Phase Behavior and Reversibility Mechanisms of Asphaltene Precipitation for High-Pressure High-Temperature CO2-Oil Systems

Net-zero carbon emission pledges by 2050 are prompting researchers to investigate alternatives to store CO2 safely. Subsurface CO2 storage faces many challenges, especially when the geological space is a depleted oil reservoir. A crucial problem is the newly formed asphaltenes during CO2 injection that could limit or even block the rock-effective zone for gas storage. In this work, a novel pressure-volume-temperature cell and a solid detection system are used to measure the phase behavior, asphaltene onset pressure, and asphaltene precipitation. Results show four different equilibrium phases during the isothermal depressurization processes. Likewise, asphaltene precipitation increases with CO2 fractions and decreases with temperature, reaching its maximum at the bubble point. More importantly, the redissolution of asphaltene particles back into the solution is observed, revealing the formation process reversibility at certain conditions (25 mol % CO2 at 60, 90, and 120 degrees C; 35 mol % CO2 at 90 and 120 degrees C). The Peng-Robinson equation of state model is employed to model the fluid phase, while the solid phase equilibrium is modeled with a solid model. A new correlation for the asphaltene molar volume is proposed, which predicts the experimental data with an AARD of 0.7%.

Automated summary

Net-zero carbon emission pledges by 2050 are driving research on safe CO2 storage alternatives. Subsurface CO2 storage in depleted oil reservoirs faces challenges due to the formation of asphaltenes during CO2 injection. This study investigates the phase behavior and precipitation of asphaltenes using a novel pressure-volume-temperature cell and solid detection system. The results demonstrate multiple equilibrium phases, increasing asphaltene precipitation with CO2 fractions and decreasing precipitation with temperature, along with the reversibility of asphaltene formation under certain conditions.