Abstract：

With the rising energy demand in the world, tight reservoirs have been seen as potential alternative resources. Such tight formations are generally exploited with multistage hydraulic fracturing technology and fracturing fluids imbibed into rocks by spontaneous imbibition is an important mechanism of oil production. Based on NMR technique, the characteristics of spon- taneous imbibition in oil-water and gas-water system for tight sandstones were studied. T₂ spectra obtained from these samples were used to reflect the migration patterns of fluid in different pores under various imbibition conditions. In addition, the influ- ences of boundary conditions on imbibition recovery were also explained via T₂ spectra. The results show that tight sandstone exhibits multiscale pore structure, which is mainly dominated by micropores and small mesopores. Once the imbibition process begins, white oil in micropores will be preferentially displaced by water and flow out through interconnected smaller pores due to the larger capillary pressure. Majority of the production by imbibition can be attributed to the contribution of micropores. For the gas-water system, however, water could not enter mesopores readily if only driven by capillary pressure owing to the snap-off effect of NWP. The boundary conditions pose a significant effect on imbibition rate and ultimate recovery for oil-water system, increasing the areas available for water imbibition helps to maintain a high imbibition rate and recovery. As for gas-water system, boundary conditions exert little effects on final recovery but remarkable impacts on the imbibition rates. The traditional scaling equations can be used to scale the imbibition data for both oil-water and gas-water systems to some extent, and predict the recovery by imbibition if the wettability of rock medium remains unchanged.

Key words：tight sandstone; spontaneous imbibition; NMR; boundary condition; scaling group

Received： 2018-01-15

Corresponding Authors： wq2012@cup.edu.cn

Cite this article：CHENG Zhilin, WANG Qing, NING Zhengfu, LI Mingqi, QI Rongrong, YU Xiongfei. Effect of boundary conditions on spontaneous imbibition in oil-water and gas-water systems for tight sandstone using NMR. Petroleum Science Bulletin, 2018, 03: 272-283.

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