Constraining mechanical and permeability properties of the Krafla geothermal reservoir, North-East Iceland

Abstract

Krafla volcano, located in North-East Iceland, holds an active hydrothermal system, that has been exploited for geothermal energy since 1978. Today it is exploited by Landsvirkjun National Power Company of Iceland and the system is generating ~60 MWe from ~18 wells, tapping into fluids at 200-300°C. But as the geothermal industry is heading into a new era, with aims to drill further and reach the roots of geothermal reservoirs, sourcing higher enthalpy (possibly supercritical) fluids, understanding the physical properties rocks at these conditions is vital. In relation to this, the first well of the Icelandic Deep Drilling Project (IDDP) was drilled in Krafla in 2009. Drilling was terminated at a depth of 2.1 km, when the drill string penetrated a rhyolitic magma body, which could not be bypassed despite attempts to side-track the well. This pioneering effort demonstrated that the area close to magma had great energy potential, even though the well did not reach its initial target of 4-5 km depth. In this thesis, I have employed laboratory experiments to describe the physical behaviour of reservoir rocks at Krafla under different conditions. During two field surveys in 2015 and 2016, and information gathered from drilling of geothermal wells, six main rock types were identified and sampled [and their porosities (i.e., storage capacities) where determined]: three groups of basalts (a lava with 10 to 27 % porosity, a basalt dyke with 31-36% porosity, and a porous lava with 34 to 60 % porosity), hyaloclastites (<35-45% porosity), obsidians (0-5% porosity), ignimbrites (13-18% porosity), and intrusive felsite’s and micro-gabbro’s (9-16% porosity). Samples are primarily from surface exposures, but selected samples of hyaloclastite core were sampled from cores drilled within the Krafla caldera. The permeability properties of both intact and fractured reservoir rocks were investigated using a hydrostatic cell, simulating stress conditions extant in the geothermal reservoir. The impact of thermal stimulation and pressure fluctuations was also investigated to further simulate reservoir conditions. The mechanical properties were investigated, especially how they might change in response to pressure changes, and how this might impact the effect of thermal- or mechanical stimulation. To further investigate the complex post-deposition evolution of rocks within the geothermal system, samples of hyaloclastite cores were compared to samples from the surface. As hyaloclastite gets buried within the caldera, it is subjected to increased pressure, temperature and fluid flow through the rock, causing alteration that decreases the permeability and increases the strength of the material within the reservoir with increasing depth. As volcanically active areas are constantly changing and evolving, the properties of reservoir rocks can vary greatly. Thus, site exploration relies on understanding the physical- and mechanical properties of the rocks from geophysical surveys and real-time drilling data. For a successful drilling campaign and economically viable extraction from the hydrothermal reservoir, knowledge of the rock behavior under various conditions is vital to seek ways to increase the permeability of the reservoir to enhance productivity of the exploited geothermal wells.