Cold Front Propagation in a Fractured Geothermal Reservoir

Abstract

Hungary has decided to implement its first geothermal pilot power plant for electricity production. The site chosen is in South-West-Hungary and is positioned on a fractured limestone reservoir. The reservoir’s depth is about 3000m and the temperature is 142oC. Two existing petroleum-prospecting dry holes were chosen to implement a doublet for the geothermal energy supply with a separation of approximately 1000m. The projected flow rate of the circulating water is 5000 m3/d. The lifespan of this production-injection system is one of critical concern to the industry. During a recent pre-feasibility study, an analytic model was developed to determine the propagation of the cooled region between the two wells. In this model, the reservoir is replaced by a plain equivalent fracture with a Hele-Shaw flow. The Hele-Show flow can be viewed as a quasi-potential motion. After applying complex variable functions, a theoretical flow pattern can be determined. The heat transfer between the water and the rock is calculated with a transient overall heat transfer coefficient. The cold front is not obtained as an abrupt temperature drop but, as an extended moving region with a very slight temperature gradient.