Experimental Research on Transformation from Huff-n-Puff Imbibition to Displacement of Tight Sandstone with NMR

Fluid mobility analyses during imbibition process and investigation of pressure changes synergetically associated with imbibition in tight oil formations are important due to the limited flow ability of matrix-fluid systems without manufacturing fractures. Imbibition is an important mechanism of oil recovery in rock matrix whilst many previous experiment studies were employed on spontaneous imbibition at ambient pressure. Influence factors of oil recovery need to be ascertained over imbibition interacted with cyclic fluid huff-n-puff and continuous displacement for EOR and energy replenishment. This research provides an experimental analysis method of fluid mobility within different sized pores in the combined processes of huff-n-puff imbibition and displacement in tight sandstone cores. Additionally, the experimental study on analyzing pore fluid mobility and identifying reasonable imbibition time are performed by transformation from huff-n-puff imbibition to displacement tests with different imbibition time. Furthermore, the influence factors on matrix oil recovery of several conditions are analyzed and their extent of effect on recovery of huff-n-puff imbibition and displacement are compared, including direction of displacement, time of transformation from huff-n-puff imbibition to displacement, pressure of displacement and existence of fracture. Experimental results show that: At the stage of huff-n-puff imbibition, the macropore was used as primary channel seepage for oil arising from mesopore and micropore which made major contributions to imbibition recovery, respectively, by 44.62% and 32.19%. The factors in orders of influence on oil recovery are as follows: direction of displacement, time of transformation from huff-n-puff imbibition to displacement, existence of fracture, pressure of displacement. This study suggests that displacement is in the same direction as imbibition, and the reasonable time for imbibition before displacement is 96 h. The research results provide important reference for the study of enhanced oil recovery in tight sandstone reservoirs.