High-resolution borehole resistivity images are widely used in carbonate reservoirs to obtain structural information, including fractures and faults, to compare with low-resolution seismic images. Because the resolution differences between these two technologies is too large. It is crucial to develop a new technique to fill the gap. Sonic imaging techniques using both monopole and dipole sources could extend depth of the investigation from the wellbore to determine the dip and azimuth of the geological features, which may correlate with dips from the borehole image.
The objective of this work is to identify and correlate fractures from borehole images and azimuthal dipole sonic images in a fault-dominated carbonate reservoir.
Horizontal drilling into the fault system is an important way to reveal more fractures in the carbonate reservoir of the Tarim Basin of China. Both the azimuthal dipole sonic imaging data and borehole resistivity image were acquired in a horizontal wellbore. An automated ray tracing and 3D slowness-time coherence (STC) procedure were applied to determine the dips and azimuth of the reflection. The results from the sonic imaging were compared with the dip from the borehole resistivity image to determine the geological meaning of the reflections. Finally, the fracture results and their response on the Stoneley waveforms were used to find the permeable zone in the horizontal well.
This new method of using a sonic tool has detected reflections up to 15 m away, in this case. The reflections on the sonic image show very good correlation with dips from the borehole image. The relative medium-angle reflection could be detected further than high-angle features that cut the wellbore. Fracture-related dissolution, which is typically seen on the borehole image, can also be detected on the sonic imaging result. Because of the strong heterogeneity in the fractured carbonated reservoir, some reflections may have no clear response on the borehole image.
The proposed integration method based on the dipole sonic imaging and borehole resistivity image reveals more detailed features in the fault-dominated carbonate reservoir and helps the operator to choose the proper well completion method to get better production performance. The results from this case study could be used in geological modeling and as a reference for similar field development.