Oil utilization degree at various pore sizes via different displacement methods

Abstract

A reasonable displacement method is essential to improve the oil displacement efficiency of tight reservoirs. In this study, three different displacement methods were utilized on the tight core samples obtained from the Yanchang Formation Chang 8 and Chang 9 tight oil reservoirs: spontaneous imbibition displacement, various water flooding rate displacement and water flooding displacement after spontaneous imbibitions; furthermore, the oil utilization degree of the residual oil in various pores was discussed. The oil displacement efficiency of the spontaneous imbibitions was approximately 26.91% and 29.56% for the Chang 8 and Chang 9 samples, respectively. With an increasing water flooding rate, the oil displacement efficiency features an inverse “V”-like tendency, and a water flooding rate of 0.06 mL/min was the optimal value as; the oil displacement efficiency achieved was 63.56% and 60.27% for the Chang 8 and Chang 9, respectively. When compared with spontaneous imbibitions, at a displacement rate of 0.06 mL/min after spontaneous imbibition, the oil displacement efficiency could be further increased to 50.02% and 30.35%, respectively. The differences in the oil displacement efficiency using various displacement methods are primarily related to the degree of utilization of residual oil in various pores. The progressively refined pore classification method is used to study the degree of oil utilization in various pores, and the pores in the tight reservoir can be divided into four types: P1, P2, P3 and P4. Regarding the spontaneous imbibition displacement, the displacement of the residual oil is dominantly determined by the residual oil present in the P2 and P3 pores; as the residual oil massively accumulates in the P2 pores, the discharging of the residual oil in this part finally determines the oil displacement efficiency when the water flooding rates changed. At a water flooding rate of 0.06 mL/min after spontaneous imbibition, the oil utilization degree of residual oil in various pores is enhanced, particularly for the P2, P3, and P4 pores, and the pore structure contributes to the increase in the oil displacement efficiency. Using the progressively refined pore classification method, the complexity of the distribution of residual oil in tight reservoirs could be studied quantitatively and elaborately, and the results can efficiently guide the development of residual oil in tight oil reservoirs.

Research on fluid mobility in tight-sandstone with a NMR fractal theory pore classification method

“…The pore structure of porous media has typical fractal characteristics. With the NMR fractal theory pore classification method proposed by Li et al (2022), the pores…”

“…Based on the NMR fractal theory pore classification method provided by Li et al (2022), the types of pores in the cores would firstly divided with a fractal dimension of 2, and then the pore fractal dimension correlation coefficient mutation point is used as the secondary pore classification point for the classified primary pore classification,…”

“…Dai et al (2019) selected four characteristic points on the NMR T 2 spectrum and classified the pores into nano-pores, small pores, medium pores, and large pores. Li et al (2022) classified the movable fluid pores in tight cores into four different pore types, P1, P2, P3 and P4 respectively with the fractal method of nuclear magnetic resonance. Based on the combination of multiple methods to form a "wide range, high accuracy, and large scale" pore structure system, which can provide an extremely enrichment pore structure information (Clarkson et al, 2012), such as Gao et al (2019) used a combination of SEM, HMIP and C-MP to study the pore structure of the pores.…”

“…However, these studies mainly focus on the total fluid mobility in the tight reservoir, and the refined fluid mobility in various types of pores is not studied detailed. In this study, the tight-sandstone core samples of Middle Jurassic Xishanyao Formation in the Santanghu Basin, Norwest China were selected as the subject, and the fluid movable of cores under various centrifugal speeds of saturated simulated water were carried out, combined with the NMR fractal theory pore classification method provided by Li et al (2022), the types of pores in the tight-sandstones were divided, and quantitatively characterizes the characteristics of fluid movability and the degree of movability in different types of pores were launched.…”

Research on the Differential Oil Producing in the Various Scale Pores under Different CO2 Flooding Modes with a Fluid Distribution Pore Classification Method

“…41 Li et al (2022) classified movable fluid pores in tight sandstone cores based on the NMR fractal method; the movable fluid pores are divided into four different pore types, P1, P2, P3, and P4. 42 A lot of previous studies have been done on CO 2 flooding mechanism and distinguishing of the microscopic heterogeneous characteristics of tight reservoir with NMR technique, but the use of NMR technique to clarify the differential mobilization of oil in different scale of pores volume under different CO 2 displacement methods is slightly weak. In this study, five tight sandstone core samples collected from Chang 6 Member of Yanchang Formation in Ordos Basin were launched the continuous CO 2 flooding and CO 2 huff and puff oil displacement experiments under five various CO 2 injection pressures, combining with the refined classification method of oil distribution pores, the types of pores in the tight sandstone cores were classified, and the differential producing of oil in different scale pore volume in tight reservoirs is studied.…”