Visualization of CO2 EOR by Diffusion in Fractured Chalk
- Oyvind Eide (University of Bergen) | Martin Anders Ferno (University of Bergen) | Arne Graue (University of Bergen)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 27-29 October, Amsterdam, The Netherlands
- Publication Date
- Document Type
- Conference Paper
- 2014. Society of Petroleum Engineers
- Upscaling, Diffusion, Fractures, CO2 injection, EOR
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- 471 since 2007
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This work demonstrates that diffusion may be a viable oil recovery mechanism in fractured reservoirs during injection of CO2 for EOR. The oil production rate from diffusion alone, however, depends heavily on the distribution of CO2 within the fracture network and fracture spacing. High oil recovery was observed during miscible, supercritical CO2 injection (RF=95% OOIP after 5 days) in the laboratory using an artificially fractured chalk core plug. The development in 3D fluid saturations from CT-imaging made it possible to study the oil displacement in the vicinity of the fracture, and to calculate an effective diffusion coefficient using analytical methods. A numerical sensitivity analysis, using a validated numerical model that reproduced the experiments, showed that the rate of oil production during CO2 injection declined exponentially with increasing diffusion lengths from the CO2-filled fracture and oil-filled matrix.
Naturally fractured carbonate reservoirs are highly heterogeneous in terms of porosity and permeability (Chillenger, 1983, Fernø, 2012), and the conductive fracture network usually leads to rapidly declining production and low total recoveries. The high residual oil saturation after waterflooding makes carbonate reservoirs good candidates for CO2 enhanced oil recovery (EOR). During CO2 EOR, high mobility of CO2 compared to oil and water leads, in many cases, to poor volumetric sweep efficiency limited by gravity tonguing and/or viscous fingering (Hirasaki and Zhang, 2004, Lescure and Claridge, 1986). The injected CO2 typically has a lower minimum miscibility pressure (MMP) with oil compared to the frequently used gases including nitrogen (N2) and re-injected produced HC-gas, mostly methane (CH4), and is less prone to gravity segregation (Brock and Bryan, 1989, Bui, 2013). Still, the poor sweep efficiency during CO2 injections in heterogeneous reservoirs remains a severe problem, and typically 10-20% OOIP incremental recoveries are reported on the field-scale during miscible CO2 (Enick, 2012.
The injection of CO2 for enhanced oil recovery was first implemented as a viable EOR method in 1972, and has successfully been used for EOR for a number of years. Nevertheless, CO2 EOR projects have so far primarily been realized in the US (Koottungal, 2012, Moritis, 2004), largely as a result of a large supply of CO2 from natural sources and tax incentives for EOR-projects. Injection of CO2 in the North Sea has been performed since 1996 for storage purposes only (Eiken et al., 2011. Compared with other EOR methods based on gas injection, CO2 EOR has many beneficial properties:1) CO2 will lower the gas/oil interfacial tension, and often has a lower MMP than nitrogen (N2), hydrocarbon gas or air, 2) CO2 reduces oil viscosity and density, resulting in increased oil mobility and oil swelling (Ashcroft and Isa, 1997, Moortgat et al., 2011). The main drawback with injecting gas is the high mobility, which is especially true in fractured reservoirs. Although chalk fields on the NCS have been identified as candidates for CO2 EOR and storage (Jensen et al., 2000), it was concluded that CO2 was too expensive and not available in a large enough quantities. The current focus on carbon capture utilization and storage (CCUS) might, however, provide a less expensive source of CO2, and with the help of tax relieves, or other economic incentives, governments can create a demand for CO2, making it a commodity (Hustad and Austell, 2004).
The success of a potential CO2 EOR project increases by identifying the key driving forces for oil displacement during a CO2 injection. In a highly fractured reservoir, such as e.g. the Ekofisk field, diffusion could be an important driving mechanism (Hoteit and Firoozabadi, 2006), however, it would require high fracture density. Molecular diffusion is the mixing of fluids due to random motion of molecules and can be expressed by Fick’s second law:
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