Experimental study of spontaneous imbibition and CO2 huff and puff in shale oil reservoirs with NMR

The shale oil in the Jimsar sag, Junggar Basin, NW China is developed by horizontal wells and volume fracturing, and has the characteristics of a high initial production rate followed by a rapid decline and a low oil recovery factor. Therefore, it is necessary to find effective methods for enhancing shale oil recovery rates. The shale formations in the Jimsar sag, Junggar Basin are classified into three different types: types I, II and III. Fracturing fluid imbibition, surfactant imbibition and CO2 huff and puff experiments were conducted to evaluate the EOR potential of different types of shale formations. The oil distribution inside the shale core samples with time before, during and after the experiments is monitored using low-field NMR. The experimental results show that the CO2 huff and puff method had the highest ultimate oil recovery rate, followed by the surfactant imbibition method; the fracturing liquid imbibition method preformed without the addition of a surfactant had the lowest oil recovery rate. Surfactants improved the oil recovery of spontaneous imbibition by approximately 18% more in the type II and III shale formations compared to that of the fracturing liquid imbibition method. Field applications also demonstrated that adding a surfactant into the fracturing liquid significantly improved the oil production in type II and III oil formations relative to the common slick water fracturing liquid without an added surfactant.