a graben made up of Quaternary volcanic rocks underlain by sedimentary rocks of the Shimanto Group. Results of individual fluid inclusion analyses show that the fluid inclusions comprise mainly H 2 O and a variable but small amount of CO 2 . CH 4 and other hydrocarbons are also detected in inclusions in a hydrothermal sample from the pelitic Shimanto Group. Peak ratios of CO 2 /H 2 0 in individual fluid inclusions are variable in some samples. This indicates that there is a difference in gas compositions of the fluid inclusions, and suggests that the inclusions were formed in multistages or trapped heterogeneous boiling fluids. Results of bulk analyses show that the inclusions are mainly composed of H 2 O (98–99 mol%) with small amounts of non-condensable gases, mainly C0 2 and N 2 , CH 4 and Ar. The proportion of N 2 is about one order of magnitude lower than C0 2 , CH 4 is generally two orders of magnitude lower than C0 2 and Ar is just above the detection limit of the mass spectrometer. The gas concentration in the fluid inclusions is much higher than that in the present-day discharge fluids in this area. CO 2 /N 2 and C0 2 /CH 4 ratios of the fluid inclusions from the volcanic rocks are lower than those of the present-day discharge fluids. CO 2 /N 2 and CO 2 /CH 4 ratios in residual fluids increase with progressive degassing, because N 2 and CH 4 are released from the residual fluids more easily than CO 2 . Thus, the difference in the CO 2 /N 2 and CO 2 /CH 4 ratios between the fluid inclusions and the present-day discharge fluids in the Kirishima area may be ascribed to the degree of degassing, and the fluid inclusions in the area were probably formed by trapping fluids that were weakly influenced by degassing. P co 2, values calculated from the gas compositions of the fluid inclusions are higher than that of buffer systems involving alteration minerals in the area. This suggests that the fluid inclusions might be trapped fluids which were not in equilibrium with the alteration mineral assemblages, that is, fluids prior to considerable degassing and alteration.

Synthetic fluid inclusion logging to measure temperatures and sample fluids in the Kakkonda geothermal field, Japan

Synthetic fluid inclusion logging is a new tool to measure temperatures and sample fluids in high-temperature geothermal wells. Fluid in the microcracks of a crystal can be trapped in inclusions through healing. Fluid inclusions in quartz, for example, can be synthesized easily in geothermal boreholes and can be used as long as the host crystal is stable (e.g. α-quartz is stable up to 573°C). This technique can be applied to high-temperature geothermal wells where conventional temperature measurement methods are not feasible. Cracked crystals of quartz, soaked in silica-saturated solutions in gold or platinum capsules mounted on containers, are placed in a geothermal borehole. Geothermal fluid enters the microcracks in the crystals at the selected sampling depths, and inclusions containing ambient fluid are formed through crack healing. Trapping temperatures of fluid inclusions in quartz are determined by microthermometry using a heating stage with pressure corrections. Other cracked crystals mounted in containers with rupture disks are used for fluid sampling. The first borehole experiment was conducted at WD-1, a deep research hole drilled in the Kakkonda geothermal field, northeast Japan, from September to October 1994 (24 days). Results from the experiment confirmed that temperatures measured from fluid inclusions are consistent with borehole temperatures measured by conventional logging tools.

Deep geothermal resources survey project in the Kakkonda geothermal field

Abstract The New Energy and Industrial Technology Development Organization (NEDO) has been conducting a research project named “Deep-Seated Geothermal Resources Survey” since 1992 in order to establish a desirable direction for development of deep geothermal resources that exist beneath the already developed shallow reservoirs. A deep drill hole, WD-1, reached a depth of 3,729 m in July 1995 by applying the latest drilling techniques, such as a top-drive drilling system, enabling the collection of highly valuable information for understanding the characteristics of deep geothermal systems. Side-track drilling of WD-1 was started from a depth of 2,200 m in September 1996, targeting productive fractures expected near the boundary of the granite in a depth range from 2,800 to 3,000 m. We successfully encountered large lost circulation at some depths, and the side-track drilling was terminated at a depth of 2,963 m in January 1997.

Field Reaction Experiments of Carbonate Minerals in Spring Waters: Natural Analogue of Geologic CO2 Storage

To diminish the uncertainty of the mineral trapping rate during geologic CO2 storage, the growth rate of carbonate minerals was measured in CO2-containing spring waters, which can be regarded as a natural analogue of geologic CO2 storage. The authors’ approach, using nanoscale analysis of seed crystal surfaces after immersion into spring waters, enables rapid and accurate measurement of mineral reaction rates. The results show that calcite growth rates in spring waters were lower by 1–3 orders than the values given in a database of laboratory experiment results. We verified the traditional paradigm that Mg2+ controls carbonate reaction kinetics. An increase of the Mg/Ca ratio to around 5 by adding Mg2+ to spring waters markedly reduced the calcite growth rate. However, even if effects of Mg2+ and flow rate are considered, we were unable to explain satisfactorily the difference of the calcite growth rates between those of spring waters and laboratory experiments. Therefore, other factors might also be related to the slow growth rate in nature. The present results, including the fact such that neither dolomite nor magnesite was formed even at the high Mg/Ca ratio, are expected to provide an important constraint to overestimation of the mineral trapping rate.