ABSTRACT

In the recent past, there has been a remarkable increase in the prospecting of basement reservoirs resulting in several new oil & gas discoveries. Although these crystalline formations have a minimal matrix porosity, billions of cracks have been created as these hard and brittle rocks are exposed to tectonic action. This further resulted in seismic scale faults or highly connected fracture networks, thus developing a storage system. These fractured zones are the potential targets for production because of their enhanced mobility. However, identifying them through conventional logging methods are extremely tedious. Being dominantly rich in magnetic minerals, NMR (Nuclear Magnetic Resonance) data may not provide the desired SNR (Signal-to-Noise ratio) in the basement which otherwise are quite reliable (can remove this one, highlighted in green) in conventional reservoirs. Although resistivity or acoustic images have been helpful in providing statistics and classification of the fractures and microcracks, the events on these images are mostly affected by strong imprints of the drilling process. Often time-consuming pressure tests result in failures, as there is significant uncertainty about the permeable intervals where the probes are to be set.

Mobility inversion from borehole mode Stoneley waves are not straight forward and are dependent on several unknown parameters inducing significant uncertainty. Also, the fracture indicators from the Stoneley are typically masked by rugose borehole conditions and might often contaminate the analysis due to the borehole irregularities. Even in good borehole conditions, the methodology may suffer from the insensitivity of the Stoneley reflectivity to the relatively high angle fractures.

However deep penetrating dipole flexural waves, oriented in orthogonal directions, carries a strong signal of the high dipping fractures with respect to the borehole axis. Further utilizing a Stoneley inverted shear in conjunction with the orthogonal dipole shear enables characterization of the shear moduli in a 3D-space and allows identification of zones with horizontal mobility. We introduce here a methodology combining the above two phenomena for qualitative fracture characterization and mobility evaluation which takes advantage of the larger depth of investigation of these acoustic waves. This can be used in complement with the traditional Stoneley based fracture or permeability identification methods as well as with the interpretation of electrical images for a more robust answer product.

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