(743c) Experimental and Simulation Study of CO2 EOR and Associated Storage in a Naturally Fractured Reservoir

Injecting carbon dioxide (CO₂) within deep geologic formations offers a potential means to reduce greenhouse gas emissions. Currently, dedicated CO₂ storage in saline aquifers and associated CO₂ storage in oil reservoirs with enhanced oil recovery (EOR) are the two most widely reported methods for long-term CO₂ storage. The target formations are usually characterized by good porosity and permeability to ensure sufficient injectivity and storage capacity. However, recent unconventional reservoir development gives an indication that tight formations could also be possible opportunities for CO₂ EOR and storage despite relatively low matrix porosity and permeability. Abundant fractures in the reservoir provide flow paths for CO₂ migration and greater surface area for penetration into the tight rock.

A series of experimental and simulation activities were conducted in this study to investigate CO₂ EOR and the associated storage potential of the Dickinson Lodgepole Mounds (DLM) located near Dickinson, North Dakota. These mound structures have abundant natural fractures but relatively low matrix porosity (2%–6%) and permeability (0.1–10 md). Ten individual DLM fields have been discovered and developed with prolific oil production since 1993. The fields contain some of the best oil-producing wells in North Dakota history. Based on the analysis of the production and injection history in the DLM, Stadium Field was selected for an in-depth study. The field is relatively small, with five active wells.

Core samples from the field were used in laboratory investigations. Contact angle measurements showed the reservoir is strongly oil wet. Core samples were saturated with oil and then flooded with water to mimic the production mechanisms in the reservoir. Both immiscible and miscible CO₂ flooding tests were then conducted to observe the EOR response under reservoir conditions. Experimental results showed that water could only displace minor amounts of oil from the rock while CO₂ could recover as much as 44% of the remaining oil. Also, miscible CO₂ flooding was more effective than immiscible flooding for EOR.

A compositional reservoir simulation model integrating a ten-component equation-of-state was developed to predict the CO₂ EOR and associated storage performance in the field. An innovative fracture simulation method, embedded discrete fracture modeling (EDFM), was adopted to simulate natural fractures efficiently using SimTech’s EDGS Suite. EDFM enables modeling of complex fracture geometries using structured grids. History-matching results showed the model could reproduce historical data satisfactorily.

Case studies were conducted to assess the CO₂ EOR and associated storage potential in both immiscible and miscible flooding modes. Simulation results indicated injectivity was sufficient because of the relatively high degree of fracturing in the reservoir. The results also suggested CO₂ flooding in Stadium Field could recover an additional 11%–14% of oil from the reservoir, with associated CO₂ storage potential of up to 2.05 million tonnes within 10 years.

Both experimental and simulation results showed that a fractured reservoir with relatively low matrix porosity and permeability could host an effective EOR operation and have the potential to store a significant quantity of CO₂. The findings from this study may have implications for CO₂ EOR and associated storage assessments in large-scale unconventional reservoirs.