Wave propagation resistivity is the primary resistivity measurement used in logging while drilling. In high resistivity environments, the LWD logs frequently have poorer vertical resolution and more adjacent bed effect than their wireline induction or resistivity counterparts. This paper presents a new method for enhancing the resolution of wave propagation resistivity logs. The resulting logs are fully corrected for adjacent bed effects and have a vertical response as good as wireline high-resolution induction logs, Typically, wave propagation LWD tools measure the attenuation and phase shift across a pair of receivers for one or more transmitter-to-receiver spacings. The phase shift and amplitude ratios across the receiver pair are converted to apparent resistivity logs. These apparent resistivity measurements are affected not only by the formation between the receivers but also the formation between the transmitters and the receivers, and the by the beds above or below the tool. This adjacent bed effect varies with the actual resistivity value as well as the contrast. The attenuation resistivity logs have more adjacent bed effects and poorer vertical resolution than the phase resistivity logs. The method described in this paper begins by picking significant bed boundaries from the actual log. It then performs inversion by repeatedly adjusting the resistivities in a model formation until the simulated log matches the actual log. Additionally, the technique can enhance the resolution of the attenuation resistivity logs to match the resolution of the phase resistivity logs. The result is a suite of resistivity logs with matched vertical responses but different radial depths of investigation. The resulting vertical response is virtually independent of resistivity, The technique can also be applied to dipping beds and deviated boreholes. The method is illustrated with field logs.
Enhanced Resolution Lwd Resistivity Logs Using A New Inversion Technique
Merchant, G.A., Strickland, Robert W., and C.E. Jackson. "Enhanced Resolution Lwd Resistivity Logs Using A New Inversion Technique." Paper presented at the SPWLA 37th Annual Logging Symposium, New Orleans, Louisiana, June 1996.
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