Experimental Investigation into Replacement of Crude Oil in Core Samples by Carbon Dioxide Based on Nuclear Magnetic Resonance Technology

ABSTRACT: CO₂ pre-pad energized fracturing operation was successfully pilot tested in shale oil development. The replacement of crude oil by CO₂ during the soaking period is considered to be a significant factor of EOR mechanism, but crude oil replacement by CO₂ has not been studied systematically from experiment. Aiming at this, we conduct experiments on crude oil replacement by CO₂ in core samples from Jimsar shale quantitatively by introducing nuclear magnetic resonance (NMR) technology. Through NMR technology, the amount of crude oil in core before and after replacement can be quantified. Experimental results show that supercritical CO₂ has a strong replacement capability on crude oil, and at core scale, the replacement occurs in a short time. For Jimsar core samples, the crude oil in the large pores is displaced out firstly, then the smaller pores. At high pressures, the peak of T2 spectrum curve shifts left, meaning some oil is forced into smaller pores. Oil replacement ratio increases from 21.35% to 56.70% as the pressure increases from to 40MPa, and levels off above about 30MPa. Oil replacement ratio increases with soaking time and levels off about at 18h. At 20MPa 24h, the oil replacement ratio can be 42.50%.