Abstract
Aiming at the problem of the unclear microscopic interaction mechanisms between carbon dioxide (CO2) and reservoir rocks and fluids during CO2 huff-n-puff in ultra-heavy oil reservoirs. In this study, scanning electron microscope (SEM) and visual displacement experiment were used to understand the mechanisms among them. Then, the interaction experiment and numerical simulation between CO2 and heavy oil were carried out to verify the feasibility of CO2 huff-n-puff to improve the recovery of ultra-heavy oil reservoirs. The results showed that: a) Within a certain range of pressure, the corrosion ability of carbonated water formed by CO2 and formation water gradually increased as pressure increased. When the pressure increased from 6 MPa to 12 MPa, the average corrosion rate increased from 0.36% to 0.49%. b) CO2 can effectively improve the pore structure and permeability of rocks. The injection of CO2 caused hydration expansion, surface particle peeled off, deepening of depression, the appearance of corrosion pits and cracks, and cracks extending as pressure increased. Under the pressure of 6 MPa and 12 MPa, the permeability increased by 14.67% and 41.86%, respectively. c) CO2 injection made the heavy oil form amount of dispersed secondary foam oil, which greatly reduced the viscosity of crude oil and increased the expansion ability of heavy oil. When the pressure increased to 12 MPa, the volume coefficient and viscosity reduction rate reached 1.18 and 99.62%. d) The displacement experiment demonstrated that the displacement efficiency of CO2 was 23.09% higher than water flooding. e) According to the results of numerical simulation, during five cycles of CO2 huff-n-puff, the reservoir pressure rose from 3481 KPa at the end of depletion production to 6187 KPa, which showed that pressure was effectively maintained; The viscosity of the heavy oil decreased from 12825cp to 3280.49cp, and the 10-year recovery factor was 13.08%. The understanding of the micro-action mechanism could have certain guiding significance for improving the development effect of CO2 huff-n-puff in the ultra-heavy oil reservoirs.
NSTL
Elsevier