A Method to Identify Vertical Reservoir Pressure Communication by Combining Borehole Sonic and High-Frequency Electrical Imaging Data
Authors:   Aker BP, Birkedal Knut Arne , Datir Harish ,   SLB, Zhang Tianhua ,   SLB
Published: September 2024

Natural fractures and faults are the imprint of tectonic and structural history of the rock section. A well-extended fracture network handles fluid channeling and unexpected pressure communication. At the wellbore, electrical images are often used to identify the existence of fracture/fault. In oil-based mud (OBM), interpreting open fracture/fault is difficult, as OBM-filled open features can appear resistive. When the borehole surface is full of spirals and drill marks, or the mud is not homogeneous, ultrasonic images quickly lose sensitivity to the fracture/fault. Further inside the formation, the 3D far-field sonic (3DFFS) analysis can provide a good map of reflectors representing the fractures and faults zone. But it cannot directly confirm if the fracture or faults are open or closed. In this paper, we present a new method for analyzing open fractures/faults using electrical dielectric permittivity derived Hayman image. Fracture instability is further analyzed based on eccentering-corrected multi-radius image and borehole shape. Then, overlapping the wellbore fracture/fault planes with 3DFFS processed reflectors, we can characterize the extension of fracture/fault from the wellbore into the formation and identify potential pressure communication channels. In OBM, the MHz frequency imager data, through advanced inversion processing, can generate resistivity, dielectric permittivity, and standoff images. Through post-processing, a new Hayman image can be generated by combining dielectric and resistivity information. Joint analyzing resistivity and Hayman image can help resolve open fracture/faults when borehole rugosity is very high when ultrasonic and standoff images lose sensitivity to fracture/faults. The OBM imager can also provide multi-radius images and be transformed into a borehole shape. Eccentering-corrected borehole cross section can indicate drilling and stress caused borehole shape deviation from circular. Combining the above two analyzes, we can not only identify open/closed fractures/faults but also get an indication of fracture stability based on the borehole shape deformation. Once identified, the open fractures/faults and the unstable closed fractures can be mapped away from the wellbore wall via the 3DFFS data. This allows us to identify the orientations and alignments of the wellbore and far-field events. In the case study pilot well, joint interpretation of the resistivity and Hayman image identify two more open fractures in the upper-middle section of the well. Many closed fractures and two closed faults are identified. The eccentering-corrected borehole radius shows a maximum borehole diameter direction rotation at the very bottom of the well. Comparing the maximum borehole diameter direction and the closed fracture/fault azimuth direction, most of the closed fractures and the top fault can be subject to drilling, resulting in borehole stress and becoming unstable. Overlaying those fractures/faults with 3DFFS reflectors, we can see the borehole image unstable interval has the highest density of sonic reflectors. There is azimuth orientation between the wellbore image fractures and sonic reflectors in the top part of the well interval. The azimuth alignment increases with increased TVD. The new method of Hayman-aided open fracture detection, fracture instability using multi-radius borehole shape image, and fracture extension combining sonic far-field processing methods are applied on a pilot well. The fracture network is mapped. Through the fracture network analysis, we identified open fractures and unstable closed fractures, which extend into the formation. The open fractures and unstable fault opened during the water injector can form a pressure communication channel vertically along the well. The observations agree with the field pressure communication observed while the well is put into injection. These spatially resolved fractures, faults, and fracture network are essential for subsurface understanding and future well placement in this field. It is a critical input to the dynamic reservoir model.