Effect of pressure pulse stimulation on imbibition displacement within a tight sandstone reservoir with local variations in porosity

Abstract

The pressure pulse technique has been suggested as a potential method to enhance oil recovery from tight oil reservoirs. To investigate the variations in the pore structure of tight sandstone samples from the Triassic Yanchang Formation in the Ordos Basin (China), a combination of casting thin sections, high-pressure mercury intrusion, and nuclear magnetic resonance was employed. Imbibition experiments combined with the imposed pressure pulse were performed to quantitively examine the effects of the primary parameter relevant to pressure pulse on displacement efficiency and oil recovery contribution of the various pore systems. By testing the effect of different pulse pressures, it was investigated which mechanisms controlled the oil recovery. It was thus found that a series of four pulses could enhance the oil recovery significantly, and that the pressure should be 2 MPa，4 MPa and 6 MPa for the three investigated types of reservoirs, respectively. In these cases, their corresponding pulse times are 48 h, 96 h, and 144 h, respectively. The oil displacement efficiency increases with the increasing pressure and pulse cycles; however, the amplitude in oil recovery gradually diminishes. It was also found that an increasing pulse pressure is favorable for mobilizing oil that is trapped in pore spaces, especially for the low-permeability samples, despite the longer pulse time. Employing the pressure pulse method could markedly improve the imbibition oil recovery, and the simultaneous pulse pressure with imbibition leads to a higher oil recovery than the approach that adopts the pulse pressure after imbibition experiments. The main cause is that the former method facilitates oil recovery, particularly from intermediate-sized pores in relatively porous media. Furthermore, there is a significant difference in oil displacement within the various pore systems for the various types of reservoirs. The developed microfractures greatly contribute to oil recovery during the early imbibition stage, and the pressure pulse method appears most suitable to tight reservoirs with a relatively uniform pore size distribution and a higher proportion of medium-large pores.