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Water-Rock Interaction in the Coso Geothermal System

Hwang, Bohyun
http://rave.ohiolink.edu/etdc/view?acc_num=osu1417724456

Abstract Details

2014, Master of Science, Ohio State University, Geological Sciences.
The overarching objective of this study is to document the water-rock interaction at the Coso geothermal area where the U.S. Department of Energy Enhanced Geothermal Systems (EGS) experiments are conducted to better understand the thermal evolution of this system. This effort quantifies the relationship between mineralogy and the pore/fracture network in order to track the evolution of water-rock interaction as a function of space and time. The relationship among the size, shape and distribution of pores relevant to mineralogy and mineral abundance are used as indicators of how fluids migrate through and react within the micropore environment. This paper summarizes petrologic and geochemical characterizations on well cuttings from three East Flank wells, and one West Flank well, with the focus on one of the East Flank wells, Navy II well 42A-16, to better understand the geologic process of hydrothermal alteration, its evolution, and how changes in mineralogy impacted porosity and permeability. Several alteration types in these well cuttings are observed with the two most important being: NaAlSi3O8 • CaAl2Si2O8 + H4SiO4 + Na + = 2NaAlSi3O8 + Al+3 + Ca+2 + 4OH- (1) (xNa, yCa)AlSi3O8 + K+ + Na+ + Fe+3 + Mg+2 + H2O -> (2) (K, Na)2(Al, Fe, Mg)4Si6O20(OH)4 + (x+mNaAlSi3O8, y-mCaAlSi2O8) +2mCa+2 where albitization of calcic-plagioclase is represented by (1), and sericitization after plagioclase is represented by reaction (2). Collectively these reactions are two of the most significantly occurring over the range of depths in Navy II well 42A-16. This albite-sericite alteration tends to be associated with the potassic alteration, and often occur together especially in quartz diorite and granodiorite host rocks. Secondary albite and sericite formed at the mid-stage of mineral paragenesis in this system. The earliest minerals to form were epidote, titanite, rutile/anatase, and chlorite (perhaps of earlier metamorphic origin); whereas the most recently formed mineral was calcite. Porosity modification in the form of dissolution pits in plagioclase are observed using SEM/QEMSCAN method. Plagioclase has the largest area percent of pores within the grains or adjacent to them with respect to the equation (1) and (2). The plagioclase dissolution pits are dominant especially from the sample at 2000-2010 ft in Navy II well 42A-16. These dissolution type pores are sometimes filled with either clay (e.g. sericite) or calcite which is the last alteration phase to form in this system. Pore features were investigated using standard BET gas adsorption which interrogates pores from about 1.7 nm to 0.3 microns. These results indicate the current matrix porosity is extremely low – ranging from <0.01 to about 0.1 percent. A comparison of BET data with neutron scattering results indicate that a significant fraction of matrix pores are not part of any organized network. This is supported by a preliminary micro-CT scan of one key sample from Navy II well 42A-16 which revealed numerous very small unconnected pores. Micro-fracture porosity and permeability were estimated from a nomogram approach developed by North and Knapp (1977) which also suggests very low values for these parameters during the late stage development of calcite veining.
David Cole, Prof. (Advisor)
Michael Barton, Prof. (Committee Member)
Ann Cook, Prof. (Committee Member)
145 p.
Geochemistry
Geothermal field; water-rock interaction; mineralogy; porosity; permeability

Recommended Citations

Citations

  • Hwang, B. (2014). Water-Rock Interaction in the Coso Geothermal System [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1417724456

    APA Style (7th edition)

  • Hwang, Bohyun. Water-Rock Interaction in the Coso Geothermal System. 2014. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1417724456.

    MLA Style (8th edition)

  • Hwang, Bohyun. "Water-Rock Interaction in the Coso Geothermal System." Master's thesis, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1417724456

    Chicago Manual of Style (17th edition)

osu1417724456
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This open access ETD is published by The Ohio State University and OhioLINK.
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