Physicochemical Characterization of Produced Oils and Gases in Immiscible and Miscible CO2 Flooding Processes

Abstract

In this paper, the oil and gas samples produced from tight sandstone reservoir core plugs in immiscible, near-miscible, and miscible CO2 flooding processes at the reservoir temperature are experimentally characterized. First, the vanishing interfacial tension (VIT) technique is applied to determine the minimum miscibility pressure (MMP) of the light crude oil–CO2 system. Second, a total of five CO2-coreflood tests are conducted under immiscible, near-miscible, and miscible conditions to thoroughly study the characteristics of the produced oils and gases during each CO2 flooding process. In the immiscible case (Pinj < MMP), the produced oil becomes heavier and heavier during CO2 injection in terms of its density, viscosity, molecular weight, and hydrocarbons (HCs). The produced gas contains up to 84–96 mol % HCs (dominantly C4–7) extracted from the light crude oil by CO2 at the beginning and 85–100 mol % CO2 at the end of CO2 injection. In the near-miscible case (Pinj ≈ MMP), the heaviest oil is produced at 0.4 pore volume (PV) of the injected CO2, which is followed by a relatively lighter produced oil from CO2 breakthrough (BT) at 0.6 PV to the end of CO2 injection. The produced gas has 40–60 mol % HCs (dominantly C5–8) at the beginning and then consists of almost 100% CO2 after CO2 BT. In the miscible case (Pinj > MMP), the produced oil becomes much lighter and lighter with the injected PV. The produced gas contains 20–75 mol % HCs (mainly C5–8) at the very beginning and quickly becomes almost 100% CO2 after CO2 BT. In summary, this laboratory study provides the comprehensive characterization of the produced fluids in three distinct CO2 flooding processes. These new experimental data will help to better evaluate the field-scale CO2-enhanced oil recovery (EOR) processes and design the future CO2-EOR projects as well.