THE HYDROTHERMAL FLUID IMPACT UPON ROCKS MINERALOGICAL, PHYSICAL AND MECHANICAL PROPERTIES

Several high-temperature geothermal systems at the region of modern volcanic activity (Kuril-Kamchatsky region, Far East, Russia) have been studied. These geothermal systems are located in depressions, composed of well-permeable tuffs (porosity is about 30-45%). These tuffs contain a great amount of water, heated by high heat flow from diorite intrusions. The heated stratum causes the convective motion of waters induced by difference of their density. Heated chloride-natrium waters, distributed at the Kuril-Kamchatsky region, while rising to the surface transform into solutions of various composition, acidity-alkalinity and temperature. Result in geothermal reservoirs undergo significant changes. As a result of mineralogical and structural alterations physical and mechanical conditions of geothermal reservoir are changes significantly.

The following petrophysical properties have been determined: density of rock, mineral density, total and open porosity, hygroscopic moisture, velocities of longitudinal (for dry and water-saturated samples) and transversal waves, acoustic anizotropic parameter, strength and magnetic susceptibitily. The dispersion of petrophysical properties allows to describe physical heterogeneity of geothermal reservoirs.

The rocks properties alterations during hydrothermal process have different directions. Depend on types and amount of secondary minerals and the structure transformations there may be both an increase as well as a decrease of petrophysical properties value. The properties of new rocks distinguish from the primary rocks and range widely: from dense (2.5-2.7 g/cm3), strong (150-200 MPa) and acoustically rigid (Vl=4000-5500 m/sec) secondary quartzites formed within fault zones until loose, high porous (40%) hydrothermal argillites and zeolite propylites (N=35-40%, Vl=1000-1500 m/sec and strength is about 1-3 MPa). The rock properties changes have zonal character according to the types and temperature of fluids. Result in primarily homogeneous reservoir acquires zonal structure: the dense and strong layers alternate with highly porous, loose and fragile rocks.