Production Monitoring as a Tool for Field Development - a Case History from the Nesjavellir Field, Iceland

Abstract

A tailor-designed monitoring programme has enabled step-wise escalation of production at the Nesjavellir geothermal field in SW-Iceland. A frequent update of a numerical model of the geothermal system has secured that the demand of the market for increased production of hot water for space heating and electricity has been met in a sustainable way. The scientific work has enabled an expansion of the Nesjavellir Power Plant fitting the resource; from a modest 100 MWt thermal power plant in 1990 to an impressive co-generation power plant in 2005 with a production of 290 MWth and 120 MWe. The present understanding of the system through monitoring and modelling makes drilling of wells for expansion and replacement a low-risk operation.

The monitoring programme, which has been conducted since the early 1980s, includes measurements of the mass extraction and discharge enthalpy from the reservoir and regular discharge measurements of production wells and water level in non-discharging wells. Chemical sampling, and annual temperature and pressure loggings are used to monitor the response of the reservoir to utilization. The cumulative fluid extraction from the reservoir since 1975 is of the order of 141 million tons, no significant temperature changes have been observed during this period but a pressure drawdown of 6-8 bars has developed in the production zone.