Abstract
Carbon dioxide (CO<sub>2</sub>) fracturing technology is introduced to improve the development effect of low permeability reservoirs. To understand the advantages of CO<sub>2</sub> fracturing and the mechanism of CO<sub>2</sub> assisting oil displacement efficiency, laboratory core displacement experiments and nuclear magnetic resonance (NMR) tests are conducted. For core displacement experiments, a water flooding, a water flooding-CO<sub>2</sub> huff and puff experiment are conducted. Water flooding simulates the oil displacement after hydraulic fracturing and the latter simulates the effect after CO<sub>2</sub> fracturing. Furthermore, an NMR test is conducted in each stage of the experiments to quantify the recovery degree of crude oil in different pore throats for a low-permeability reservoir. The results show that the water flooding recovery of the two cores is 31.26&#37; and 30.14&#37;, respectively. The CO<sub>2</sub> huff and puff enhanced oil recovery and subsequent water flooding are 11.48&#37; and 5.19&#37;, respectively. Compared with conventional hydraulic fracturing, CO<sub>2</sub> fracturing enhanced oil recovery by 15.55&#37;. Furthermore, the crude oil recovery degree of water flooding in macro-pores is 75.71&#37;, while that in mesopores is 62.17&#37; and that in small pores is 7.49&#37;. After CO<sub>2</sub> huff and puff, the recovery degrees in macropores, mesopores, and small pores are 99.10&#37;, 67.51&#37;, and 23.21&#37;, respectively. It demonstrates CO<sub>2</sub> can penetrate deeply into the tiny pores that cannot be affected by water flooding for oil displacement, effectively increasing the swept volume and improving the oil recovery. Research shows the advantages of CO<sub>2</sub> fracturing and may help to enlarge its application in low-permeability reservoirs.
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