Experimental study on replacement behavior of crude oil by CO2 based on nuclear magnetic resonance technology

Abstract

Due to the high viscosity of Jimsar crude oil and the lipophilicity of the reservoir, CO2 has an excellent performance in development of Jimsar shale oil. However, previous studies mostly focused on CO2 fracturing, but the replacement of crude oil by CO2 is rarely reported. Therefore, we conducted research on oil replacement by CO2 through systematical experiments. Firstly, we designed an experimental approach based on nuclear magnetic resonance to quantify the replacement ratio of oil. Next, a series of experiments were conducted for analyzing the effect of pressure, soaking time, permeability, and fractures on replacement ratio. Finally, oil-water imbibition experiments were conducted to compare its development effect with CO2 replacement for target reservoir. The study shows that the replacement capacity of CO2 for crude oil increases with pressure. In the experiment, the replacement ratio increases from 27.46% (10 MPa) to 56.70% (40 MPa) after 24 hours. On the microscopic scale, the crude oil in large pores is replaced out firstly, then the smaller pores. And on the macroscopic scale, the replacement of crude oil is from outside to inside. High permeability and fractures in the core promote the intrusion of CO2 into the core, raising the replacement ratio significantly. Since Jimsar reservoir is oil-wet, the oil recovery of oil-water imbibition is far less than that of CO2 replacement. The minimum pore through which oil can be exploited by oil-water imbibition is 1 μm, while for CO2 replacement the value is 0.08 μm. This study provides fundamental for CO2 increasing oil recovery in Jimsar shale oil.