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Mathematical modelling of geothermal carbon dioxide production in a specific field in Turkey.

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2020
Kumsal, Beril
Turkey’s non-condensable gases production from geothermal fields is very high when compared to other countries’ average production values. A big predominance of these gases is generally carbon dioxide (CO2) and the origin of this CO2 is generally meteoric for the studied area as reservoir rocks are carbonate-dominated metamorphic rocks such as dolomitic marbles and marbles.The dissolution of calcite mineral within the reservoir rocks, where it equilibrates with water, results in CO2 release from the system. And this release occurs because of meteoric waters. When a field is put on production, a CO2 decline is observed during the production life time and this decline can be addressed in three different scenarios. First, re-injected brine does not include any CO2 as it is released to the atmosphere after production. When this brine reaches to the production wells due to the strong hydraulic connectivity, a sharp CO2 decline occurs in the reservoir. Second, there might be a weak hydraulic connectivity between the production and re-injection wells and a gradual CO2 decline may be observed with time due to the natural recharging. Last, a CO2 decline may occur as a result of a sharp pressure decline in an excessively producing well because of the water invasion that comes from the upper part/shallow part of the geothermal system and this sub-surface water has less amount of dissolved CO2 in it. This study aims to clarify modelling of CO2 declines for an Alaşehir geothermal field. It has been observed that CO2 declines show the best matches with the hyperbolic decline method introduced by Arp’s in 1945. In this study, the reasons of the observed declines in Alaşehir geothermal field showed that a strong hydraulic connectivity between the re-injection and production wells resulted in a sharp CO2decline. On the contrary, a gradual CO2 decline has been observed when there is a weak hydraulic connectivity between the wells.