Fracture And Permeability Evaluation In A Fault Zone From Sonic Waveform Data

Identification and evaluation of fracture systems are important in oil and gas exploration in hard-rock areas and for scientific drilling. In fracture systems, faults are major events that impact not only the fracture distribution but also the rock structure and stresses. Near faults are significant rock deformation, fracturing and variations of the stress field. Faults have, therefore, large effects on the producibility and stability of a reservoir and must be accounted for when completing the well. We integrated various sonic interpretation techniques to evaluate these effects. Stoneley wave reflections and attenuation analysis are known techniques for fracture evaluation. They have the advantage of showing open permeable fractures but also are sensitive to borehole irregularities. Stoneley modeling, when added to these techniques, estimates the effect of the borehole and improves the reliability of Stoneley fracture evaluation. Stoneley permeability analysis evaluates the slowdown of the Stoneley wave to indicate fluid mobility in the pore space, both from fractures and porosity. The recently introduced dipole shear anisotropy evaluation provides information on aligned cracks and stress directions. The technique evaluates shear wave splitting resulting from acoustic anisotropy. These techniques react in different ways to the presence of fractures in the formation. Combining these indications provides additional information on the reservoir characteristics, especially the location of fault zones. We applied these techniques in two wells. The first well was drilled for scientific purposes to intercept a known large active fault in a granodiorite formation. In this data set, near the fault the Stoneley wave shows evidence of strong fracturing and deformed zones. Acoustic anisotropy indicates significant perturbations of the stress field. Stoneley permeability analysis detects high fluid mobility. The second well was drilled in a granite formation in an oil field. We observed similar signatures in the Stoneley and anisotropy results that strongly suggest the presence of a fault intercepting the wellbore. In this data set, systematic variations of the fracture?s dip and strike, consistent with sonic evaluation results, are also observed on the microresistivity images. High mobility indications from Stoneley analysis are confined by the production data. Integrating anisotropy information with conventional fracture evaluation techniques uncovers new possibilities for reservoir evaluation. Fractures can be identified and better understood with the resulting fracturing and rock alteration to give new insights on reservoir properties and producibility.