Temperature and pressure analysis of wells NJ-13 and NJ-28 in the Nesjavellir geothermal system, SW-Iceland

Abstract

Temperature and pressure analysis of geothermal wells is one of the vital steps in evaluating a geothermal reservoir during exploration, production and monitoring the wells. By analysing temperature and pressure logs from the warm up period of a well, the formation temperature and initial pressure can be deduced. Injection well testing is usually done at the end of drilling to find out physical parameters of the well and the reservoir surrounding it and get information on the connection of the well to the reservoir. Well NJ-13 is a 1609 m deep vertical production well drilled in 1985. It is located in the Nesjavellir high-temperature geothermal field of the Hengill geothermal system located in SW-Iceland. The total enthalpy of well NJ-13 is around 2500 kJ/kg. At well head pressure (P0) of 16 bar-g, the power output for well NJ-13 is 15 MWe, as estimated from the original production test data. Well NJ-28 is a directional well drilled from the same well pad as NJ-13 in MayJune 2015 as a makeup well to a depth of 1301 m with respect to the platform. The intended depth of this well was 2100 m but the drilling was stopped at 1301 m after several attempts to continue when the drill string repeatedly got stuck due to intrusions at the bottom of the well. The total enthalpy of well NJ-28 is around 2600 kJ/kg. At a well head pressure (P0) of 20 bar-g, the power output of well NJ-28 is 5 MWe estimated from production test data from September 2015. Well NJ-28 has lower transmissivity and injectivity than well NJ-13 which might explain why NJ-28 is a poor producer compared to NJ-13.