Occurrence characteristics of movable fluids based on the division of pore throat system in tight gas reservoir by fractal theory

s: Tight sandstone gas reservoirs are of strong heterogeneity, and the distribution and mobility of reservoir fluids vary substantially, so it is quite necessary to accurately and reliably evaluate the seepage capacities and occurrence characteristics of fluids in reservoirs. To explore the occurrence characteristics and seepage capacity differences of movable fluids in different qualities of reservoirs, this paper selects eight typical samples of the Shanxi Formation along the southeastern margin of the Ordos Basin. Based on the pore throat distribution in the reservoir characterized by the pseudo-capillary pressure curve converted from NMR, different grades of pore throat systems are defined according to the fractal theory, and their effects on the occurrence characteristics of moveable fluids are analyzed. And the following research results are obtained. First, according to the pore type, mercury injection curve shape and parameter, the reservoirs in the study area can be classified into three types. From type I to type III, larger dissolution pores get more, intercrystalline micropores get fewer, and consequently effective reservoir space and seepage capacity reduces continuously. Second, type I and type II samples' pore throat distribution curves obtained on the basis of pseudo-capillary pressure curves are morphologically similar to that of high-pressure mercury injection, and are consistent in the peak values. Type III sample has more nanoscale pore throats that can't be detected by means of mercury injection, so its peak pore throat distribution obtained by NMR shifts to the small pore throat. Third, according to the fractal turning point obtained by the fractal theory, the pore throat space can be classified into three relatively large, medium, and small pore throat systems. Porosity is in the best correlation with the absolute space content of large pore throat system, and permeability and movable fluid saturation are closely correlated with the percentage of the large pore throat system in the reservoir space. In conclusion, the growth degree of the large pore throat system dominates the seepage capacity of the moveable fluids in the reservoirs. The research results can provide the basis for the selection of tight gas reservoir evaluation parameters and the reference for the further research on unconventional oil and gas reservoirs. © 2023 Natural Gas Industry Journal Agency. All rights reserved.