Changes

==Electrical borehole scanning: the formation microscanner==

==Electrical borehole scanning: the formation microscanner==

Electrical borehole scanning is an extension of the dipmeter technique. In this method, a large number of closely spaced electrodes of 0.2-in. diameter are mounted on a conductive pad and pressed against the borehole wall. The amount of current emitted from each electrode is recorded as a function of azimuth and depth (Figure 6). The tool thus produces a microresistivity map. Currently on the market are versions with two and four imaging pads perpendicular to each other. The resulting image stripes are about 2.5 in. wide and are oriented azimuthally by a downhole magnetometer unit. In a 8.5-in. borehole, the four-pad Formation MicroScanner provides a 45% circumferential coverage. Repeat logging passes can often increase this percentage. By convention, darker gray tones are used for lower resistivities.

Electrical borehole scanning is an extension of the dipmeter technique. In this method, a large number of closely spaced electrodes of 0.2-in. diameter are mounted on a conductive pad and pressed against the borehole wall. The amount of current emitted from each electrode is recorded as a function of azimuth and depth ([[:file:borehole-imaging-devices_fig6.png|Figure 6]]). The tool thus produces a microresistivity map. Currently on the market are versions with two and four imaging pads perpendicular to each other. The resulting image stripes are about 2.5 in. wide and are oriented azimuthally by a downhole magnetometer unit. In a 8.5-in. borehole, the four-pad Formation MicroScanner provides a 45% circumferential coverage. Repeat logging passes can often increase this percentage. By convention, darker gray tones are used for lower resistivities.

Formation MicroScanner images record changes in rock resistivity caused by variations in [[porosity]] and clay content of a small rock volume in the vicinity of the borehole wall. Increased pad stand-off due to mudcake buildup on the borehole wall may decrease the spatial resolution, while abrupt changes in tool movement may produce a local misalignment or sawtooth effect in the layers ([[:file:borehole-imaging-devices_fig7.png|Figure 7]]). Important bedding types and surfaces can be identified and measured for their dip and azimuth ([[:file:borehole-imaging-devices_fig8.png|Figure 8]]), as are fractures ([[:file:borehole-imaging-devices_fig9.png|Figure 9]]) and stylolites ([[:file:borehole-imaging-devices_fig10.png|Figure 10]]). Faults are often readily recognized on Formation MicroScanner images because of the offset of rock types across the fault plane.

 

Formation MicroScanner images record changes in rock resistivity caused by variations in [[porosity]] and clay content of a small rock volume in the vicinity of the borehole wall. Increased pad stand-off due to mudcake buildup on the borehole wall may decrease the spatial resolution, while abrupt changes in tool movement may produce a local misalignment or sawtooth effect in the layers (Figure 7). Important bedding types and surfaces can be identified and measured for their dip and azimuth (Figure 8), as are fractures (Figure 9) and stylolites (Figure 10). Faults are often readily recognized on Formation MicroScanner images because of the offset of rock types across the fault plane.

 

[[file:borehole-imaging-devices_fig7.png|thumb|{{figure number|7}}Formation MicroScanner images of cross-bedded sequence showing drill marks (d.m.) at WSW and sawtooth effects (s.e.) on some layers.]]

 

[[file:borehole-imaging-devices_fig8.png|thumb|{{figure number|8}}Formation MicroScanner images showing dipping unconformity at contact (arrow) of Cretaceous clastic rocks with Mississippian carbonates.]]

 

[[file:borehole-imaging-devices_fig9.png|thumb|{{figure number|9}}Formation MicroScanner images of open conductive fracture (arrow).]]

 

[[file:borehole-imaging-devices_fig10.png|thumb|{{figure number|10}}Formation MicroScanner images of stylolitic limestone.]]

==See also==

==See also==