WELLOG FOCUSED RESISTIVITY
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THE PROBLEM AND THE SOLUTION:
In order to measure high formation resistivities in a borehole containing conductive mud, focused resistivity tools are required. Focused resistivity tools offer good vertical resolution and excellent penetration. A problem with conventional resistivity tools is that they become inaccurate when they are used to log relatively high resistivity formations in low resistivity mud. The reason for the inaccuracy is that the current produced by the tool follows the path of least resistance. The path of least resistance is the low resistivity mud in the borehole. Because of these conditions, most of the current will flow in the borehole. The result is that high resistivity formations lack character and bed boundaries become difficult to define. In 1951, H. Doll (Schlumberger) is credited with demonstration of a tool that used guard electrodes above and below the current electrode. This tool commonly referred to as a guard tool provides a solution to the problem of borehole current. The guard electrodes having the same voltage potential as the current electrode oppose borehole current flow and force the current from the measuring electrode (M) to flow outward into the surrounding formation. The longer the guard electrodes are, the deeper the current will flow into the surrounding formation.
View image of focused tool.
The guard electrodes are placed above and below the current (measuring) electrode. The depth of penetration is about three times the length of one of the guard electrodes. If a guard section is 6 feet in length, then the current will generally penetrate 18 feet into the formation.
The measuring electrode has a series resistance connected within the tool. Current flow through the measuring electrode also flows through the series resistor and develops a voltage directly related to formation conductivity. Formation conductivity is inversely related to formation resistivity, therefore the tool response can result in calculated resistivity. The thickness of the current beam is approximately equal to the length of the measuring electrode.
COMPARISON TO E-LOG:
In a conventional E-log tool, when the current electrode is in a relatively thin bed having resistivity greater than the surrounding bed, the current will tend to flow into the surrounding (lower resistivity) bed. The guard tool provides better thin bed definition because the current is focused into the formation being logged. It is interesting to note, that the lower resistivity surrounding formation also serves to “guard” or focus the current.
Limitations are imposed due to the inherent design of the guard tool. Because the tool must have long guard electrodes above and below the measuring electrode, it is not possible to log resistivity to the bottom of the borehole. The longer the guard electrodes are, the farther the measuring electrode will be from the bottom of the hole when the tool reaches TD. Another limitation is imposed because of the current flow in vicinity of the borehole, that will influence measurement of SP. It is difficult to obtain a reliable measurement of SP within 25 feet of the guard tool. It is because of this limitation, that SP cannot be measured to the bottom of the hole.
LATEROLOG®: (Note: The name Laterolog is Registered by Schlumberger)
The Laterolog tool uses multiple conventional (short) electrodes above and below a central current electrode (A0). Typical electrode arrangement is two monitoring electrode pairs (M1, M2) above and below Ao. Two additional “bucking electrodes”, A1 above and A2 below the M1, M2 electrode pairs. A reference electrode (N) is placed farther above the current and measuring electrodes. Two electrodes are placed at the surface, one for current return and a second for voltage reference. Laterolog tools in several versions have been released, Laterolog 7, Laterolog 8, Laterolog 3, Dual laterolog, and Spherically Focused Log (SFLtm Schlumberger).
Microlaterolog and others:
Pad mounted versions of the laterolog have been used for measurement of resistivity of the flushed zone (Rxo). Service companies have marketed versions also known as FoRxo tool, Minilog, Microlog, Microlaterolog. These electrode arrays are mounted on pad mounted caliper tools and pressed against the borehole wall. In situ measurement of mud filte resistivity (Rmf) and flushed zone resistivity (Rxo) are obtained.