Re-injection process of the produced fluids back into the reservoirs is a widely recognized method to maintain the reservoir pressure and prolong the life-cycle of the geothermal fields. In addition, environmental awareness throughout the world requires returning the effluent geothermal fluids into the reservoir back to protect both surface and subsurface freshwater resources. Under geothermal conditions, chloride typically behaves as a conservative species remaining in the liquid phase during boiling thus, it can be treated as a tracing element for the injectate. Due to pressure drop along the fluid gathering systems; boiling and gas formation occurs in produced fluids which yields a gradual enrichment in the re-injection fluid in terms of chloride concentration. Therefore, the chloride contents of produced fluids and injectate significantly increase over the lifetime of the field, which indicates substantial returns of injectate to the production wells, enabling the treatment of the conservative chloride species as a continuous tracer injection process. In this work, our aim is to track the carbon dioxide injection by modeling chloride concentration change in the Kizildere geothermal field, located in the west of Turkey, which is performed by using the TOUGH2 simulator. The reservoir model parameters are calibrated with measured static pressure-temperature values and fitted through monitored water chemistry. In the absence of tracer tests, modeling chloride concentration change becomes beneficial to understand the connectivities between wells and the estimation of parameters such as permeability and porosity thus, it provides a reliable model for tracking the carbon dioxide injection process.

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