Filtration index of the DRILLING fluid is an important parameter of well DRILLING, especially at DRILLING-in the reservoirs. Standard parameter in the study of DRILLING fluids is static filtration index. At the same time, it is more feasible to determine dynamic filtration index, if considering actual conditions of the well. Use of polymer carbonate-weighted DRILLING fluids is advisable at DRILLING-in oil and gas formations; but, it is also necessary to take into account fractional composition of the weighting agent. Aim of the work is to study the influence of weighting-bridging agent's fractional composition on DRILLING fluid's dynamic filtration index. Taking into account main theories of bridging agent selection, such as the theory of Abrams, Kaeuffer and Vickers, will make it possible to define the influence of carbonates “ blend” on the filtration losses of the DRILLING fluid. Dynamic high pressure high temperature (HPHT) filter was used in this research, which allows measuring filtration properties of the DRILLING fluid as in standard HTHP test, with fluid circulating inside the cell. In order to get results that are closer to real conditions, in addition to the standard paper filter, studies were carried out on ceramic disks with a known porosity. Theories on selection of bridging agents considered in the paper have their advantages and disadvantages. Depending on available data on the reservoir (for example, data availability on porosity and permeability) and access to high-quality bridging material, a more effective theory is chosen and the negative effect of the DRILLING fluid on the productivity of the formation is reduced.