Chapter 8 - Naturally Fractured Reservoirs

Abstract

The fractures represent mechanical failures of the rock strength to natural geological stresses such as tectonic movement, lithostatic pressure changes, thermal stresses, high fluid pressure, drilling activity, and even fluid withdrawal, since fluid also partially supports the weight of the overburden rock. The natural fractures are classified on the basis of stress/strain conditions in laboratory samples and fractures observed in outcrops and subsurface settings. They are classified as shear fractures, which exhibit a sense of displacement parallel to the fracture plane; extension fractures, which exhibit a sense of displacement perpendicular to and away from the fracture plane; and tension fractures that also exhibit a sense of displacement perpendicular to and away from the fracture plane. Fractures may have either a positive or a negative impact on fluid flow, depending on whether they are open or sealed as a result of mineralization. The fractures play multiple roles in exploration and exploitation of naturally fractured reservoirs. Fractures could alter the matrix porosity or the permeability, or both and if the fractures or connected vugs are filled with secondary minerals, they may restrict the flow. Even in rocks of low matrix porosity, fractures, and solution channels increase the pore volume by both increasing porosity and connecting isolated matrix porosity and, therefore, help the recovery of petroleum fluids economically.