IMAGE-D4.03 Final report on: High temperature (>380°C) measurement

The aim of this task is the development of a method to measure high reservoir temperature (≥380°C) by the production of synthetic fluid inclusions within an apparatus that will be placed in high-temperature geothermal wells. Super-hot geothermal systems in magmatic areas are, in fact, a possible target for the future geothermal exploration either for the direct exploitation of fluids or as a potential reservoirs of Enhanced Geothermal Systems. Reservoir temperature determination are crucial for the assessment of the geothermal resources, however, measurements of temperature (T) in high-temperature (>380°C) geothermal wells are difficult or impossible by using conventional logging tools. In fact, mechanical temperature sensors (KTG bi-metal, KTG LV based on bourdon tube principle) and electronic high-temperature tools (thermocouples, resistance thermometers, thermally shielded) have a maximum operating temperatures of 380°C and 350°C, respectively (HiTI, 2010). Furthermore, conventional optical fibers device can be used to maximum temperature of 275°C. For temperature >380°C “melting tablets” with known melting temperature (Zn: 419ºC) were sometimes used, however such method gives minimum temperature of temperature ranges. Thus, un-conventional methods should be developed in order to overcome this problem. 

The European HITI project (High Temperature Instruments for Supercritical Geothermal Reservoir Characterisation & Exploitation) which was carried out in 2007-2010 developed, built and tested in the field new downhole tools and developed chemical approaches for deep high temperature boreholes. In particular, during the HiTI a wire-line temperature probe with a possible use up to 500ºC was developed and was tested at temperature up to 320ºC. Moreover, a PLT multi-sensor within a heat shielding flask tolerating 400ºC was also developed. 

In the framework of IMAGE project, we developed a possible method alternative to the above methodology for borehole high-temperature measurement by the production of synthetic fluid inclusions (F.I.) within an apparatus that will be placed in geothermal wells. 

This is based on the entrapment of aqueous fluid (of known chemical composition and salinity) in pre-fractured mineral chips; for a given composition, the density and the molar volume of synthetic F.I. entrapped in the fractures will be function of Pressure (P) and T. Therefore, the density (and molar volume) of the synthetic F.I. can be obtained from microthermometry, and isochores can be computed and the T of trapping of synthetic F.I. can be estimated with a good approximation from isochores if P is known. 

Synthetic F.I. as logging tools was first used for temperature measurement and fluid sampling in the Continental Scientific Drilling Program in the Valles caldera, New Mexico, U.S.A. (Bethke et al., 1990). In this case F.I. were synthesized for 21 days and their formation temperature corresponded to that measured using conventional techniques (293.6°C at 1762 m depth). Then, Sawaki et al. (1997) utilized the synthesis of F.I. to measure temperatures and sample fluids in high-temperature geothermal wells; they tested this methodology in a hole drilled in the Kakkonda (Japan) geothermal field and after 24 days, they got temperatures measured from F.I. consistent with borehole temperatures measured by conventional logging tools. 

Later, Sekine et al. (2004) through lab experiments estimated trapping conditions of fluids in pre-fractured quartz utilizing different sets of synthetic F.I. with different salinities. They got synthetic F.I. after 5 days’ autoclave experiments at T= 375-475°C, P=39-62 MPa. 

The method elaborated in this task is based on the production of synthetic fluid inclusions within an apparatus that will be placed in geothermal wells. This apparatus consist of gold capsules placed within a stain steel vessel (micro-reactor), the latter will be partially filled with an amount of distilled water such that the the P-T conditions in the micro-reactor will follow the liquid-vapor curve of H2O and critical isochore of H2O (characterized by a density of 0.322 g/cm3) above the critical point of H2O. The gold capsules contain: i) a pre-fractured quartz fragment (devoid of natural fluid inclusions) in which synthetic F.I. will be trapped, ii) an alkaline, saline aqueous solution (9.1 wt.% NaCl + 0.4 wt.% NaOH), i.e. the solution that will be trapped in the synthetic F.I. and iii) powdered silica. The utilization of alkaline aqueous solutions (in quartz) decrease the time necessary to form F.I., this is a crucial point for measurements in high-T geothermal well 9 (>380°C), as the resistance of the cable needed for lowering devices in geothermal wells decrease with time at high-T and with the prolonged exposure to high-T aggressive fluid. 

A set of experiments have been performed in the laboratory either by placing the gold capsules in an externally heated pressure vessel and by placing a commercial micro-reactor containing the gold capsules within a furnace. Such experiments demonstrated that synthetic F.I. form within a relatively short time (even in 48 hours) and that trapping temperatures of synthetic F.I. give a good estimate of the experimental temperatures. Finally, a test was carried out by lowering the apparatus in the KJ-35 geothermal well (Krafla, Iceland) at about 1750 m below the ground level, at such depth conventional method recorded a T of about 335°C. The test showed that synthetic F.I. formed and that the trapping temperature of synthetic F.I. (339-343°C) closely approach the measured temperature. 

In conclusion, the proposed method can be used to measure the temperature in relatively high-temperature geothermal well (<380°C) but it can be potentially applied for temperature measurements of geothermal well with temperature up to 427°C (i.e. the working temperature limit of the utilized commercial pressure vessel). This limit can be extended to higher temperature by using different pressure vessels.