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Electrode resistivity devices
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.
Normal and lateral devices
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 Rxo and Rt. Nevertheless, the conventional ES logs (consisting of SP; 16-in. normal; 64-in. normal; and 18-ft, 8-in. lateral) are stored in log archives all over the world. 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. These were, in concept, extensions to laboratory four-terminal resistivity-measuring cells. 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. 1), the distance AM is small: 1 to 6 ft as compared with MN, MB, and BN. 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 A), referred 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 than the normal tools, while at the same time improving the detection of thin beds. For lateral devices (Fig. 2), measuring electrodes M and N are close to each other and located several feet below current electrode A. 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. A and B are moved to M and N, and N and M are moved to B and A. This tool is called the "inverse," and it records the same resistivity 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. Fig. 3 shows a computed response of the 16- and 64-in. normals for a series of beds with and without invasion. The separation is not a clear function of invasion, but is also a function of bed thickness. Fig. 4 shows the 18-ft, 8-in. lateral tool in the same series of beds. The relation of the curve to the bed is not clear at all. Many charts (called departure curves) were published to aid in interpretation 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.<html><parsererror style="display: block; white-space: pre; border: 2px solid #c77; padding: 0 1em 0 1em; margin: 1em; background-color: #fdd; color: black">
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To estimate Rt 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 Rxo and a sharp boundary at diameter di, with the uninvaded zone of resistivity Rt. Three independent, borehole-corrected resistivity measurements with appropriately chosen depths of investigation contain enough information from the formation to reliably solve for Rt using this model. Measurements with the following features should be chosen: small, correctable borehole effects; similar vertical resolutions; and well-distributed radial depths of investigation—one reading as deep as practical, one very shallow reading, and one intermediate reading.
In conductive muds, the Dual Laterolog (DLL) Resistivity– Rxo combination tool provides simultaneous measurements suitable for evaluating Rt, Rxo, and di. It should be said that the value of Rt in a given bed is an interpreted parameter, and is almost never measured. As long as the formation is invaded, assumptions about the invasion profile must be made to estimate Rt.
Dual laterolog resistivity measurements
Fig. 5 shows the electrode array used for deep and shallow laterolog measurements (LLd and LLs, respectively). 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 (I0) is emitted from the central A0 electrode, returning to an "infinitely distant" electrode, usually at the surface. The focusing current (Ia) flows from the A1 and A2 (and A1′ and A2′) electrodes to a distant electrode for the LLd measurement and from A1 to A2 (and A1′ to A2′) for the LLs measurement. The focusing current is adjusted so that electrodes M1 and M2 (and M1′ and M2′) remain at the same potential.
A constant-power measuring system ensures measurement accuracy over a wide range of resistivities (from 0.2 to 40,000 ohm•m). Both the measure current and measure voltage (V0) are varied and measured, but their product power I0V0 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 28-ft [8.5-m] total length of the LLd electrode array. A beam thickness of only 28 in. [0.7 m], however, ensures good vertical resolution.
The LLs measurement shares most of the electrodes with the deeper measurement. This is achieved by operating LLs and 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 is an anomalously high resistivity reading that occurs for approximately 100 ft [30 m] below a thick, highly resistive bed such as the thick evaporitic Zechstein caprock at the Groningen field. The effect is maximum around 1 ohm•m. Because the DLL measure current is AC (albeit very low frequency), skin 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." This problem severely limits the use of drillpipe conveyance of the DLL in high-angle or horizontal wells in many reservoirs.
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. This provides an LLg curve that separates from the LLd curve when Groningen effect is present. 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.
Azimuthal dual laterologs
In the early 1990s, a new dual laterolog that had an additional azimuthally segmented current electrode was introduced. The Schlumberger ARI* Azimuthal Resistivity Imager has a set of segmented azimuthal electrodes incorporated in a conventional dual laterolog array. The tool records azimuthal resistivity variations around the borehole and produces an image of the variations. The azimuthal electrodes are placed at the center of the A2 electrode of the DLL tool and do not interfere with the standard LLd and LLs measurements.
