Porosity structure of Icelandic basalt

Abstract

Systematic sampling of igneous rocks from extinct geothermal systems in the Icelandic crust has resulted in a database covering most types of geothermal reservoir rocks presently under exploitation in Iceland. At present, the database includes more than 500 samples spanning the basaltic to rhyolitic composition and hydrothermal alteration from literally fresh rocks to greenschist facies rocks. The database is used to study the relationship between different reservoir parameters, such as thermal conductivity and porosity, alteration, and permeability. The present paper describes the relationship between porosity and permeability and the information that can be derived from this data set. It is found that the average difference between effective and total porosities is about 2% and that this difference is independent of porosity over the range 0-50%. This demonstrates that the whole pore volume in these igneous rocks is effective for fluid flow through them. The permeability measured with liquid brine is systematically lower than that obtained by measurements using air or gas. Brine permeability is on average three times lower than gas permeability measured on the same sample. We explain this discrepancy as a result of the formation of a connate liquid film that adheres to the rock in combination with capillary forces that reduce and block portions of the flow paths for the liquid flow. Reasonable conversion of gas permeability to brine permeability can be obtained by taking into account the effect of immobilization of part of the brine. This supports the view that a film of connate brine causes the difference in the observed permeability values. The bundle capillary tube model seems to be the best model describing the porosity structure of the Icelandic rocks, whereas the Kozeny-Carman model shows very weak correlation to the data. The calculated tube diameters of the bundle capillary tube model are in the range 0.03-80 μm. while laboratory measurements on only a few samples gave values in the range 0.05-100 μm. The calculated diameters of the majority of the samples are less than 0.7 μm. or smaller than the wavelength of visible light.