The resolution of commonly used oil-based resistivity imaging devices is often insufficient in resolving complex structures, especially when the resistivity of the formation goes below several ohms (e.g., in the Gulf of Mexico and offshore Borneo Malaysia). Efforts were focused on development of an improved imager providing high-resolution resistivity images in vertical and azimuthal directions for these environments with a target operating resistivity range above 1 ohmm.
The instrument employs multi-frequency impedance measurements to overcome dielectric effects due to the displacement current excited by the transmitter in the formation. Such an effect, if not addressed, can confuse image interpretation because a single real-part apparent resistivity can refer to more than one true formation resistivity value (i.e., non-unique results). Multi-frequencies are recorded while logging and a processing algorithm selects the optimum acquisition frequency to merge and display the best-quality image log. Each pad simultaneously measures all individual button currents at multiple frequencies. The data are then converted from respective voltages into amplitude and phase values by comparison to a transmitter signal.
Proof of the concept is demonstrated by presenting field examples from typical low-resistivity, low-contrast environments found in the Gulf of Mexico and Southeast Asia. Case studies are presented covering a wide range of geological features typical in deepwater deposits including thin-beds, slumping, faulting and fracturing. Detailed geological features are clearly resolved in high-definition image logs, despite being acquired in nonconductive borehole fluid. We show that the images provide the necessary information for interpreting structural and sedimentary processes. Furthermore, the high-definition images are used to optimize zones selected for pressure testing, and reduce uncertainty when calculating hydrocarbon saturations in low-resistivity, low-contrast thin-bed successions by providing more precise and accurate net-to-gross calculations.