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. They are focused by the current from the A2 electrode’s upper and lower portions and by currents from the other current electrodes. 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. 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 Ryo is less than Rt. An LLhr log can therefore replace an LLd log for interpretation, especially where its vertical resolution is an advantage. 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 borehole and to irregular borehole shape. Auxiliary measurements are made that are very shallow, with current paths close to the tool. Most of the current returns to the A2 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 the borehole.
A High-Resolution Azimuthal Laterolog Sonde (HALS) resembles an ARI or a dual laterolog array. Although it is shorter than either of these tools, the HALS is not just a scaled-down version. 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 ARI tool, the azimuthal array of the HALS makes deep and shallow resistivity measurements around the borehole with a 1- or 2-ft [0.3- or 0.6-m] vertical resolution. (In this chapter, vertical resolution is defined as the 90% width of the vertical response function.) Formation resistivity images can be derived from either the deep or the shallow measurements. Mud resistivity and tool standoff are also measured. 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.
Because the HALS is shorter than ARI and DLL sondes, the borehole effect of the shallow measurement is larger. Combined with the slightly reduced depth of investigation of the deep array, this reduces the precision of the invasion correction in cases of invasion where d i > 50 in. [1.3 m]. The inherent vertical resolution is sharper—24 in. [0.6 m] for the HALS deep and shallow resistivity curves (HLLD and HLLS, respectively) compared with 40 in. [1 m] for the LLd and LLs curves of the DLL log. The HALS provides high-resolution deep and shallow curves (HRLd and HRLs, respectively) with the same 12-in. vertical resolution as the LLhr curve of an ARI log.
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 Rt and di, with Rxo provided by the microcylindrically focused log (MCFL) measurements of the Platform Express tool.
* Throughout this chapter, tool names are service marks of the referenced companies.
Array electrode tools
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. All current is returned to electrodes above and below the array, so no bridle is used. Because it has no bridle, it does not suffer from Groningen effect.
Borehole and shoulder effects 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 270 Hz) to acquire six simultaneous measurements. The six modes are focused by a combination of hardware and software focusing. The hardware injects the currents 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 sensitive to the formation, becoming progressively deeper in investigation. Fig. 6 shows the radial response of the optimized HRLA tool compared to the HLLd and HLLs measurements from the HALS tool. Fig. 7 shows the HRLA logs compared to the DLL, LLd, and LLs logs.
The shoulder-bed and invasion responses of laterolog tools are combined in the tool response in ways that are difficult to separate. For this reason, some sort of 2D inversion is necessary to determine the formation parameters accurately in the case of invaded thin beds. An automatic 2D inversion is available that yields the best results from the data in invaded thin beds. Fig. 8 shows the field logs, 1D inversion results, and 2D inversion results in the same well.
The Baker Atlas high-definition lateral log (HDLL) tool acquires 8 potential and 16 first differences, and computes 14 second differences. These are used as inputs to a 2D inversion to solve for formation resistivities. The inversion process begins with the raw data input and an initial estimate of a parametric model describing the formation resistivity distribution. Simulated logs or synthetic tool responses for each of the array sensors are derived for the selected initial formation parameters as a solution to Maxwell’s equations.
The inversion process is initiated by using the shallow measurements (associated with short-spacing sensors) to identify and evaluate the shallow-formation resistivity structure. Deep measurements (associated with the longest sensor spacings) are used in evaluating the uncontaminated formation resistivity structure. The intermediate measurements are used to derive the radial-invasion profile. Using inversion processing, measurements of different vertical resolution and depths of investigation are combined in a single interpretation process to provide an accurate resistivity distribution image.
Final inversion results consist of the estimated resistivity structure satisfying all HDLL data and the statistical quality indicators represented in terms of the importance of the formation parameter along with its corresponding error bounds. Fig. 9 shows a comparison of HDLL Rt and Rxo inversion results with conventional Dual Laterolog (RS is DLL shallow and RD is DLL deep) and Micro Laterolog (MLL) measurements.
Shallow focused laterologs (SFLs)
The SFL device measures the conductivity of the formation near the borehole. It uses small electrodes that can be combined with the dual-induction tool to provide shallow-investigation data for invasion evaluation.
The SFL device uses two independent current systems. A focusing current system establishes constant-potential spherical "shells" around the current electrode, even in the presence of a conductive borehole, and the I0 survey current flows through the volume of investigation.
