Three-dimensional model of the Niutuozhen geothermal system, Heibei province, China

Abstract

The Niutuozhen geothermal system, located in North China, is composed of the Jixian system dolomite reservoir and a Neogene sandstone reservoir. A three-dimensional model with 5437 elements and seven layers based on the geological and geothermal data was built using the AQUA3D package designed for solving flow and transport problems. An area of 1050 km2 confined by four faults is included in the model. Production history of forty five wells and pressure monitoring data from six wells were used for calibration. Permeability was estimated to range from 2.5 to 25×10-16 m2 in layer 6 after a satisfying agreement was obtained between the measured and computed water level data. The maximum predicted drawdown after 20 years of production without any reinjection ranged from 30 to 120 m in the north and south, respectively. The rate of drawdown was predicted to be kept at a lower level if 50% of the produced water was reinjected into the system. Cones of depression were predicted to gradually enlarge due to an increased production rate without reinjection, but were effectively limited and even diminished with a reinjection ratio of 50%. A maximum temperature decrease of 2.75°C was predicted to occur in production well A34, located near the reinjection well R3, for a production rate of 162 l/s with 50% reinjection. The maximum temperature decrease of 24.81°C was predicted to occur in the southern reinjection well R2. The cooling plumes surrounding the reinjection wells were predicted to gradually enlarge with an increasing reinjection fraction. Prediction of temperature response indicated that the reinjection rate, the temperature of the reinjected water and the distance between the reinjection and production wells are the three factors affecting the reservoir temperature change.