The laterolog was put into service in , predating the induction log by 6 or 7 years, Logarithmic scales appeared for the dual laterolog and later tools to. Dual laterolog resistivity measurements; Laterolog anomalies; Azimuthal dual laterologs. 2 Array electrode tools; 3 Shallow focused. Description. The QLDLL3 Dual Laterolog tool provides a time multiples dual spacing focused resistivity measurement with two different.

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Resistivity logging is the recording,in uncased or recently cased sections of a borehole, of the resistivities or their reciprocals, the conductivities of the subsurface formations, generally along with the spontaneous potentials SPs generated in the borehole. These recordings are used for correlation of the strata and detection and quantitative evaluation of possibly productive horizons. This article discusses the types of resistivity logging devices that rely on electrodes.

During the first quarter century of well logging, the only electrical surveys ES available were the resistivity logs made with so-called lateral and normal devices plus the spontaneous potential SP. Thousands of them were run each year all over the world. Since then, new logging methods have been developed to measure values much closer to R xo and R t. Nevertheless, the conventional ES logs consisting of SP; in. Because new information can often be obtained by reinterpreting old ES logs, this chapter includes discussion of the principles and responses of the ES measurements.

The first resistivity devices were the normals and laterals. Current is injected in the formation from a single electrode and returned to a point remote from the well. The current near the injection electrode spread out radially from the electrode. Two voltage—measuring electrodes M and N on the sonde approximated the measurement of a constant-voltage spherical shell around the injection electrode.

The measurements of voltage and current are converted to a resistivity measurement. For normal devices Fig. In practice, N or B may be placed in the hole at a large distance above A and M [the voltage measured is practically the potential of M because of current from Areferred to an infinitely distant point]. The distance AM of a normal device is its spacing.

The point of measurement is midway between A and M. The most common normal spacings were 16 and 64 in. The lateral device was designed to provide a deeper resistivity measurement latefolog the normal tools, while at the same time improving the detection of thin beds.

For lateral laaterolog Fig. Current-return electrode B is at a great distance above A or at the surface. The voltage measured is approximately equal to the potential gradient at the point of measurement 0, midway between M and N. The distance AO is the spacing of the lateral device. In an alternate version of the lateral, the positions of the current and voltage electrodes are interchanged. This tool is called the “inverse,” and it records the same lsterolog values as the lateral by reciprocity.

The inverse arrangement made it more practical to record measurements by the two normals and the lateral simultaneously. Interpretation of laterals and normals is very complicated because the response is a complicated function of the formation being measured. The separation is not a clear function of invasion, lxterolog is also a function of bed thickness.

The relation of the curve to the bed is not clear at all. Many charts called departure curves were published to aid in laterolov of the ES logs. Modern interpretation methods include 2D inversion after the curves are digitized and iterative forward modeling for when they are not digitized. To laterologg R t under a variety of different logging conditions and in different formations, a simple three-parameter, step-profile invasion model is often used.

This model consists of a flushed zone of resistivity R xo and a sharp boundary at diameter d iwith the uninvaded zone of resistivity R t. Three independent, borehole-corrected resistivity measurements with appropriately chosen depths of investigation contain enough information from the formation to reliably solve for R t using this model. Measurements with the following features should be chosen: As long as the formation is invaded, assumptions lateroolg the invasion profile must be made to estimate R t.


Both logs share the same electrodes and have the same current-beam thickness, but different focusing currents give them different depths of investigation. The measure current I 0 is latsrolog from the central A0 electrode, returning to an “infinitely distant” electrode, usually at the surface. A constant-power measuring system ensures measurement accuracy over a wide range of resistivities from 0. Both the measure current and measure voltage V 0 are varied and measured, but their product power I 0 V 0 is kept constant.

Long guard electrodes are required to achieve the desired depth of investigation and measurement range. For electrode tools focused using guard electrodes, the depth of investigation increases only as the square root of the length of the guard electrodes.

This requirement results in the ft [8. A beam thickness of only 28 in. The LLs measurement shares most of the electrodes with the deeper measurement. This is achieved by operating LLs sual LLd at different frequencies. The LLs and LLd measurements have the same vertical resolution, but the LLs device uses a less constrained focusing condition in which the focusing current returns to electrodes on the array instead of to a remote electrode.

The LLs measurement therefore has a shallower depth of investigation and responds more strongly to the region around the borehole that is usually affected by invasion. The Groningen effect was named after the large Dutch gas field where the anomaly was first identified. The effect [6] is an anomalously high resistivity reading that occurs for approximately ft [30 m] below a thick, highly resistive bed such as the thick evaporitic Zechstein caprock at the Groningen field.

