Petrophysical and Mechanical Characterization of the Volcanic Rocks in the Hellisheiði Geothermal Field and implications of Thermal Fracturing in CO2 mineralization

Abstract

The level of advancement in the understanding of the mechanical properties of volcanic rocks is comparatively lower than that of sedimentary rocks. As part of the SUCCEED Project (Synergetic Utilisation of CO2 Storage Coupled with Geothermal Energy Deployment), which aims to investigate the feasibility of injecting captured and produced CO2 into the reservoirs to enhance geothermal production and achieve permanent CO2 storage at the Hellisheiði Geothermal Field in Iceland, this experimental research provides significant insights into the petrophysical and mechanical properties of the volcanic rocks collected from surface outcrops. The subsurface in Hellisheiði is mainly built up of hyaloclastite formations and interglacial basaltic lavas. During a field campaign samples were collected in different outcrops, ensuring that the samples were of high quality and sufficiently diverse to enable comprehensive analysis. Four samples per block and rock type have been prepared from the collected blocks, and they have been subjected to different laboratory tests to evaluate their petrophysical properties, such as porosity, density, and permeability, and their geomechanical behavior, using Unconfined Compression Test (UCS), Active-Source Acoustic Test, and Splitting Tensile Strength Test. Additionally, laboratory experiments have been conducted to investigate the impact of rapid cooling on rock damage due to thermal fracturing. The results show that there are interdependent relationships between porosity, bulk density, ultimate strength, Young's modulus, and wave velocities that can be observed when considering average values per rock. The rocks studied showed a negative correlation between porosity and other parameters and a direct correlation between ultimate strength and Young's Modulus. When examining individual rock samples, no significant correlations were observed between porosity and other parameters, however, those correlations where evident when comparing between different rock types, emphasizing the importance of analyzing rock properties from a broader perspective. The rocks studied could be classified into five units based on their petrophysical and mechanical properties. Ordered from higher porosity and lower mechanical parameters, these units are: unit 1 consists of hyaloclastite HH-1, unit 2 includes porous basalts HBA-18 and HPB-23, unit 3 consists of low-porosity basalts HB-4 and HBimp-9, unit 4 is made up of dike ND-6, and unit 5 comprises gabbro G. This implies that there is a notable variation in the properties of rocks between different units, but the properties of rocks within the same unit do not differ significantly. Volcanic rocks have a significant amount of unconnected porosity, which, if connected, can enhance the storage capacity of the reservoir and improve the reactive surface area of the rocks in contact with the reinjected fluid, leading to a more efficient mineral storage process. The results of a thermal shock conducted to simulate reservoir and injection temperatures (270ºC and 60ºC) have shown no significant changes in the petrophysical and mechanical properties of the rocks, indicating that this temperature difference does not increase the effective porosity nor compromise the integrity of the reservoir. This study validates the potential use of certain rocks collected from surface outcrops in Hellisheiði as reservoir analogs for future geological models, particularly those with lower porosity.