Mathematical Modeling of Heat Transfer During the Retrieval of a Downhole Sampler from an Ice Borehole: The Case of Borehole 5G-5, Vostok Station

During the drilling of deep ice boreholes in Central Antarctica, one of the key tasks is to collect representative samples of the borehole fluid. The principal challenge is that, during retrieval of the downhole sampler to the surface, the sample is exposed to steep negative temperature gradients, which alter its physical properties and distort the representation of the actual borehole conditions at the sampling depth. For this study, an analytical review of current downhole sampler designs was carried out. For mathematical modeling, the finite difference method was used to solve the two-dimensional axisymmetric heat conduction equation for the "fluid sample–sampler wall" system. The initial temperature distribution was adopted from thermometric data obtained in borehole 5G-5, Vostok Station. The model incorporates actual trip-speed logs recorded during tripping operations. After modeling it was established that the temperature of the near-wall layer of the sample decreases significantly faster than that of the central region and that by the time the sampler reaches the surface, the difference between the sample temperature and the temperature of the surrounding borehole fluid is substantial enough to affect rheological properties of the fluid. The developed model makes it possible to justify the introduction of corrections to the results of direct measurements of fluid properties at the wellhead.