Fracture geometry and basic characteristics (orientation, length, density, aperture, porosity) of fractures in a number of Middle East reservoirs (from Egypt to Pakistan), have been recently studied. These studies confirm that fracture orientation, distribution and characteristics important to reservoir behaviour do not vary randomly nor chaotically. Moreover, there are commonly lower densities of fractures in reservoirs than are seen in the same formations outcropping at the surface. The simple cubic models used in reservoir studies, are sometimes appropriate for reservoir studies but often more complex geometric models are needed.
The factors which determine the occurrence of natural, open, permeable fractures within Middle East reservoirs are nature and degree of folding and/or faulting, in-situ stresses, and changes in rock properties such as porosity, bedding, and lithology, especially shaliness. These geological factors can and are often already mapped in reservoir studies. By accurately defining the relationship of these various factors to the fracturing analyzed in a number wells within a field it becomes possible to extrapolate the fracture data through out a reservoir.
There is no routine direct evaluation of fracture porosity and permeability available at the present time. However, borehole imagery allows the fracture porosity and flow potential to be calculated by multiplying the fracture density and the aperture widths measured using a modelling package available on a computer workstation. The use of aperture and fracture density data to also calculate permeability (approximately the value of the fracture aperture cubed) means that fracture aperture and density are critical parameters in defining the reservoir potential of a fracture system intersected by a well bore. The observations concerning the variations of the major fracture characteristics especially aperture and density along with orientation are summarized.
The short-coming to fracture studies using either borehole imagery or oriented cores, however, is that the fractures detected may not be representative of the large-scale fracture network which will determine reservoir production. Thus, it is necessary that the geometrical characterization of any borehole data be related to geological studies (structure, stratigraphy, sedimentology, diagenesis, and geostatistics) of that reservoir.
The borehole-scale characterization and the geological modelling also need to be integrated with the dynamic testing and production data more than just compared as a type of quality control. Even when they are radically different, both sets of data may be correct. In fact, how the two types of data fit is a key to proper modelling of the reservoir.