WELL LOGGING ANALYSIS FOR ESTIMATING POROSITY IN FRACTURED BASEMENT RESERVOIRS

Low porosity fractured reservoirs have been successfully described, using a combination of high
resolution geometrical information from borehole images, together with deeper penetrating log evaluation
methods. Borehole images from acoustic or electrical scanning tools provide statistics of the fracture
distribution, first order estimates of fracture opening and porosity, and a basis for geological inferences.
Their drawback is that, in this environment, the events on the images bear a strong overprint of the drilling
process. Deeper penetrating, but lower resolution techniques, such as Stoneley wave reflectance and deep
resistivity log inversion are used to distinguish the deep and permeable fractures that may contribute to
flow. By making some assumptions about the nature of porosity in basement reservoirs, we develop a new
method to estimate the porosity and the fraction of this secondary porosity is developed due to fractures.
This method makes the use of the Kuster-Toksoz acoustic scattering model and requires low frequency
measurements of compressional and shear velocities.