Well test analysis and temperature and pressure monitoring of Krafla and Nesjavellir high-temperature geothermal fields, Iceland

Abstract

The Krafla high-temperature geothermal field is located within the Krafla caldera lying within the active NE‐SW striking rift zone of North‐East Iceland, whereas the Nesjavellir geothermal field is a high-temperature geothermal system, part of the Hengill central volcano in SW-Iceland. Reservoir assessment and monitoring was conducted on wells NJ-15 and NJ-18, located in the Nesjavellir high-temperature geothermal field. For Krafla geothermal field, wells K-37 and K-38 were considered to assess the reservoir, and well K-18 was used for monitoring. Temperature and pressure logs, measured during the warm up of the wells, were analysed to estimate formation temperature and initial pressure. In order to understand the parameters that characterise the reservoir and the wells, injection tests were analysed and parameters such as the injectivity index, transmissivity, storativity, skin, wellbore storage, etc. were evaluated.

Transmissivity estimated for the wells selected in Nesjavellir were of the same order of magnitude as for the wells in Krafla, i.e. 10-8 m3 /(Pa∙s). Storativity for Krafla was higher than that of Nesjavellir, as can be expected in a two-phase reservoir. The wells in Nesjavellir are located at the outer boundaries of the geothermal reservoir and are liquid-dominated. This was further established by analysing the formation temperature and initial pressure. Temperature and pressure monitoring analyses at various depths were performed for Krafla on well K-18 from 1981 to 2013 and for well NJ-15 from 1985 to 2013; and for Nesjavellir well NJ-18 from 1988 to 2013. No significant change in temperature was observed in wells NJ-15 and NJ-18, but a linear constant pressure draw down of about 13 bar was observed in well NJ-15 from 1985 to 2013 and a rapid decline in pressure (20 bar) was observed in well NJ-18 from 2006 to 2013. In well KJ-18 in Krafla, a slow pressure decrease was observed.