Characterizing the Response of a New LWD Micro-Electrical Imager for Oil Based Mud and Large Boreholes in Terms of Formation Wave Impedance and Mud Properties

A new LWD micro-electrical imager for OBM and large boreholes measures the radar signal reflected off the borehole wall. The new sensor operates at microwave frequencies (3.8GHz). It is important to understand what parameters govern the feasibility of the measurement and how these parameters affect the quality of the signal. Naturally, the characteristics of the borehole fluid, the borehole size, and the electrical properties of the formation each play an important role. In particular, the electromagnetic impedance of the formation and the electrical properties of Oil Based Mud (OBM) emerge as new key parameters. The object of this study is to develop a coherent predictive model to characterize the response of the new electric imaging sensor. The study integrates theoretical methods, recorded logs, and laboratory measurements and verifies them against each other when possible. Because the reflection of the miniature radar echo is directly related to the impedance contrast between the OBM and the formation, it is natural to compute the wave impedance of the formation in terms of matrix permittivity, water-filled porosity, and water salinity. The traditional Archie’s equation and its variations are known to fail in the microwave range because of the phenomenon of resistivity dispersion, and because of the impact of the dielectric effect. A new formulation is proposed for the formation properties in the microwave range, based on the electromagnetic wave impedance and the Complex Refractive Index Method. The properties of OBM must also be modeled. They play an important role in the attenuation of the wave between the sensor and the borehole wall, and in the reflection coefficient at the borehole wall. Modeling the OBM is accomplished using the Hanai-Bruggeman equation. OBM emulsions lend themselves almost ideally to the Hanai- Bruggeman model, with oil being the continuous phase and the brine the discontinuous phase. The Wave Impedance of the OBM is derived in terms of the Oil/Water ratio for a range of brine salinities. The results of the modeling of OBM agree with experimental laboratory data. OBM is in fact conductive at microwave frequencies. Furthermore, the amplitude from a recent image log exhibits a strong correlation with the wireline resistivity logs. The characterization of the radar sensor indicates that the range of operation of the miniature radar-based imager is quite large. A weaker signal is observed in tight intervals and in hydrocarbon bearing intervals. A stronger signal is present in high water-filled porosity intervals and in shales. Modelling and field test results show that the new LWD radar-based micro-electrical tool provides accurate and insightful images in OBM in a wide range of hole sizes, including large surface holes.