We present a multi-level discrete fracture model (MLDFM) to guarantee a robust and efficient solution for naturally fractured reservoir simulation. In MLDFM, we apply a triple continuum model using structured grid for forward simulation where large-scale fractures are represented with numerical embedded discrete fracture model (EDFM) and the secondary fractures are upscaled as third continuum. What makes the triple continuum model different from the previous work is that both the numerical EDFM and the third continuum are treated in a dynamic approach by considering the effect of flow direction on the complex local-scale flow response. For that purpose, we construct a finer unstructured discrete fracture matrix (DFM) grid which represents all fractures explicitly and is conformal to the boundary of coarse structured grid. During a simulation run, we apply a basis function to generate the local boundary conditions at fine scale using the global solution. Benefit from that, we can use a more accurate flow-based approach in the extended local upscaling to re-compute the transmissibility in triple continuum model. Moreover, we apply a local-global upscaling formalism to guarantee dynamically updated local boundary conditions for upscaling. Besides, we present several cases using synthetic and realistic fractured networks to demonstrate the performance of MLDFM. The results prove that the proposed MLDFM approach more accurately captures the flow in complex fractured systems than EDFM solutions by comparing against fine-scale DFM. At the same time, MLDFM is more computationally efficient in comparison with fine scale DFM.

The potential of well test technology is discussed in this paper to estimate the miscible condition and displacement fronts position during CO₂ flooding. To interpret the multiphase well test curve of CO₂ flooding process, an accurate compositional numerical model is developed in this study. The model includes fully EoS-based compositional nonlinear formulation, unstructured gridding and multi-segmented well. A systematic well test analysis of CO₂ flooding at different regimes, including immiscible, multi-contact miscible and first-contact miscible gas injection, was performed for hydrocarbon systems with different number of components. Based on the interpretation root cause analysis, proposed in this work, the specific characteristics of the well test curve of CO₂ flooding have been identified and described. These characteristics provide the guidance for the distinction among the different regime of CO₂ displacement. It was demonstrated that the most important characteristics stay invariant from the number of components involved into numerical study. Finally, a tangent line method has been proposed to detect the key point on the pressure derivative curve corresponding to a CO₂ front. This method allows to predict the displacement front position for problems of practical interest.