Scale Dependence of Diffusion in Naturally Fractured Reservoirs for CO2 Injection

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Abstract Based on detailed compositional simulations of matrix/fracture systems, we present results of the impact of diffusion on oil recovery by CO2 injection for small-scale laboratory experiments and field-scale systems. We consider reservoir conditions that lead to a range of recovery mechanisms from clearly immiscible to near-miscible. Laboratory experimentation is a desirable approach to understanding the recovery mechanism of CO2 injection in fractured oil reservoirs because an unusual combination of complex physical phenomena exists. A fundamental problem with this approach is scaling the results to field conditions, and particularly matrix block size which is often many (5–100) times larger than laboratory cores. Molecular diffusion is clearly affected by matrix block dimensions, and any affect of diffusion on the recovery mechanism would also be scale dependent. In this paper we provide guidelines and computational examples of laboratory tests to study CO2 recovery processes, including diffusion. We also study the change in recovery processes as matrix/fracture dimensions approach field scale. We recommend using reservoir core or outcrop samples similar to the actual reservoir rock, with dimensions (5–25 in2 horizontal area and 10–30 in high). A synthetic model oil can be constructed to mimic PVT properties of the actual reservoir oil, and in particular the minimum miscibility pressure (MMP) as defined by a traditional 1D multi-contact displacement process – MMP1D, reduced gas-oil interfacial tensions (IFT) and oil-gas density differences. The core should be placed in a container with annular space representing the fracture. CO2 is injected at various rates into the annular space, at relevant reservoir pressures both above and below the MMP1D. In this work, the effect of several key parameters are studied – matrix permeability, matrix block size, CO2 injection rate and reservoir pressure. One of the key results is the effect of diffusion on oil recovery, and how it varies with matrix block size and CO2 injection rate. We show that diffusion has a dominant effect on recovery mechanism in experimental tests, except at very low CO2 injection rates. For field-scale matrix/fracture systems, diffusion has a significant effect on the rate of recovery, the effect becoming noticeable for low reservoir pressures and/or high fracture densities. Compared with earlier studies on the Iranian Haft-Kel reservoir considered in this paper, our current work shows that the rate of high recoveries by CO2 injection reaches more than 90% 2–10 years with diffusion, versus >100 years in the absence of diffusion, for an 8x8-foot matrix grid.