Direct Downhole Temperature Measurement and Real Time Pressure-Enthalpy Model Through Photon Counting Fibre Optic Temperature Sensing

Abstract

Temperature, pressure and enthalpy information is very important and valuable, in both geothermal drilling phase and well maintenance. We research the possibility of a direct downhole measurement of temperature, which will result in a real time pressure and enthalpy model. We report on the development of downhole geothermal brine pressure and enthalpy model, in application of the state-space T-p-X delineations and density (ρ) correlations in flowing gas, liquid and two-phase system [1][2]. The model was built through C language programming, running in NI Lab Windows/CVI user interface, using the H2O–NaCl geothermal brine thermodynamic formulation. The model is highly dependent on our fibre optic temperature sensor system for a direct temperature measurement. We are working to extend the range of a calibration-free fibre optic temperature sensing technique based on photon counting measurements of Raman backscatter (previously developed in collaboration with the US National Institute of Standards and Technology) [3][4]. We aim to extend the range of the system to kilometre length, by increasing the optical power to 1.2 µW per pulse and reducing the repetition rate of the excitation laser. We will employ superconducting single photon detectors with improved efficiency (20%) housed in a practical, closed cycle cooling system. Following laboratory tests, we aim to field test in the one of geothermal boreholes in the north of England [5], to demonstrate our real-time pressure and enthalpy model, based on temperature sensing result, down to 1 kilometre depth and over the temperature interval 270 – 321 K. References [1] Palliser and McKibbin. Transport in Porous Media 33 65 (1998) [2] Palliser and McKibbin. Transport in Porous Media 33 155 (1998) [3] Dyer et al. Optics Express 20 3456 (2012) [4] Tanner et al. Applied Physics Letters 99 201110 (2011) [5] Younger et al. Proceedings of the Geologists’ Association 126 453 (2015)