Impact of Thermo-Mechanical Stimulation on the Reservoir Rocks of the Geothermal System at Krafla, Iceland

Abstract

Krafla volcano is a magmatic-hydrothermal system located in North-East Iceland. The hydrothermal reservoir at Krafla has been exploited for many decades, but as the geothermal industry is heading into a new era, with aims to drill further and reach the roots of geothermal reservoirs for higher enthalpy fluids, possibly supercritical, understanding of the rock properties at these conditions is vital. Here we employ laboratory experiments to describe the thermo-mechanical behaviour of reservoir rocks at Krafla; in particular to constrain conditions favouring the generation of fractures and improve our ability to enhance fluid flow via thermo-mechanical stimulations. Geological survey and drilling activity have helped identify the presence of six main rock types in the hydrothermal reservoir: basalts (9-60% porosity), hyaloclastites (35-45% porosity), obsidians (0.25-5% porosity), ignimbrites (13-18% porosity), and intrusive felsites and microgabbros (9-16% porosity). These samples were selected for mechanical testing, and their porosities where determined with a helium pycnometer. Samples were collected primarily from surface exposures. Thermal expansion coefficients for dense basalt (11% porosity), hyaloclastite (36% porosity) and felsite (10% porosity) were measured using a thermo-mechanical analyser. The data shows a range of expansivities depending on the lithology. The basalt expands uniformly with temperature, whereas the felsite shows different expansion rates below and above the alpha – beta transition of quartz at around 575°C. The hyaloclastite, however, shows a strong contraction resulting from mineral breakdown during heating. Uniaxial and triaxial deformation tests were used to constrain the strength, Poisson’s ratio and Young’s modulus of each rock type. The results show that the rock strength is approximately inversely proportional to the porosity and is strongly affected by the abundance of micro cracks; some of the rocks are unusually weak considering their porosities, especially at low effective pressure as constrained at Krafla. The presence of cracks also strongly influences the rock permeability, which was measured in a hydrostatic cell at pressure conditions relevant for the Krafla hydrothermal system. Thermal stressing variably impacts the strength of rocks, depending of the resilience of the mineralogy at high temperature. The data allows a complete description of the rocks’ thermo-mechanical and permeability properties, and suggests that moderate temperature fluctuations induce sufficient thermal stresses (in dense, unfractured rocks, primarily) and/ or mineral reactions, which generate thermal cracks and voids that affect the resultant permeability. We will discuss how the data may be integrated to into future fluid flow simulations to increase our understanding and potential for exploitation of hydrothermal reservoirs for geothermal energy.