High Noncondensible Gas Liquid Dominated Geothermal Reservoir, Kizildere, Turkey

Abstract

Noncondensible gas in the Kizildere geothermal reservoir is 99% carbon dioxide (CO2). CO2 concentrations dissolved in the geothermal reservoir brine vary from 0.02-0.03 kg/kg of brine in the deep >225oC reservoir and 0.01-0.02 kg/kg of brine in the intermediate <200oC reservoir. The partial pressure of CO2 at the top of the deep reservoir is greater than 50% of the total reservoir pressure. While the presence of marble within the reservoir indicates that the source of CO2 may be thermal decomposition of carbonates, decarbonisation by reaction of carbonates with alumina silicates may also contribute to the concentration of CO2 in the reservoir brine. The dissolved CO2 in Kizildere is of sufficient concentration to affect the behaviour of the fluids, specifically the transition to two-phase conditions. As pressure is released or temperature decreases, dissolved CO2 decreases and calcium carbonate precipitates. Specifically, this transition to two-phase and/or cooler conditions may decrease the effective permeability of the cap rock and thus increase the separation between reservoir zones. The depth at which gas breakout occurs is deep into the reservoir at the approximate depth of the impermeable section between the two reservoirs.