All reservoirs contain a transition zone below the oil water contact (OWC) which varies in thickness depending on the reservoir rock properties. In some reservoirs that have undergone geological and hydrodynamic titling, there could be an additional, sometimes significant, residual oil zone (ROZ) below the transition zone. This zone sometimes referred to as the ‘paleo’ oil zone and may contain significant quantities of hydrocarbons. Traditionally, this has not received much attention as its oil is immobile and does not normally produce through primary and secondary recovery methods. However, paleo oil has the potential to be mobilized by CO2 injection.
The objective of this study is to show the ability of CO2 injection to mobilize paleo oil in the ROZ. Different laboratory experiments on actual reservoir sponge cores from wells intersecting the ROZ have been conducted at reservoir conditions. The study includes static CO2 saturation (PVT cell) and dynamic CO2 coreflooding experiments, and Nuclear Magnetic Resonance (NMR) analysis. The mobility of paleo oil by CO2 flooding was investigated using both static and dynamic CO2 injection experiments on sponge cores containing paleo oil. The static experiments showed, through images and videos, how the CO2 can mobilize the paleo oil in the cores. The coreflood experiments showed the potential to mobilize the original oil in cores from these two wells intersecting the ROZ. It was found that CO2 soaking is a critical factor to mobilize oil because it allowed more time for the CO2 to interact with the available components. Core samples were further analyzed microscopically using NMR analysis (before and after CO2) to complement coreflood interpretations. NMR showed the exact places in the core where the oil was mobilized by CO2 and the type of components extracted after the experiments (mostly intermediate).