Sveinbjörn Björnsson

The use of gas chemistry to evaluate boiling processes and initial steam fractions in geothermal reservoirs with an example from the olkaria field, Kenya

A method has been developed to evaluate boiling processes in the producing aquifer of “high-enthalpy” geothermal wells using data on the concentrations of CO2, H2S and H2 in steam discharged. The extent to which water and steam are separated in the producing aquifer is evaluated as well as the amount of enhanced evaporation due to heat flow from the rock to the boiling water. Further, the initial steam fraction in the reservoir fluid is calculated. Results are presented for the Olkaria geothermal field, Kenya, to demonstrate the use of our method. They show that the initial steam fraction in the reservoir is very small: up to 0.25% of the mass, or about 10% by volume. Segregation of water and steam in the producing aquifers is rather extensive for some of the wells. Thus, water which has boiled and yielded steam into wells amounts to more than two times the mass of the fluid discharged from the well. The larger part of the exploited steam (34) is generated by flow of heat from the rock to the boiling water.

Seismicity of Iceland

Earthquakes in Iceland located teleseismically are mainly confined to a zone off the northern coast (the Tjornes Fracture Zone), a narrow E-W zone in S-Iceland (including the Reykjanes Peninsula), and the eastern volcanic zone. Destructive historic earthquakes of magnitude up to 7 or 8 are known to have occurred in these zones, except in the eastern volcanic zone and the western part of the Reykjanes Peninsula. Local instrumental observations show that significant seismic activity occurs in other parts of the country. Smaller earthquakes and earthquake swarms occur frequently on the Reykjanes Peninsula and in the western and the eastern volcanic zones.

Exploration of the Reykianes thermal brine area

Abstract The Reykjanes thermal brine area is located in the extreme southwest of Iceland on the subaerial continuation of the Reykjanes Ridge. This area is unique among thermal areas for its fluid composition. In the reservoir, where temperatures are between 250 and 290 °C the brine has the same salinity as sea water. However, the concentrations of some ions are different and can be explained by relatively simple interaction of sea water with the rock. The content of trace metals is low. In the upflow zone hot brine has been exchanged many times during the lifetime of the hydrothermal system. Surface activity covers an area of 1 km 2 and resistivity survey indicates that the system is of similar area above 900 m within the hyaloclastite formation surrounded by cold ground water of sea water composition. By contrast the system is more extensive in the underlying basalt formation consisting of lava flows and thick interbeds of hyaloclastites and sediments. At 2600 m depth P-velocity increases from 4.2 to 6.5 km/sec. At this depth the basalt formation is succeded by denser formation considered to be similar to the ⪡ oceanic layer ⪢. Due to its temperature and composition feasibility studies indicate that the brine could be exploited economically for the production of NaCl, KCl, CaCl 2 and possibly other components. Aquifers are abundant in the basalt formation. It is therefore recommended that production wells penetrate to depths of about 2600 m to withdraw brine within this permeable formation so as to ensure highest mass flow and minimize risk of cold sea water intrusion.

Crust and Upper Mantle Beneath Iceland

Except for greater thickness and more pronounced lateral variations the Icelandic crust is very similar to the crust beneath the Reykjanes Ridge. Layer 3 can be traced without interruption from the 10 m.y. old oceanic crust on the southeastern flank of the ridge through Iceland. It exists also beneath the eastern neo- volcanic zone which is supposed to be the present spreading axis. Wide-angle reflection surveys across the active volcanic zones and the adjacent plateau basalts have given a detailed picture of the crust. The cross sections bear a certain resemblance to the crustal structure deduced from geological observations in Eastern Iceland. The dip of refractors and reflectors toward the axial zone and the absence of reflecting horizons below 8 km depth in the central part of the zone, support the predictions of models of crustal accretion mechanism. The episode of crustal rifting in the NE Iceland axial zone, which started in 1975 and still continues, has demonstrated how intimately the rifting is related to episodic magmatic processes at central volcanoes.

Heat and Mass Transport in Geothermal Reservoirs

Geothermal reservoirs are generally more complex than reservoirs of groundwater or petroleum. Physical states of the hydrothermal fluid fall into four categories: vapor-saturated, two-phase boiling, liquid-saturated and supercritical. Liquid-saturated reservoirs and liquid-dominated or vapor-dominated reservoirs of the two-phase boiling type are the most common types exploited so far. There is growing interest in submarine geothermal systems and heat extraction from hot rock or magma bodies, where the hydrothermal fluid circulates at supercritical temperatures and pressures. Meteoric water dominates in continental systems and ocean water in submarine systems. The contribution of magmatic water is small at upper levels in the crust, but may increase as magma bodies are approached. The larger fumarolic fields have magma as a heat source. The rate of heat transfer required to sustain the intense heat output of such fields remains problematic, unless an intimate contact between circulating fluids and hot boundary rock of the magma is maintained over the lifetime of the activity. Convective downward migration of fluid along existing fractures and water penetration by thermal cracking of hot rock are important processes in this respect. Two-phase convection is of major importance in geothermal reservoirs. The phase change instability mechanism induces convection prior to the onset of ordinary buoyancy-driven thermal convection. Mathematical modelling of geothermal systems has greatly advanced the understanding of the dynamic nature of geothermal reservoirs and their response to exploitation.

Tectonic Control of the Reykjanes Geothermal Field in the Oblique Rift of SW Iceland: From Regional to Reservoir Scales

This paper presents a multidisciplinary structural analysis of the Reykjanes Peninsula where Holocene deformation of a young oblique rift controls the geothermal processes in presence of a transform segment. The new structural map from aerial images and outcrops is correlated with selected surface and subsurface data and shows a complex pattern: NNE extensional rift structures, N-S dextral and ENE sinistral oblique-slip Riedel shears of the transform zone, and WNW and NW dextral oblique-slip faults. Shear fractures are more common, and along with the NNE fractures, they compartmentalise the crustal blocks at any scale. The fractures are within two ENE Riedel shear zones, indicating a minimum 7.5 km wide transform zone. The greatly deformed Southern Riedel Shear Zone is bounded to the north and the south by the 1972 and the 2013 earthquake swarms. This shear zone contains the geothermal field in a highly fractured block to the west of a major NW structure. Some of the deformations are: a) clockwise rotation of rift structures by the 1972 earthquake zone, inducing local compression; b) magma injection into extensional and oblique-slip shear fractures; c) reactivation of rift structures by transform zone earthquakes; d) tectonic control of reservoir boundaries by WNW and ENE shear fractures, and the distribution of surface alteration, fumaroles, CO2 flux, reservoir fluid flow and the overall shape of pressure drawdown by N-S, ENE, WNW/NW and NNE fractures. Results demonstrate the role of seismo-tectonic boundaries beyond which fault types and density change, with implications for permeability.