The application of integrated petrophysical analysis to geothermal field studyтезисы доклада

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[1] Frolova Y., Ladygin V. The application of integrated petrophysical analysis to geothermal field study // Abstract. The 33-d International Geological Congres, August 5-14 2008. — Oslo, Norway, 2008. This report generalizes the results of petrophysical studies obtained by the authors on a number of geothermal systems in the Kuril-Kamchatka island arc and Iceland. The advantages of petrophysical analysis application for an investigation of evolution and mechanisms of geothermal process are discussed. Petrophysical parameters (density, porosity, permeability, hygroscopy, ultrasonic velocity, magnetic susceptibility, mechanical parameters), their interconnections and correlation with petrography are informative for a definition of geothermal system structure and an evaluation of intensity of geothermal process that is shown in the report with a number of examples.<BR> Physical and mechanical properties of host rock in geothermal systems are described and compared. Effusive and subvolcanic rocks are dense and low permeable with high strength and elastic modulus. They form water-confining horizons in the structure of geothermal fields and rarely fracture-type reservoir. Volcanoclastic rocks differ significantly from effusive rocks by lower physical and mechanical properties, higher magnitude of alteration and permeability character. They are the most common type of the host rocks of geothermal reservoirs. Typically they form porous or fracture-porous aquifers, rarely – water confining layers (for example, fine grained argillized tuffs). Due to high porosity and permeability they are greatly exposed to thermal fluids resulting in changes of composition, structure and properties.<BR> Petrophysical properties of rocks are changes intensively during hydrothermal process. Low-temperature fluids lead to rocks leaching and substitution by clay minerals, opal, tridymite and high-silica zeolites. These mineral alterations result in properties decrease and secondary porosity formation. In some cases hard basalts transform to loose plastic clays. Basically altered rocks are highly porous and hygroscopic, characterized by low values of density, ultrasonic velocity and strength. In spite of high porosity permeability can be low due to: 1) ultra-small size of pores in clay minerals; 2) swelling of smectite. The petrophysical indicator of argillization zone – potentially cap rocks in geothermal system - is high values of hygroscopic moisture. High-temperature alterations lead to the opposite tendency in rocks properties changes – an increase of density, strength, elastic characteristics, and a decrease of porosity and permeability. This is the result of development of secondary minerals (chlorite, epidote, zeolites, quartz, sericite, albite, adularia, prehnite, calcite) which fill pores, substitute matrix and phenocrystals. Sometimes secondary fractures are formed but they are filled by high-temperature minerals and don’t decrease rocks properties. The magnitude of alteration can be so high that the rocks loose their primary features. In this case initial difference in properties between tuffs and effusive rocks is disappeared.

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