To gain insight into the influence of geologic factors on the oil recovery from the Monterey Formation, a 3D slice model was constructed to simulate the Monterey Reservoir using a single-porosity reservoir simulator.
The complex geology of the Monterey Formation, which resulted from nonuniform mineralogic diagenesis, was first simplified by grouping the different lithologies into two categories: Fractured lithologies and unfractured lithologies. This simplification reduced the highly fractured and interbedded Monterey geometry into a thinly layered system with drastically different physical properties which can be easily handled with conventional single-porosity reservoir simulators.
In the modeling, emphasis was given to the understanding of the effects of this highly laminated reservoir geometry, represented by layering of the model, on the primary oil recovery due to solution gas drive, gravity drainage and partial pressure maintenance. The diffusion and imbibition forces were not considered here.
The sensitivity of oil recovery from the Monterey Formation to the ratio of fracture to gross pay and to the permeability contrast between the different lithologies were also investigated.
The sensitivity study indicated that the fracture content of the Monterey formation has a strong influence on the producing life of the fields. The shale matrix contribution to oil recovery remained significant at matrix permeabilities equal or greater than one microdarcy. The gravity drainage substantially increased the oil recovery from the fractured layers. Similarly, partial pressure maintenance by injection of the produced gas also increased the oil recovery from the high conductivity layers. However, this improvement in recovery from fractured layers was more than balanced by the reduction in oil migration from the matrix. Thus, gas reinjection did not improve the overall oil recovery. Actually, partial pressure maintenance reduced the ultimate oil recovery from the Monterey Formation.