W.H. Meyer
Teleco Oilfield Services, Inc.
ABSTRACT
A technique has been developed to invert the phase difference and amplitude ratio apparent resistivity data from a 2 MHz propagation resistivity tool. The result of the inversion is to improve the apparent vertical resolution of the measurements and produce more accurate resistivity values in thin beds. The inversion uses a two step process which first produces an approximate answer that is then used as an initial estimate for the more accurate second step. The process is both fast (43 seconds for 100 feet of log data on a 6 megaflop workstation) and robust.
To test this inversion process, several synthetic logs were calculated and then inverted. In all cases where the borehole was perpendicular to the beds, the inversion greatly improved the vertical resolution of the log. This method works best for vertical boreholes because the model for the inversion is a layered formation with a vertical borehole. When the beds dip steeply relative to the borehole, the inversion is less successful, but the logs are still improved.
Synthetic logs representing invaded zones were also successfully inverted, and the indications of invasion were still evident on the inverted logs. The inverted amplitude ratio and phase difference resistivities from an invaded thin bed were virtually identical to the resistivities that would have been expected from a thick bed with the same Rt and invasion profile. In this case, therefore, invasion correction charts could have been used to determine Rt.
Field logs acquired by the Conoco Borehole Test Group in Summer of 1990 were also processed with very good results. These results compare favorably with two different deconvolved induction logs from the same borehole.