Fracture Imaging and Response Characterization of the High-Definition Oil-Based Mud Borehole Imagers Through Modeling and Inversion

Abstract

The new-generation, high-definition oil-based mud borehole imagers measure button impedances, which are often inverted to produce images of the formation resistivity, formation permittivity, and sensor standoff. These images, each reflecting a unique aspect of the downhole media, can provide a comprehensive understanding of the reservoir’s secondary porosity, i.e., fractures and vugs. To understand and validate the inversion behavior on fractures and vugs, synthetic logs of axisymmetric 2D fractures and vugs are generated and inverted. While the inverted medium properties follow the variation of the fracture-filling materials, the inverted standoff is shown to be a reliable indication of the fracture open/closed conditions. Specifically, an increased inverted standoff would always appear for mud-filled open fractures, which is further validated by laboratory measurements on artificial fractures and vugs. Numerical tests indicate that mineral-filled fractures may also lead to some variations on the inverted standoff when the resistivity contrast between the mineral-filling material and the host formation is high. Therefore, an elevated standoff may also be associated with a mineral-filled fracture. The modeling and inversion help reveal the connection between the response of the inverted parameters and the actual fracture characteristics. When the fractures are open and filled with mud, the inversion obtains an equivalent standoff at the fractures. This equivalent standoff increases not only with fracture width but also with the resistivity of the host formation. As a result, a fracture may have varying amplitudes in different formation layers. The fracture modeling and inversion also allow us to understand the prevalent existence of conductive fractures observed in the field. It is commonly thought that the resistivity of oil-based mud is always higher than that of the formation. However, this is only true at traditional operating frequencies. Because of mud dispersion, the mud resistivity is different at the two operating frequencies. For an open, mud-filled, resistive fracture to turn conductive, the formation resistivity only needs to exceed the mud resistivity at the higher operating frequency, which is typically on the order of a few 100 Ω·m instead of many 1,000 Ω·m at the lower operating frequency. As a result, an open fracture could be easily conductive in one layer but resistive in another formation layer.