Origin and Impacts of High Concentrations of Carbon Dioxide in Geothermal Fluids of Western Turkey

摘要

Most explored and exploited geothermal systems located in Western Turkey have relatively high (>1% by weight) concentrations of non-condensible gas in reservoir fluids. The gas is typically 96-98% or greater carbon dioxide (CO_2) and is dissolved in the moderate temperature (200°C±50°C) liquid-dominated reservoirs. Carbonate-dominated metamorphic rocks including marbles, dolomitic marbles and calc-schists dominate reservoir rocks; the calcite in these rocks provides a large potential source of CO_2 when the calcite equilibrates with water. The dissolution of calcite to calcium and carbonate, bicarbonate, or water and CO_2 is accelerated by acidic conditions, temperature and depressed by salt concentrations. The CO_2 produces high-pressure, gas-driven geothermal systems. When the systems are produced, the CO_2 causes relatively deep two-phase conditions or boiling, requiring deep delivery of scale inhibitor and limiting pumping. When the CO_2 partitions into the vapor phase, it is removed from the reservoir and discharged to the atmosphere. Recharge (either from re-injected brine or meteoric water) is relatively low in CO_2. Without re-equilibration of recharge, reservoir CO_2 concentrations could decline if recharge mixes with reservoir fluids in the production zone. Understanding the source of CO_2 in these geothermal fluids, and monitoring CO_2 concentrations in reservoir fluids and in produced fluids is critical to managing and sustaining production and injection from these high-gas geothermal reservoirs.