Title:

IDDP. The Chemistry of the Krafla Geothermal System in Relation to the IDDP Well

Authors:

Halldór Ármannsson

Key Words:

IDDP, Krafla, geochemistry, acid wells, Iceland

Geo Location:

Krafla, Iceland

Conference:

World Geothermal Congress

Year:

2010

Session:

39. Iceland Deep Drilling Project

Language:

English

Paper Number:

3906

File Size:

1619KB

View File:

Abstract:

The first IDDP well was drilled in Krafla in the first half of 2009. The drill rig hit magma at about 2100 m depth and drilling was stopped. The well was designed to be drilled into a high temperature hydrothermal system with the goal of finding a 400 – 600 °C hot superheated or supercritical fluid. The chemistry of the Krafla geothermal system at shallower depths has been thoroughly studied over the last 35 years. The area has been divided into several sub-fields: Leirbotnar (Lower Leirbotnar, Vítismóar), Sudurhlídar, Vesturhlídar, Sandabotnar, Hvíthólar, Vestursvædi, Leirhnúkur. In the Leirbotnar field the system is divided into an upper zone down to 1000-1400 depth which is liquid dominated with a temperature 190-220°C and sulphate is the major anion in the fluid, and a lower two phase zone at about 300°C (on the boiling point curve) with chloride as the main fluid anion. Hvíthólar is a a two phase system following the boiling curve down to about 1000 m depth where chloride is the main fluid anion but is cooler and liquid dominated below that where sulphate is the main anion. One well has discharged in Sandabotnar suggesting a two phase fluid (boiling point curve) from a reservoir at about 260°C with chloride as the main fluid anion . In Suðurhlídar and Vesturhlídar the boiling point curve is followed and a two phase fluid of about 300°C is delivered whose main anion is chloride. The same characteristics were observed for the one well drilled in the Leirhnúkur area. No well has discharged from the Vestursvædi area which seems cool and unproductive. In most areas very deep wells (>>2000 m deep) have been observed with acid fluids near the bottom which have caused corrosion and deposition of iron compounds (sulphides, silicates, oxides) and silica. The recharge in Leirbotnar and Vesturfhlídar is local but in Hvíthólar and Sudurhlídar either from nearby high ground or from far south. The acidity is connected with the chloride anion and it has been suggested that at depth there may be a brine pool from which molecular hydrogen chloride is boiled. The hydrogen chloride rapidly forms hydrochloric acid when saturated with water. It has also been speculated that the acid fluids may be a phenomenon which occurs at a certain depth and might have been cased off in a deeper IDDP well. During the Krafla fires 1975-1984 an excess of magmatic gas entered the geothermal system but this appears for the most part to have disappeared. This gas however facilitated reactions between the fluid and the rock which may be responsible for the acidity and the deposition problems encountered. The origin of the acidity and ways to deal with it will be among the prime challenges of the IDDP well. The results of the chemical work on the fluids from this well and its interpretation in the light of the previous knowledge about the area is due to be reported.


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