Natural state modelling of the Mutnovsky geothermal field, Kamchatka, Russia

Abstract

The Mutnovsky geothermal field is located on a volcanic plateau at 700-900 m a.s.l. Numerous hot springs and fumaroles are found, often at intersections of faults and fracture systems. Systematic analysis of downhole pressure and temperature data in over 30 wells, together with a careful review of older literature on the Mutnovsky resource, has led to the following conceptual reservoir model. An upflow zone of over 300°C resides underneath the Mutnovsky volcano, some 3-4 km to the south of the present well field. The hot fluid flows laterally to the north, towards the Dachny site. There, a shift occurs in the flow direction and the geothermal system is elongated towards northeast, where the Upper-Mutnovsky site is located. Another hot upflow zone to the reservoir may be located there. The reservoir areal extent, as defined by a 240°C temperature contour, is about 10 km2, and the reservoir thickness exceeds 1 km. The field is generally liquid-dominated, and at 240-280°C temperature. A steam-cap is found near the top of the reservoir in Dachny. Based on this conceptual model, a natural state numerical TOUGH2 reservoir model has been developed, made of five horizontal layers, each consisting of 160 elements. Three model variants were considered for calibration. All have in common an upflow zone to the south, and an outflow zone to the northeast, while additional upflow zones were modelled beneath the Dachny and the Upper-Mutnovsky areas. Calibration was done against the initial pressure and temperature distribution. It suggests a well field permeability of 30-50 mD, while the outer boundaries and the base layer have permeability below 1 mD. Total source strength of 50-60 kg/s of 1650 kJ/kg enthalpy was needed to reasonably match the available data, while a better match to observed temperature distribution was obtained by spreading out the source instead of considering only one upflow zone.