Decoupling of electromagnetic MWD tool 's signals and its application for ahead-of-bit detection

Key words:  ahead-of-bit detection  dyadic Green 's functions  measurements-while-drilling (MWD)  decouple

Abstract:

The response of the electromagnetic MWD tool was computed via the dyadic Green 's functions in horizontally stratified anisotropic media. A method of decoupling anomalous formation signals ahead of the bit from formation signals around the tool to a maximum extent has been obtained for both alternating arrangement and serial arrangement of transmitter and receiver antennas. A method to maximiaze decoupling of horizontal conductivity from vertical conductivity in the two sets of signals above has also been obtained for anisotropic formations. The results show that the induced electromotive force from around-the-tool formations is only affected by the formation resistivity in between the lowest and highest antennas, and is not sensitive to the presence of layers ahead of the bit, therefore the formation conductivity can be accurately obtained. The induced electromotive force from ahead-of-bit formations is only affected by the layers suggesting that it can be used for ahead-of-bit detection to accurately detect the presence of the layer. The decoupling extent of the ahead-of-bit signal from the around-the-tool signal is related to the dip angle of the formation with respect to the tool 's axis, and large dipping angle leads to significant degradation in the decouping effect.