ADVANCEMENTS IN ACOUSTIC TECHNIQUES FOR EVALUATING OPEN NATURAL FRACTURES

摘要

Open natural fractures, when present, can provide the conduit for the majority of natural gas in low porosity, hard-rock environments. Therefore, a complete understanding of fracture attributes, including aperture and permeability, is required for optimal production as well as cementing and completion design, well placement and reservoir modeling. Sonic Scanner*, the latest of a new generation of waveform sonic acquisition tools with its 13 axial receivers, can provide a Stoneley measurement with improved broadband frequency response, higher waveform fidelity and increased energy. Stoneley reflectivity is minimal for high angle fractures so the energy loss or attenuation of the waveforms is required to evaluate their permeability. The lower end of the frequency response reaches 300 Hz, which significantly improves the detection of highly dipping fractures using existing techniques such as normalized differential energies (NDE). The improved Stoneley measurement is critical when trying to distinguish between drilling induced fractures and open natural fractures. For cases where both sets of fractures are striking in-line with the present day stress field and dipping at high angles, Stoneley reflections may be very small or absent. An attenuation technique such as NDE proves to be the optimal way to evaluate fracture aperture and permeability. Slowness frequency analysis (SFA) of the rotated dipole flexural dispersion curves can also be used to identify open natural fractures. SFA is a projection of the flexural wave frequency and amplitude profile on the slowness plane at each depth. The traditional STC (Slowness Time Coherence) or semblance processed shear slownesses are compared with the SFA projection to confirm that the waveforms processing is consistent with the frequency content. The amplitude and frequency information provides an image of the shear characteristics of the formation from the face of the well bore extending several feet into the formation. Variations in the signal strength and frequency content are evident when examining naturally fractured zones. Comparisons of the SFA projection with the Stoneley NDE provide a 3-dimensional acoustic characterization of the formation identifying fractures and borehole stress regimes. A case study is presented using the latest generation Stoneley measurements, SFA projection images and micro-resistivity images. The additional acoustic information compared with the resistivity image data provides a much clearer and powerful characterization of the formation for identifying open natural fractures.