LAUSR

Abstract The micropore-throat structure of a tight oil reservoir is complex, and its movable fluid distribution differs from that of a conventional reservoir. By using nuclear magnetic resonance (NMR) and centrifugation, the movable fluid distribution in the Chang 7 tight reservoir in the Xin'anbian Block of the Ordos Basin was quantitatively evaluated. Combined with the results of the physical property and high-pressure mercury intrusion experiments, the effects of the porosity, permeability, pore size distribution and micropore-throat structure on the movable fluid distribution in the Chang 7 tight reservoir were analyzed. The NMR results show that the T2 spectral morphology of the Chang 7 tight oil reservoir in the Xin'anbian Block can be characterized by three main patterns with different amplitudes of front and post peaks. The front peak corresponds to the scale of small pores and the post peak corresponds to the scale of large pores. Based on the three T2 spectrum patterns, the Chang 7 tight reservoir can be divided into three types: type 1 reservoir with more small pores and fewer large pores, type 2 reservoir with similar amounts of small and large pores, and type 3 reservoir with fewer small pores and more large pores. Thus, the characteristics of these three types of reservoirs can be determined. The movable fluid parameters were calculated by NMR and centrifugation. The movable fluid saturation of the type 1 reservoir is low, with an average of 20.38%, and the average movable fluid porosity is 1.47%. The type 2 and type 3 reservoirs have higher movable fluid contents of 40.79% and 47.04%, respectively. The average movable fluid porosities of the type 2 and type 3 reservoirs are 3.37% and 3.48%, respectively. The greater number of small pores and smaller number of large pores in the type 1 reservoir result in weak correlations of the movable fluid with the physical properties and pore-throat structure parameters. The movable fluid saturation is negatively correlated with the maximum pore radius, mainstream throat radius and maximum mercury saturation and is strongly positively correlated only with the porosity of the large pores. As the number of large pores increases, the movable fluid saturation strongly correlates with the property parameters, pore size distribution, and pore-throat structure parameters in the type 2 and type 3 reservoirs. The movable fluid saturations are positively affected by the maximum pore radius, median radius, mainstream throat radius, and effective porosity but are strongly negatively correlated only with the radius mean value. These results indicate that the fluid mobility of these two types of reservoirs can be promoted by the pore-throat system to obtain favorable seepage characteristics.