Utilizing supercritical geothermal systems: a review of past ventures and ongoing research activities
Supercritical geothermal systems are very high-temperature geothermal systems that are located at depths near or below the brittle–ductile transition zone in the crust where the reservoir fluid is assumed to be in the supercritical state, that is for pure water, temperature and pressure are, respectively, in excess of 374 °C and 221 bar. These systems have garnered attention in recent years as a possible type of unconventional geothermal resource due to their very high enthalpy fluids. Supercritical conditions are often found at the roots of volcanic-hosted hydrothermal systems. More than 25 deep wells drilled in geothermal fields such as The Geysers, Salton Sea, and on Hawaii (USA), Kakkonda (Japan), Larderello (Italy), Krafla (Iceland), Los Humeros (Mexico), and Menengai (Kenya) have encountered temperatures in excess of 374 °C, and in some cases have encountered magma. Although fluid entries were documented for some of these wells, it remains an open question if permeability can be maintained at high enthalpy conditions. The IDDP-1 well at Krafla encountered magma, and ended up producing very high enthalpy fluids; however, these fluids were very corrosive and abrasive. Innovative drilling and well completion techniques are therefore needed to deal with the extreme temperatures and aggressive fluid chemistry compositions of these systems. New efforts are underway in Japan (northern Honshu), Italy (Larderello), Iceland (Reykjanes peninsula and Krafla), Mexico (Los Humeros), USA (Newberry), and New Zealand (Taupo Volcanic Zone) to investigate supercritical systems. Here, we review past studies, describe current research efforts, and outline the challenges and potential opportunities that these systems provide for international collaboration to ultimately utilize supercritical geothermal systems as a geothermal energy resource.
Sub-Surface Geology , Petrology and Hydrothermal Alteration of the Menengai Geothermal Field , Kenya : Case Study of Wells MW-02 , MW-04 , MW-06 and MW-07
Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-temperature geothermal activity. The field is currently in its initial stages of development for geothermal energy in Kenya following Olkaria and Eburru fields. Regional surface geology of Menengai is largely composed of late Quaternary volcanics. The building of a 200,000 year old trachyte shield volcano was followed by piecemeal subsidence through two paroxysmal eruptions 29,000 and 8,000 B.P to produce a caldera of about 84 km in size that has subsequently been largely filled by recent trachyte lavas. Twenty four exploration and production wells, some of them hotter than 390C, have been drilled in Menengai caldera by Geothermal Development Company (GDC). The aim of this study is to reveal the evolutionary history of the Menengai volcano and to describe its igneous lithostratigraphy and secondary mineralization in order to characterize hydrothermal processes within the field. Analytical methods used include binocular microscope analysis, petrographic analysis, X-ray Diffractometer analysis and inductively coupled plasma (ICP-ES) analysis. Studies of drill cutting from wells MW-02, MW-04, MW-06 and MW-07 have provided information on the stratigraphy, hydrothermal alteration and present state of the geothermal system. Petrochemistry of these wells revealed subsurface lithostratigraphy that includes, consolidated pyroclastic tuff, pyroclastics, basalt, trachybasalt, phonolite, trachy-andesite, syenitic intrusive units, while trachyte constitutes over 90% of the total. Reaction of geothermal fluids with the host rocks has resulted in a progressive hydrothermal alteration with increasing depth. A number of studies have recognized characteristic alteration zones at Menengai based on distribution of key index alteration minerals. These zones, in order of increasing alteration grade, are zeolitesmectite, quartz-illite and wollastonite-actinolite zone. The study reveals that Menengai evolved a complex compositional variation with time as a result of magma fractionation, possibly in combination with crustal contamination and hydrothermal activities en route to the surface. The study defines the geology of the Menengai into, post-caldera, syn-caldera, upper and lower pre-caldera volcanics. Petrography and mineral chemistry of the basaltic and trachytic end members of the Menengai rocks indicate that two or more distinct magma types were involved in the formation of the volcano. This study reveals that magma exists beneath the caldera floor, at depth slightly over 2000 m within the summit area. This is evidenced by fresh glassy, quenched cuttings at these depths in wells MW-04 and MW-06. This implies that the thickness of the geothermal reservoir above the magma-containing domain may be no more than 1.5 km from the hydrostatic surface at ~400 m.
Integrated two-beam antenna array featuring dual-circular polarization
A concept of an integrated antenna array featuring two independent beams and dual circular polarization has been described. The antenna array consists of two radiating elements which are excited at two orthogonally placed feeding ports with the use of four appropriately connected 3-dB/90° directional couplers. Therefore, such an array generates two symmetrically located beams having right-handed circular polarization (RHCP) and identical two beams having left-handed circular polarization (LHCP). The presented concept has been verified by the design and measurements of an antenna array operating at 2.4 GHz frequency range.