The effects of ion diffusion on imbibition oil recovery in salt-rich shale oil reservoirs

Abstract

During the fracturing fluid imbibition into shale reservoirs, partial minerals could dissolve and diffuse into the fracturing fluid.However, few studies are conducted on the effects of ion diffusion on shale oil migration, especially in salt-rich shale oil reservoirs. The salt-rich shale oil reservoirs are regarded as the potential hot targets for shale oil exploration in China. In this study, a series of water imbibition and ion diffusion experiments are conducted on salt-rich shale samples. The nuclear magnetic resonance method is used to monitor the oil migration and demonstrate the effects of ion diffusion. The experimental results show that the salt-rich shale contains a large amount of halite, resulting in a much higher salt ion diffusion capacity than conventional shale (about 15-45 times higher). Salt ion diffusion can induce lots of pores, caves and fractures that can act as channels for oil migration. However, the dimensionless imbibition rate has a negative relationship with ion diffusion capacity, indicating that too many salt crystals cannot contribute to the oil production. The salt crystals dissolution may block matrix pores and generally reduce the oil migration rate, despite these new migration channels. This study can help understand the effects of fracturing fluid retention on oil production in salt-rich shale oil reservoirs.