The SFL electrode array consists of current-emitting, current-return, and measure electrodes. Two equipotential spheres are established around the I0 survey current electrode; the first sphere is approximately 9 in. [0.2 m] from the electrode, and the other is approximately 50 in. [1.3 m] from it. The volume of formation between these two surfaces is constant, and because a potential difference of 2.5 mV is maintained between the spheres, the conductivity of this volume of formation is proportional to the I0 survey current intensity.
Cased-hole resistivity tools
Despite the apparent paradox of measuring formation resistivity through the highly-conductive steel casing, tools are now available that can measure the formation resistivity to considerable accuracy. The idea originated in the 1930s and was revisited by Kaufman and Vail in the late 1980s. Commercial tools were introduced in 2000 by Schlumberger and Baker Atlas. Both of these tools operate on the Kaufman-Vail principles.
The Schlumberger cased-hole formation resistivity (CHFR) tool has three sets of four arms that contain electrodes that are forced into contact with the inside of the casing. A current generator on the surface is connected to an electrode at the top of the tool. The current is injected into the casing and returns to an electrode in the earth some distance from the casing. Although most of the current returns through the casing, some small fraction of it will leak off from the outside of the casing and will return through the earth. This leakoff current forms the basis for the CHFR measurement.
The leakoff current is determined by measuring the voltage drop along a section of the casing. The double-differenced voltage Δ contains both the leakoff term and the voltage drop produced by the current flowing through the casing and the resistance, R, of the casing. The current switch is changed to position 2. Now a current from inside the tool is sent from the upper electrode to a lower electrode. The voltage difference is now measuring the resistance of the casing, R1. All of the measurements are combined in the equation
to produce a formation resistivity measurement. The measurements are taken while the tool is stationary and take approximately a minute per station.
Fig. 10 shows a log of the CHFR in a newly cased well compared with open-hole HALS and AIT logs. The comparison is very good, and in the zone from 865 to 900 ft, with Rxo > Rt, the CHFR agrees well with the AIT 90-in. log, showing the great depth of investigation of the CHFR log.
Cased-hole resistivity is becoming accepted for applications including contingency logging, reservoir monitoring, and evaluation of old producing wells. One application combines CHFR with pulsed-neutron logs to do cased-hole formation evaluation.
The Baker Atlas Through-Casing Resistivity (TCR) tool operates on a similar measurement principle as the CHFR. It is smaller in diameter (2 1/8 vs. 3 1/8 in. for the CHFR).
GVR resistivity-at-the-bit tool
The Schlumberger Geovision Resistivity (GVR) tool is an electrode resistivity tool that measures five resistivity values—bit, ring, and three button resistivities—as well as gamma-ray and shock measurements.
A 1500-Hz alternating current passes through the toroidal-coil lower transmitter that is 1 ft from the bottom of the tool, inducing a voltage in the collar below. Current flows through the collar and bit and into the formation in front of the bit, returning to the collar farther up the drillstring. The resistivity at the bit is derived from the axial current, which is measured by a ring monitor toroid, and the induced voltage, which is a function of the transmitter current.
When the GVR tool is positioned directly above the bit, the resistivity measurement has a resolution of approximately 2 ft [61 cm], which is usually adequate for "geostopping"—stopping drilling precisely at casing or coring depths.
Focused multidepth resistivity
There are four focused-resistivity measurements incorporated in the RAB tool. These include the ring electrode measurement, with a depth of investigation of approximately 9 in. [0.23 m], and three button electrode measurements, with depths of approximately 1, 3, and 5 in. [2.5, 8, and 13 cm, respectively] into the formation.
The button measurements are radial, acquiring azimuthal resistivity profiles as the tool rotates in the borehole. A rotational speed of at least 30 rpm is required for full profile recording, with each button recording 56 resistivity measurements per rotation. The data are usually stored downhole for later retrieval, although a compressed image and selected button data may be transmitted to the surface in real time together with the ring and bit resistivities and gamma-ray measurements. Fig. 11 shows a recorded GVR image compared with an image from the wireline FMI borehole resistivity image tool (see the following for a description of this tool).
