FIXED-DEPTH OF INVESTIGATION LOG FOR MULTI-SPACING MULTI-FREQUENCY LWD RESISTIVITY TOOLS

摘要

The specification discloses a system and related method for obtaining electromagnetic resistivity logs of a formation having fixed depths of investigation that differ from the actual depths of investigation for the tool. The system and related method comprises calculating actual invaded zone and formation resistivities, and then using least-squares curve fitting techniques to find a function that models the relationship between the physical spacing of the transmitters and the depths of investigation. Using the function, it is possible to find a transmitter-to-receiver spacing that, though not actually used, would give the desired depth of investigation. This transmitter-to-receiver spacing is then used in combination with a second function, determined using least-squares curve fitting methods, which models the relationship between the transmitter-to-receiver physical spacing and the measured resistivity to obtain the resistivity reading at a fixed depth of investigation. This may be done for a set of desired depths of investigation, and may also be done for a set of elevations within the borehole, to create a fixed depth of investigation log that may be easily compared to other resistivity logs, such as those created by a wireline device.