Definition of FractureS
Fracture porosity is usually very small. Values between 0.0001 and 0.001 of rock volume are typical (0.01% to 0.1%). Fracture-related porosity, such as solution porosity in granite or carbonate reservoirs, may attain much larger values, but the porosity in the actual fracture is still very small.
Fracture porosity is found accurately only by processing the formation micro-scanner curves for fracture aperture and fracture frequency (fracture intensity). Reservoir matrix porosity and permeability, including that attributed to fracture related (solution) porosity, can be found by normal porosity calculation methods.
Reservoir simulation software that accounts for the fracture system is often termed a “dual porosity” model. While this is strictly true, it would be better to think of them as “dual permeability” models, since the fracture permeability fed by the matrix or reservoir permeability is far more important than the relative storage capacity of the fractures and matrix porosity. A reservoir with only fracture porosity is quickly depleted; a decent reservoir in the matrix rock feeding into fractures will last much longer.
Fractures are caused by stress in the formation, which in turn usually derives from tectonic forces such as folds and faults. These are termed natural fractures, as opposed to induced fractures. Induced fractures are created by drilling stress or by purposely fracturing a reservoir by hydraulic pressure from surface equipment. Both kinds of fractures are economically important. Induced fractures may connect the wellbore to natural fractures that would otherwise not contribute to flow capacity.
Note: Equations 2, 3, and 4 give identical results.
Df = 10 fractures per meter
examples represent well fractured reservoirs. You can see that
the volume of hydrocarbon is very small but the permeability is
Calculating Fracture APERTURE FROM IMAGE LOG DATA
A more accurate approach is based on finite element analysis of the resistivity image data. as described in "Fracture Apertures from Electrical Borehole Scans", S. M. Luthi and P. Souhaite, Geophysics, Vo1.55, No.7, July, 1990, pp.821-833. The math is beyond me and beyond the scope of this article. The method is available from some service companies. An example of a fracture aperture log from a program called Frac-View is shown below.
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