Volume Expansion Behavior of CO2 in Various Types of Hydrocarbon Systems Under Reservoir Conditions

As a process fluid in oil reservoirs, carbon dioxide plays a dominant role in enhanced oil recovery by increasing volume and decreasing interfacial tension. To study the volume expansion behavior of a CO2–oil system under reservoir conditions, ten hydrocarbon components with carbon numbers ranging from 8 to 26 were selected to represent crude oil. Systems of CO2 with normal alkanes, cycloalkanes, and aromatic hydrocarbons were measured using a self-assembled high-pressure visible cell, with a temperature range of 313.15 K to 353.15 K and a pressure up to 25 MPa. Experimental results demonstrate that pressure and temperature significantly influence the relative volumetric expansion behavior. The expansion rate exhibits a positive correlation with pressure, whereas it shows a negative correlation with temperature. Among different molecular configurations, normal alkanes exhibit the most pronounced swelling effect. This study establishes that the volumetric expansion behavior of crude oil under CO2 exposure is predominantly governed by n-alkane components with carbon numbers less than 16. In the heavy hydrocarbon (carbon number > 16) and CO2 system, the influence of hydrocarbon structure and carbon number on the expansion extent is considerably reduced. This paper delivers critical theoretical foundations for elucidating the microscopic interaction mechanisms in CO2 enhanced oil recovery and optimizing injection parameter strategies.