This report aims to find the underlying cause of pressure fluctuations in five Icelandic geothermal wells. These fluctuations affect the productivity of the power plant; making it relevant to understand their causes and mitigation measures. Based on the available data we concluded that the fluctuations do not originate in the reservoir or surface infrastructure but in the well bore. We then proceeded to follow two hypotheses to explain fluctuations within the well bore. The first one, we called Internal Flow Instability. It describes oscillating behavior as a result of Ledinegg instability, where small instabilities in momentum balance become amplified along the well. To further explore this theory we built a 1D numerical model for two-phase flow. We plotted the characteristic curve (i.e. the relation between pressure drop and flow rate) for an example stable and unstable well. Our results show that the operating condition of the stable well lies in the stable region of the curve, and vice versa. The curves do predict the stability of the wells, correctly but do not give an explanation for the instability.
The second hypothesis examined in this report is the Multiple Feed Zones theory. This hypothesis investigates how different feed zones supplying fluids of different enthalpies mix within a well bore; focusing on the interactions between two phase flow in the well bore and single phase flow from the feed zones. A STARS model was developed to test whether single-phase flow could exist above a two-phase region in the well bore, and the simulations confirmed that this is feasible. To further investigate the mechanism, we applied the model from Matsumoto et al. (2020). This model was used as a proof of concept to demonstrate that interactions between multiple feed zones with different enthalpies can produce the type of fluctuations observed in the wells. While the simulated fluctuations do not match the observed frequency, the results support that, under the modelled conditions, the Multiple Feed Zones theory could provide an explanation for the pressure oscillations observed in the wells.