The poor quality of reservoir fluid samples is a recognized industry problem. The paper presents new experimental data indicating that a significant contribution to this is made by the imperfection of the designs of the used downhole samplers. In 2019-2020, the authors developed a dowhhole sampler with the function of measuring pressure and temperature inside and outside the sample chamber and forcibly closing the sample chamber to cut it off from the borehole space. During the 3 years of operation of the sampler, unique information has been accumulated that significantly changes the understanding of the operation of deep samplers and the requirements imposed on them. The sample chambers of most modern samplers are sealed by a check valve pressed against the landing seat by a spring and, when the sample is raised to the surface, by the overpressure of the sample itself; wherein the well fluid is in direct contact with the valve. Since the 1940s, it has been believed that in the process of lifting a sample, the pressure in the chamber will always be higher than the pressure in the well. The authors experimentally obtained dozens of pressure curves inside and outside the receiving chamber for various which show that the cooling of the chamber during its rise often leads to such a temperature decompression of the fluid that the pressure from the well becomes greater than the pressure in the chamber (the maximum pressure drop was obtained during water sampling and was in some cases 9 MPa, which is two orders of magnitude higher than the force of the valve springs of any known samplers). This means that most modern samplers allow fluid to flow from the well into the sample chamber after its nominal closure. The obtained experimental results are confirmed by modeling the behavior of various fluids using cubic equations of state. The authors describe the phenomenon of self-depressurization of the chambers of downhole samplers, conducted a thermodynamic analysis of this phenomenon, collected actual experimental data confirming its presence in practice. Potential effect of equipment modernization: reduction of the number of rejected downhole samples by 47%. The assessment is based on the results of the analysis of historical data on 289 downhole samples.
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