QUASI-STATIONARY EFFECT OF FREE THERMAL CONVECTION IN WATER-FILLED BOREHOLES

Abstract

The relevance. Temperature measurements in water-filled boreholes and wells are used for solving a wide range of exploration, geophysical, environmental, hydrogeological, and geodynamic problems. Free thermal convection that occurs in boreholes with a geothermal gradient exceeding a critical value causes two types of thermal effects – non-stationary effect and quasi-stationary one. The non-stationary effect is manifested by non-periodic temperature fluctuations relative to a certain average value (temperature noise) and operates in a wide range of frequencies – from seconds to hourly. The quasi-stationary effect is associated with long-term deviations of temperature and gradient in the borehole relative to the undisturbed characteristics in the surrounding rocks. The last effect leads to erroneous estimates of formation temperatures and heat flows. The main aim of the research is justification of the applicability of the Ramey’s approximation model describing the thermal effect of forced fluid flows to assess the quasi-stationary effect of free thermal convection. Adaptation and verification of the model by experimental borehole temperature data. Methods: analysis of geothermal and technological parameters determining the quasi-stationary effect of free thermal convection described by the Ramey’s model; comparison of calculations based on the Ramey’s model with data from experimental studies in boreholes. Results. The application of the Ramey’s model for evaluating the quasi-stationary thermal effect of free thermal convection in water-filled boreholes has been substantiated and experimentally verified. The decrease in the measured temperature gradient in comparison with the undisturbed gradient in the surrounding rocks is localized in the upper and lower intervals of the borehole. The effect is more pronounced and intervals are wider as the convective flow velocity increases, which in its turn depends on the Rayleigh number and the borehole diameter. The effect is less dependent on the total depth of the borehole.

Keywords

• borehole
• temperature logging
• temperature gradient
• free thermal convection
• quasi-stationary effect
• ramey’s model