Experimental investigation on forced imbibition characteristics of shale oil reservoir based on nuclear magnetic resonance method

After large-scale fracturing of shale reservoirs, the fracturing fluid will imbibe in the reservoir. In order to clarify the characteristics of imbibition and the distribution characteristics of crude oil in the stage of forced imbibition, this paper selects the core of some blocks in Qinghai and Gulong areas to carry out the shale forced imbibition experiment. Nuclear magnetic resonance technology is used to monitor the variation of crude oil. And the influence of core permeability and contact angle on forced imbibition is further analyzed. The results of the study show that the shale has a strong imbibition effect on the fracturing fluid. The content of oil phase in the core decreases, and the phenomenon of oil-water replacement occurs. Priority porosity for fracking fluids exists in the process of imbibition, where fracturing fluid preferentially enter smaller pores and can cause a short-term increase in the oil phase content in larger pores. With the continuous process of imbibition, the rate of imbibition oil gradually slowed down, and the fracturing fluid gradually permeates evenly into the rock sample. Both permeability and wettability affect the imbibition capacity of fracturing fluid in the forced imbibition stage.