Numerical Reservoir Modelling to Test Potential Upflow Locations of the Pico Alto Geothermal Field, Terceira Island, Azores

Abstract

To understand the characteristics of a geothermal reservoir it is important to understand fluid and rock properties. In terms of the reservoir performance, the most important quantities are temperature, pressure, porosity, and permeability. This study looks to refine the current knowledge of the Pico Alto Geothermal Field by building on the static conceptual model and developing a natural state numerical reservoir model. The aim is to understand the possible heat source, shape, and extent of the Pico Alto reservoir. In 2017 the Pico Alto Geothermal Power plant was constructed on Terceira Island, an island abundant in renewable energy. Phase A installed a 3.5MW ORC binary plant and phase B aims to expand the plant to 10MW. Developmental studies, dating back to 1970s, of the Pico Alto Geothermal Area, Terceira Island, Azores, Portugal confirm the presence of a geothermal reservoir. However, while conceptual models are constructed, uncertainty on the location of the heat source remains and is discussed below. A 3D conceptual model and natural state models of the Pico Alto Geothermal Area were created from open-source data. The geological model allowed initial inputs and hypothesis of the reservoir thermodynamics. Calibration of the natural state model tested three possible heat source location scenarios. Scenario One is caldera hosted and matched the measured well data above sea level well. However, it depicts temperatures too hot at the reservoir and at depth shows an inversion, which is inconsistent with the extrapolation of the measured well data. Scenario Two has upflow focused on the western caldera edge, resulting in a narrow high temperature upflow zone and an output similar to Scenario One with a small inversion at depth and discharge in the location of the surface manifestation. Scenario Three is caldera adjacent bounded upflow on the southeast by the caldera. This scenario gives plausible extrapolation of temperatures at depth and shows indications of the influence of cold fluid flowing laterally along the reservoir, as seen in the measured well data. Scenario Three is the probable location of heat source as it aligns with suggestions from the literature and results in the closest fit to the calibration data.