ABSTRACT

The fine scale response of oil industry imaging / dipmeter downhole logging tools in 3D faulted structures has been investigated via numerical modelling of electric current flow in and around the borehole. It has been demonstrated the tools respond to an interval near the borehole, rather than only the borehole wall itself. Consequently, the response of these widely-used tools is far more complex than often assumed. Examples are presented showing the ?loss? of response due to fine scale layering within the fault and their wide ranging implications. For example a very thin conductive layer, which might be a clay-rich fault ?seal?, may not be detected. We present results of a 3D numerical simulation of a generic borehole wall imaging tool for a simple dipping layer of contrasting resistivity. Initially we simulate dipping layers of contrasting resistivity for both conductive and resistive cases. These anomalies are shown to extend beyond the physical dimensions of the intersection of the layer with the borehole wall. Our simulation shows the button responses to be very sensitive with respect to azimuthal position in the presence of a dipping layer. A detailed simulation for continuous positions around the borehole (sampled every 2.5 degrees or 7 mm) show the detailed variability of the button response. The ?halo? effects, commonly seen on electrical borehole images, are apparent. These demonstrate that the button responds to the dipping layer even when the dipping layer is not intersecting the borehole wall immediately in front of the button electrode. Additionally, the detailed response for each position around the borehole wall shows the amplitude and shape changes continuously with azimuth. We present results for faults, including fine scale variability within the fault zone. The fine scale response of oil industry imaging / dipmeter downhole logging tools in 3D faulted structures has been investigated via numerical modelling of electric current flow in and around the borehole. It has been demonstrated the tools respond to an interval near the borehole, rather than only the borehole wall itself. Consequently, the response of these widely-used tools is far more complex than often assumed. Examples are presented showing the ?loss? of response due to fine scale layering within the fault and their wide ranging implications. For example a very thin conductive layer, which might be a clay-rich fault ?seal?, may not be detected. We present results of a 3D numerical simulation of a generic borehole wall imaging tool for a simple dipping layer of contrasting resistivity. Initially we simulate dipping layers of contrasting resistivity for both conductive and resistive cases. These anomalies are shown to extend beyond the physical dimensions of the intersection of the layer with the borehole wall. Our simulation shows the button responses to be very sensitive with respect to azimuthal position in the presence of a dipping layer. A detailed simulation for continuous positions around the borehole (sampled every 2.5 degrees or 7 mm) show the detailed variability of the button response. The ?halo? effects, commonly seen on electrical borehole images, are apparent. These demonstrate that the button responds to the dipping layer even when the dipping layer is not intersecting the borehole wall immediately in front of the button electrode. Additionally, the detailed response for each position around the borehole wall shows the amplitude and shape changes continuously with azimuth. We present results for faults, including fine scale variability within the fault zone.

This content is only available via PDF.
You can access this article if you purchase or spend a download.