Because the DLL measure current is AC albeit very low frequencyskin effect reduces the volume around the well where the measure and focusing current can flow. Little of the current is able to return to the remote electrodes through the highly resistive formation, with the majority flowing in the conductive mud in the borehole. This creates a negative potential at the far reference electrode used as the potential reference for the laterolog measurement.

If casing has been set in or below the resistive zone, it accentuates the “short circuit” effect of the borehole, and the Groningen effect is more pronounced.

Drillpipe conveyance produces the same effect, with the drillpipe becoming the “short circuit. A mild Groningen effect may be difficult to identify from the LLd curve alone. The Schlumberger DLL has a modified-geometry measurement that can also be recorded.

If the Groningen effect is positively identified, an estimate of its magnitude can be made by analyzing the signal phases in the tool, and an approximate correction can be applied to the log. The LLs measurement uses different current paths and does not suffer from the Groningen effect. The array laterolog see the following is not affected by Groningen effect.

In the early s, a new dual laterolog that had an additional azimuthally segmented current electrode was introduced.

The tool records azimuthal resistivity variations around the borehole and produces an image of the variations. The deep azimuthal measurement operates at the same frequency as the deep laterolog measurement, and the currents flow from 12 azimuthal current electrodes to the surface.

In addition, the current from each azimuthal electrode is focused passively by the currents from its neighbors. The resulting operation of the azimuthal array has no effect on the LLd and LLs measurements.

Twelve azimuthal resistivities are computed, and from their sum, a high-resolution resistivity measurement, LLhr, is derived.

Electrode resistivity devices –

This is equivalent to replacing the azimuthal electrodes with a single cylindrical electrode of the same height. The high-resolution LLhr curve reads almost as deeply into the formation as a deep laterolog LLd curve, particularly when R yo is less than R t. An LLhr log can therefore replace an LLd log for interpretation, especially where its vertical resolution is an advantage.

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Individually selected azimuthal resistivities can be used in the same manner where the logged interval is azimuthally anisotropic or includes highly dipping thin beds. The azimuthal resistivity measurements are sensitive to tool eccentering in the latrrolog and to irregular borehole shape.

Auxiliary measurements are made that are very shallow, with current paths close to the tool.

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Most of the current returns to the Duap electrode near the azimuthal array. Because the borehole is generally more conductive than the formation, the current tends to stay in the mud, and the measurement responds primarily to the volume of mud in front of each azimuthal electrode. The measurement is therefore sensitive to borehole size and shape and to eccentering of the tool in the borehole.

However, for best use of the azimuthal measurements, the tool should be well centralized in fual borehole. Its dimensions are optimized to achieve similar performance as an ARI sonde with a tool that is only approximately one-half its overall length. Like the Vual tool, the azimuthal array of the HALS makes deep and shallow resistivity measurements around the borehole with a 1- or 2-ft [0. Formation resistivity images can be derived from either the deep or the shallow measurements.

Mud resistivity and tool standoff are also measured.

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In addition to providing a visual image of formation lamination and anisotropy, the azimuthal images can be used to estimate the gross formation dip and to correct deep resistivity measurements in dipping beds. The inherent vertical resolution is sharper—24 in. Real-time corrections can be made for Groningen effect, electrical path changes imposed by tough logging conditions TLC logging in which the logging tool is transported on drillpipe, and borehole effects.

A two-parameter inversion model can also be used in real time to solve for R t and d iwith R xo provided by the microcylindrically focused log MCFL measurements of the Platform Express tool. The most recent development in electrode tools is the array laterolog or array lateral tools. These combine multiple depths of investigation with 2D inversion of the data to give much improved response in invaded thin beds with conductive mud.

The Schlumberger high-resolution laterolog array HRLA tool consists of five laterolog arrays with different depths of investigation. Because it has no bridle, it does not suffer from Groningen effect. Borehole and shoulder cual are minimized by the use of laterolog-style focusing. Focusing involves injecting current from guard or bucking electrodes to ensure that the current from the central measure electrode flows into the formation rather than along the borehole.

By having all currents return to the tool body rather than surface, Groningen effect is eliminated and shoulder-bed effect reduced. More importantly, the surface current return and insulating bridle are no longer needed. All signals are measured at the same time and logging position. This avoids horns or oscillations caused by irregular tool motion and ensures that the measurements are always exactly depth-aligned. The HRLA tool uses segmented bucking electrodes and multifrequency operation ranging from 75 to Hz to acquire six simultaneous measurements.

The six modes are focused by a combination of hardware and software focusing. The hardware duual the dul in a way that is as close to focused as possible. The shallowest mode, RLA0, is mostly sensitive to the borehole and is used to estimate the mud resistivity.

The apparent resistivities RLA1 through RLA5 are all laterilog to the formation, becoming progressively deeper in investigation.