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- Title
Bayesian Calibration of a Natural State Geothermal Reservoir Model, Krafla, North Iceland.
- Authors
Scott, S. W.; O'Sullivan, J. P.; Maclaren, O. J.; Nicholson, R.; Covell, C.; Newson, J.; Guðjónsdóttir, M. S.
- Abstract
The Krafla area in north Iceland hosts a high‐temperature geothermal system within a volcanic caldera. Temperature measurements from boreholes drilled for power generation reveal enigmatic contrasts throughout the drilled area. While wells in the western part of the production field indicate a 0.5–1 km thick near‐isothermal (∼210°C) liquid‐dominated reservoir underlain by a deeper boiling reservoir, wells in the east indicate boiling conditions extending from the surface to the maximum depth of drilled wells (∼2 km). Understanding these systematic temperature contrasts in terms of the subsurface permeability structure has remained challenging. Here, we present a new numerical model of the natural, pre‐exploitation state of the Krafla system, incorporating a new geologic/conceptual model and a version of TOUGH2 extending to supercritical conditions. The model shows how the characteristic temperature distribution results from structural partitioning of the system by a rift‐parallel eruptive fissure and an aquitard at the transition between deeper basement intrusions and high‐permeability extrusive volcanic rocks. As model calibration is performed using a Bayesian framework, the posterior results reveal significant uncertainty in the inferred permeability values for the different rock types, often exceeding two orders of magnitude. While the model shows how zones of single‐phase supercritical vapor develop above the deep intrusive heat source, more data from deep wells is needed to better constrain the extent and temperature of the deep supercritical zones. However, the model suggests the presence of a significant untapped resource at Krafla. Plain Language Summary: Geothermal systems in volcanic settings develop as a result of groundwater circulation around subsurface magmatic intrusions. The temperature of hot circulating groundwaters depends on the location/depth of the heat source and the large‐scale permeability structure of the subsurface, which controls the fluid flow dynamics. In this study we present a new reservoir model of the Krafla geothermal system in north Iceland. The model is constrained by temperature measurements and geologic data from 40 deep wells. Our model reproduces the large‐scale temperature distribution measured in the subsurface. Relatively large variations in temperature measured in nearby wells are linked to permeability heterogeneity. While most geothermal reservoir models of this kind are restricted to temperatures less than 350 °C, this model extends to even higher temperatures, evidence of which comes from certain wells in the area that punctured a deep magmatic intrusion and discharged water at temperatures up to 440 °C. Model calibration is performed using modern, machine learning techniques. The results highlight the significant uncertainty of the inferred permeability structure, which would likely be underestimated using other approaches that only find a single "optimal" solution. Key Points: We perform MCMC calibration of a 3‐D natural state reservoir model of the Krafla geothermal systemPosterior results indicate that the uncertainty of inferred permeability values is underestimated using deterministic approachesThis is one of the first reservoir models of an exploited geothermal system extending to the deep, supercritical roots near the heat source
- Subjects
ICELAND; GEOTHERMAL resources; TEMPERATURE distribution; CALDERAS; WATER temperature; VOLCANIC ash, tuff, etc.; FLUID control; GAS condensate reservoirs; WATER salinization
- Publication
Water Resources Research, 2022, Vol 58, Issue 2, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2021WR031254