In EGS reservoirs, a fracture surface area is one of key uncertainty in development. We propose a method to estimate cumulative fracture surface area for each multiple flow path based on tracer and temperature data with cold water injection. Tracer response is used to obtain the flow properties and determine the pore volume for each flow path. By substituting the pore volumes, analytical solutions of heat transfer equation can consist of measurable and/or controllable properties except fracture surface area for each path. The unknown fracture surface areas are estimated by comparison between the superposition of analytical solutions of heat transfer equations and observed temperature decline. Validation with numerical simulation results suggests that even though the number of flow paths is unknown, use of "virtual flow path" can estimate the pore volumes and fracture surface areas. For the simulation results, temperature history for 28 days (10% of temperature decline) provides reasonable estimation of fracture surface area with error of 15.0%.