Guard Log

Key Takeaways

• Guard log electrode configuration and current focusing mechanism distinguish it from the unfocused normal and lateral resistivity tools that preceded it in wireline logging history: the guard log electrode array consists of a central current electrode (A0) flanked by two guard or bucking electrodes (A1, A1-prime) above and below at specified spacings, all carried on a metal mandrel that is pressed against the borehole wall by a pad or bow spring; the bucking electrodes carry currents automatically adjusted (by a servomechanism in the surface electronics) to maintain the potential difference between A0 and A1 at zero, which requires that the guard currents be equal to each other and equal in magnitude to the primary current from A0, forcing the primary current from A0 to flow radially into the formation in a narrow disk rather than diverging up and down the borehole; the resistivity measured by the guard log is calculated from the potential at a monitoring electrode near A0 divided by the primary current from A0, with a geometric correction factor determined by the tool geometry; the focusing depth (the radial distance into the formation at which the measurement primarily responds) is controlled by the guard electrode spacing, with longer guard spacing producing deeper focusing but coarser vertical resolution, and the standard guard log design represents a specific trade-off between depth of investigation and vertical resolution optimized for shallow flushed zone measurement.
• Guard log applications in invasion analysis use the resistivity of the flushed zone measured by the guard log (Rxo) in combination with the deeper-reading laterolog or induction tool (Rt) to evaluate the invasion profile and to correct the deep resistivity measurement for invasion effects: the ratio Rxo/Rt (the ratio of flushed zone resistivity to true formation resistivity) reflects the ratio of residual oil saturation to original oil saturation in the flushed zone, and combined with the mud filtrate resistivity (Rmf), provides an estimate of the invasion depth and the movable oil saturation from the crossplot of Rxo versus Rt using the empirical Tornado chart invasion correction charts; in a water-bearing formation where the flushed zone has been displaced by fresh mud filtrate, Rxo is higher than Rt (the fresh filtrate is more resistive than the saline formation water), and the ratio Rxo/Rt can be used to estimate the formation water resistivity Rw from Archie's relation when both the flushed zone saturation and the formation resistivity factor are known; in a hydrocarbon-bearing formation where both the original pore fluid and the filtrate are relatively resistive, the contrast between Rxo and Rt may be smaller and the invasion correction less significant, but the guard log measurement still provides the shallow data point in the multi-tool invasion analysis that enables determination of the true formation resistivity Rt from the combination of shallow, medium, and deep resistivity measurements.
• Guard log vertical resolution compared to other resistivity measurements determines its utility for thin-bed evaluation in finely laminated reservoirs: the vertical resolution of a guard log is controlled by the spacing between the guard electrodes and the size of the survey current beam in the formation, typically providing a vertical resolution of approximately 2 feet (0.6 meters) in standard configurations, which is substantially better than the 4 to 8 feet vertical resolution of the medium and deep laterolog tools but coarser than the 1 to 2 inch resolution of the microlog pad tools; in finely laminated sequences (interbedded sands and shales with individual layer thicknesses of 1 to 3 feet), the guard log with 2-foot resolution provides a useful intermediate-scale measurement that resolves individual sand laminae better than the deep tool but without the extremely shallow investigation of the microlog; the shoulder effect (the influence of adjacent formation resistivity on the guard log measurement in beds thinner than the guard electrode spacing) introduces systematic errors in the guard log reading in thin beds, causing the reading to be pulled toward the resistivity of the adjacent thick beds and potentially misrepresenting the thin bed's true resistivity by 20 to 50 percent in extreme cases of high-contrast adjacent beds.
• Guard log environmental corrections required for accurate Rxo determination include borehole diameter correction, mud resistivity correction, and formation dip correction in highly deviated wells where the guard log current sheet is not perpendicular to the bedding planes: the borehole size correction for guard logs addresses the systematic error introduced by the portion of the guard log current that flows through the borehole mud rather than the formation, which increases as the borehole diameter increases relative to the tool's contact area and current focusing geometry; the mud resistivity correction accounts for the fact that the guard log's measured apparent resistivity includes the contribution of the mud in the borehole annulus between the tool and the formation, which must be subtracted to obtain the formation contribution alone; in wells with high formation dip (greater than 30 to 40 degrees) or in horizontal wells, the guard log current sheet that is intended to flow radially in the plane perpendicular to the borehole axis instead intersects multiple formation beds at acute angles, causing the measured apparent resistivity to reflect an average of the different formation resistivities along the current path rather than the resistivity of the single formation at the measurement depth.
• Guard log historical role in the development of modern resistivity logging illustrates the progressive improvement of formation evaluation technology from the unfocused tools of the 1920s through the focused tools of the 1950s to the modern array laterolog and array induction tools that provide simultaneous multiple depth of investigation measurements from a single logging pass: the unfocused normal and lateral resistivity tools introduced by Schlumberger in the late 1920s were the first generation of resistivity logging devices, providing qualitative formation evaluation but suffering from severe borehole and adjacent-bed effects that limited their quantitative accuracy; the guard log and laterolog-3 introduced in the late 1940s and early 1950s demonstrated that current focusing could substantially reduce borehole and adjacent-bed effects and improve the accuracy of resistivity measurement, particularly in salt-mud environments where the induction log was less effective; the development of the dual laterolog (shallow and deep laterolog in a single tool) in the 1970s, the array laterolog with multiple electrode spacing in the 1990s, and the high-definition laterolog array (HDLL) in the 2000s represents the successive improvement of the focused resistivity measurement concept that the original guard log introduced, progressively increasing the depth of investigation, vertical resolution, and environmental correction accuracy while reducing the acquisition time through simultaneous multi-depth measurements.