All four focused resistivities use the same measurement principle: Current from the upper transmitter flows down the collar and out into the formation, leaving the collar perpendicular to its surface and returning to the collar above the transmitter. Low-impedance circuits measure the current at each button electrode, and the axial current flowing down the collar is measured at the ring electrode by the ring monitor toroid and at the lower transmitter by the lower monitor toroid. These resistivity measurements are repeated using current from the lower transmitter.
In a homogeneous formation, the equipotential surfaces near the button and ring electrodes on the RAB tool are cylindrical. However, in layered formations, there is a tendency for current to flow preferentially in the more conductive beds and avoid the more resistive beds. This effect is known as "squeeze" for conductive beds and "antisqueeze" for resistive beds, and it leads to horn-like distortion of resistivity readings at bed boundaries.
A cylindrical focusing technique (CFT) is used to measure and compensate for this distortion by restoring the cylindrical geometry of the equipotential surfaces in front of the measurement electrodes. This is achieved by regulating the currents generated by the upper and lower transmitters for zero axial current flow at the ring monitor electrode, which avoids current flow along the borehole and focuses the ring current into the formation. This focusing technique produces a response very similar to that of a wireline laterolog.
Environmental effects on laterolog tools
Laterolog and SFL log readings are influenced by the borehole mud, adjacent shoulder beds, and the invaded zone as well as the uninvaded formation. If automatic corrections are not available, log-interpretation charts provided by the service company are used to manually correct the log readings for these influences. The borehole corrections must always be made first, followed by bed-thickness corrections and finally invasion corrections of the determination of Rt, Rxo, and di.
The "geometric factor" relates the effect of a portion of formation on the logging tool reading to its position relative to the tool in an infinite homogeneous medium. It has a particular application to induction logging tools, but pseudo-geometrical factors are a useful comparative tool for other resistivity devices.
Fig. 12 is a plot of integrated pseudogeometrical factors for several focused resistivity logs. It graphically compares the relative contributions of the invaded zone to the tool responses and their relative depths of investigation. The good spread in radial characteristics of the LLd and LLs measurements enables accurate resistivity analysis over a wide range of invasion conditions.
To evaluate the three unknowns of the simple step-profile invasion model (Rxo, Rt, and di), a combination of at least three carefully chosen resistivity measurements is required. LLd and LLs curves, with a very shallow resistivity measurement that reads Rxo directly, may be sufficient. See the section on invasion interpretation for more details on the determination of Rt, Rxo, and di.
Induction vs. laterolog measurements
Laterolog and induction logging tools each have unique characteristics that favor their use in specific situations and applications.
The induction log is generally recommended for holes drilled with only moderately conductive drilling muds or nonconductive muds (e.g., oil-based mud (OBM)) and for empty or air-drilled holes. The laterolog is generally recommended for holes drilled with very conductive drilling muds (i.e., salt muds).
Induction tools are conductivity-sensitive devices, which are most accurate in low- to medium-resistivity formations. Laterolog tools are resistivity devices, which are most accurate in medium- to high-resistivity formations. In practice, both modern laterolog and induction-logging tools are suitable for most logging conditions, and it is no longer practical to make a specific recommendation for one type in preference to the other, except in extreme conditions.
Laterolog devices see the more resistive zones, and induction tools see the more conductive zones. Therefore, when Rxo is greater than Rt, an induction tool is preferred for Rt determination because laterolog tools will be affected mostly by Rxo. Conversely, a laterolog tool is preferred when Rxo is less than Rt. Conductivity in the borehole has a strong influence on an induction measurement, but little influence on a laterolog measurement.
|I||=||electrical current, Amperes|
|Rxo||=||resistivity of the invaded zone|
|Rt||=||resistivity of the uninvaded formation|
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- Béguin, P. et al. 2000. Recent Progress on Formation Resistivity Through Casing. Paper CC presented at the 2000 SPWLA Annual Logging Symposium, Dallas, 4–7 June.
- Maurer, H.M. and Hunziker, J. 2000. Early Results of Through-Casing Field Tests. Paper DD presented at the 2000 SPWLA Annual Logging Symposium, Dallas, 4–7 June.
- Rosthal, R.A., Young, R.A., Lovell, J.R. et al. 1995. Formation Evaluation and Geological Interpretation from the Resistivity-at-the-Bit Tool. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22–25 October. SPE-30550-MS. http://dx.doi.org/10.2118/30550-MS
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