High temperature instruments and methods developed for supercritical geothermal reservoir characterisation and exploitation—The HiTI project

•New Na/Li geothermometric relationships were discovered and tracer tests performed.•Televiewer and gamma tool with casing thickness evaluation field tested to 300°C.•Pressure, temperature and spinner tool made for 400°C.•Wireline temperature sensor field tested to 300°C.•Laboratory core deformation studies and rock resistivity measurements.