Investigation on electromagnetic measurement ahead of drill-bit

In drilling for oil, overpressured shale zones, which can be disastrous to drilling, are occasionally encountered. The overpressured zones usually exhibit lower electrical resistivity. Electromagnetic methods are therefore proposed to predict the location and intensity of the overpressured zones at tens of meters ahead of a drill-bit. The existing methods in petroleum well logging are sensitive to the formation volume up to 3 meters lateral to the borehole. To predict the formation ahead of drill-bit, it is essential to use a time-domain or a wide-band frequency domain approach. In a conductive medium, the high frequency fields are more confined to the volume around the source while the low frequency components reach larger volume of formation. In the time-domain situation, early time response is closely related to the formation property near the sensor while later time measurement is more sensitive to the property of the far away formation. It is found that the presence of an overpressured zone at 10´s of meters ahead of a drill-bit can be detected. Besides the prediction of overpressured zone, the method can also provide better geosteering to guide the drilling trajectory and to improve formation evaluation