A Paleocene dolomite in northeastern Libya was modeled using three porosity types: matrix porosity (intercrystalline plus the separate vugs), horizontal-to-subhorizontal solution-enlarged vuggy porosity generated by dissolution and named "touching vugs," and fracture porosity.
The touching vugs, which acted like fractures in their dynamic behavior, are modeled as a very permeable discrete fracture network (DFN) in PETREL* seismic-to-simulation software. Their presence is known primarily due to pressure buildup analysis, and it is expected that they would be interconnected over hundreds of meters.
The primary challenge in dual-media modeling is being able to characterize and predict the dynamic behavior of the complex matrix + fracture + touching vug system. There are three wells in this area of northeastern Libya with pressure buildups that have an unusually distinct and pronounced dual-porosity signature. Each well has been matched by adjusting the five primary parameters: matrix and fracture porosity and permeability, and the sigma shape factor. Aquifer strength also had to be adjusted.
There are some aspects of the dynamic behavior of this reservoir that are of particular interest:
The highly fractured low-porosity parts of the reservoir are the least productive, and the least fractured high-porosity parts with touching vugs are the most productive.
The well test shows pronounced dual-porosity behavior and indicates a high level of contrast between the matrix and secondary porosity systems, both in terms of permeability and porosity.
There is a strong aquifer and in reservoirs with moderate fracture permeability, we would expect to see the fractures provide a pathway for rapid water breakthrough. Owing to the very high permeability associated with the fracture system, this does not happen as expected; i.e., the reservoir acts to some extent like a tank with a level oil/water contact (OWC). There is not much coning or advancing of water in the secondary porosity system.
This paper presents the dynamic features of the reservoir and demonstrates how the simulation model was calibrated using all available information, in particular, the pressure buildups.